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Message-ID: <zwbuflyk2fmauewhian7hcqibcbyjjofqowbddrud4ohkbora4@yrdd4rjt47tt>
Date: Mon, 13 Jan 2025 14:18:57 +1100
From: Alistair Popple <apopple@...dia.com>
To: "Darrick J. Wong" <djwong@...nel.org>
Cc: akpm@...ux-foundation.org, dan.j.williams@...el.com,
linux-mm@...ck.org, alison.schofield@...el.com, lina@...hilina.net,
zhang.lyra@...il.com, gerald.schaefer@...ux.ibm.com, vishal.l.verma@...el.com,
dave.jiang@...el.com, logang@...tatee.com, bhelgaas@...gle.com, jack@...e.cz,
jgg@...pe.ca, catalin.marinas@....com, will@...nel.org, mpe@...erman.id.au,
npiggin@...il.com, dave.hansen@...ux.intel.com, ira.weiny@...el.com,
willy@...radead.org, tytso@....edu, linmiaohe@...wei.com, david@...hat.com,
peterx@...hat.com, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org, nvdimm@...ts.linux.dev,
linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
linux-xfs@...r.kernel.org, jhubbard@...dia.com, hch@....de, david@...morbit.com,
chenhuacai@...nel.org, kernel@...0n.name, loongarch@...ts.linux.dev
Subject: Re: [PATCH v6 21/26] fs/dax: Properly refcount fs dax pages
On Fri, Jan 10, 2025 at 08:54:55AM -0800, Darrick J. Wong wrote:
> On Fri, Jan 10, 2025 at 05:00:49PM +1100, Alistair Popple wrote:
> > Currently fs dax pages are considered free when the refcount drops to
> > one and their refcounts are not increased when mapped via PTEs or
> > decreased when unmapped. This requires special logic in mm paths to
> > detect that these pages should not be properly refcounted, and to
> > detect when the refcount drops to one instead of zero.
> >
> > On the other hand get_user_pages(), etc. will properly refcount fs dax
> > pages by taking a reference and dropping it when the page is
> > unpinned.
> >
> > Tracking this special behaviour requires extra PTE bits
> > (eg. pte_devmap) and introduces rules that are potentially confusing
> > and specific to FS DAX pages. To fix this, and to possibly allow
> > removal of the special PTE bits in future, convert the fs dax page
> > refcounts to be zero based and instead take a reference on the page
> > each time it is mapped as is currently the case for normal pages.
> >
> > This may also allow a future clean-up to remove the pgmap refcounting
> > that is currently done in mm/gup.c.
> >
> > Signed-off-by: Alistair Popple <apopple@...dia.com>
> >
> > ---
> >
> > Changes since v2:
> >
> > Based on some questions from Dan I attempted to have the FS DAX page
> > cache (ie. address space) hold a reference to the folio whilst it was
> > mapped. However I came to the strong conclusion that this was not the
> > right thing to do.
> >
> > If the page refcount == 0 it means the page is:
> >
> > 1. not mapped into user-space
> > 2. not subject to other access via DMA/GUP/etc.
> >
> > Ie. From the core MM perspective the page is not in use.
> >
> > The fact a page may or may not be present in one or more address space
> > mappings is irrelevant for core MM. It just means the page is still in
> > use or valid from the file system perspective, and it's a
> > responsiblity of the file system to remove these mappings if the pfn
> > mapping becomes invalid (along with first making sure the MM state,
> > ie. page->refcount, is idle). So we shouldn't be trying to track that
> > lifetime with MM refcounts.
> >
> > Doing so just makes DMA-idle tracking more complex because there is
> > now another thing (one or more address spaces) which can hold
> > references on a page. And FS DAX can't even keep track of all the
> > address spaces which might contain a reference to the page in the
> > XFS/reflink case anyway.
> >
> > We could do this if we made file systems invalidate all address space
> > mappings prior to calling dax_break_layouts(), but that isn't
> > currently neccessary and would lead to increased faults just so we
> > could do some superfluous refcounting which the file system already
> > does.
> >
> > I have however put the page sharing checks and WARN_ON's back which
> > also turned out to be useful for figuring out when to re-initialising
> > a folio.
> > ---
> > drivers/nvdimm/pmem.c | 4 +-
> > fs/dax.c | 212 +++++++++++++++++++++++-----------------
> > fs/fuse/virtio_fs.c | 3 +-
> > fs/xfs/xfs_inode.c | 2 +-
> > include/linux/dax.h | 6 +-
> > include/linux/mm.h | 27 +-----
> > include/linux/mm_types.h | 7 +-
> > mm/gup.c | 9 +--
> > mm/huge_memory.c | 6 +-
> > mm/internal.h | 2 +-
> > mm/memory-failure.c | 6 +-
> > mm/memory.c | 6 +-
> > mm/memremap.c | 47 ++++-----
> > mm/mm_init.c | 9 +--
> > mm/swap.c | 2 +-
> > 15 files changed, 183 insertions(+), 165 deletions(-)
> >
> > diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> > index d81faa9..785b2d2 100644
> > --- a/drivers/nvdimm/pmem.c
> > +++ b/drivers/nvdimm/pmem.c
> > @@ -513,7 +513,7 @@ static int pmem_attach_disk(struct device *dev,
> >
> > pmem->disk = disk;
> > pmem->pgmap.owner = pmem;
> > - pmem->pfn_flags = PFN_DEV;
> > + pmem->pfn_flags = 0;
> > if (is_nd_pfn(dev)) {
> > pmem->pgmap.type = MEMORY_DEVICE_FS_DAX;
> > pmem->pgmap.ops = &fsdax_pagemap_ops;
> > @@ -522,7 +522,6 @@ static int pmem_attach_disk(struct device *dev,
> > pmem->data_offset = le64_to_cpu(pfn_sb->dataoff);
> > pmem->pfn_pad = resource_size(res) -
> > range_len(&pmem->pgmap.range);
> > - pmem->pfn_flags |= PFN_MAP;
> > bb_range = pmem->pgmap.range;
> > bb_range.start += pmem->data_offset;
> > } else if (pmem_should_map_pages(dev)) {
> > @@ -532,7 +531,6 @@ static int pmem_attach_disk(struct device *dev,
> > pmem->pgmap.type = MEMORY_DEVICE_FS_DAX;
> > pmem->pgmap.ops = &fsdax_pagemap_ops;
> > addr = devm_memremap_pages(dev, &pmem->pgmap);
> > - pmem->pfn_flags |= PFN_MAP;
> > bb_range = pmem->pgmap.range;
> > } else {
> > addr = devm_memremap(dev, pmem->phys_addr,
> > diff --git a/fs/dax.c b/fs/dax.c
> > index d35dbe1..19f444e 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -71,6 +71,11 @@ static unsigned long dax_to_pfn(void *entry)
> > return xa_to_value(entry) >> DAX_SHIFT;
> > }
> >
> > +static struct folio *dax_to_folio(void *entry)
> > +{
> > + return page_folio(pfn_to_page(dax_to_pfn(entry)));
> > +}
> > +
> > static void *dax_make_entry(pfn_t pfn, unsigned long flags)
> > {
> > return xa_mk_value(flags | (pfn_t_to_pfn(pfn) << DAX_SHIFT));
> > @@ -338,44 +343,88 @@ static unsigned long dax_entry_size(void *entry)
> > return PAGE_SIZE;
> > }
> >
> > -static unsigned long dax_end_pfn(void *entry)
> > -{
> > - return dax_to_pfn(entry) + dax_entry_size(entry) / PAGE_SIZE;
> > -}
> > -
> > -/*
> > - * Iterate through all mapped pfns represented by an entry, i.e. skip
> > - * 'empty' and 'zero' entries.
> > - */
> > -#define for_each_mapped_pfn(entry, pfn) \
> > - for (pfn = dax_to_pfn(entry); \
> > - pfn < dax_end_pfn(entry); pfn++)
> > -
> > /*
> > * A DAX page is considered shared if it has no mapping set and ->share (which
> > * shares the ->index field) is non-zero. Note this may return false even if the
> > * page is shared between multiple files but has not yet actually been mapped
> > * into multiple address spaces.
> > */
> > -static inline bool dax_page_is_shared(struct page *page)
> > +static inline bool dax_folio_is_shared(struct folio *folio)
> > {
> > - return !page->mapping && page->share;
> > + return !folio->mapping && folio->share;
> > }
> >
> > /*
> > - * Increase the page share refcount, warning if the page is not marked as shared.
> > + * Increase the folio share refcount, warning if the folio is not marked as shared.
> > */
> > -static inline void dax_page_share_get(struct page *page)
> > +static inline void dax_folio_share_get(void *entry)
> > {
> > - WARN_ON_ONCE(!page->share);
> > - WARN_ON_ONCE(page->mapping);
> > - page->share++;
> > + struct folio *folio = dax_to_folio(entry);
> > +
> > + WARN_ON_ONCE(!folio->share);
> > + WARN_ON_ONCE(folio->mapping);
> > + WARN_ON_ONCE(dax_entry_order(entry) != folio_order(folio));
> > + folio->share++;
> > +}
> > +
> > +static inline unsigned long dax_folio_share_put(struct folio *folio)
> > +{
> > + unsigned long ref;
> > +
> > + if (!dax_folio_is_shared(folio))
> > + ref = 0;
> > + else
> > + ref = --folio->share;
> > +
> > + WARN_ON_ONCE(ref < 0);
> > + if (!ref) {
> > + folio->mapping = NULL;
> > + if (folio_order(folio)) {
> > + struct dev_pagemap *pgmap = page_pgmap(&folio->page);
> > + unsigned int order = folio_order(folio);
> > + unsigned int i;
> > +
> > + for (i = 0; i < (1UL << order); i++) {
> > + struct page *page = folio_page(folio, i);
> > +
> > + ClearPageHead(page);
> > + clear_compound_head(page);
> > +
> > + /*
> > + * Reset pgmap which was over-written by
> > + * prep_compound_page().
> > + */
> > + page_folio(page)->pgmap = pgmap;
> > +
> > + /* Make sure this isn't set to TAIL_MAPPING */
> > + page->mapping = NULL;
> > + page->share = 0;
> > + WARN_ON_ONCE(page_ref_count(page));
> > + }
> > + }
> > + }
> > +
> > + return ref;
> > }
> >
> > -static inline unsigned long dax_page_share_put(struct page *page)
> > +static void dax_device_folio_init(void *entry)
> > {
> > - WARN_ON_ONCE(!page->share);
> > - return --page->share;
> > + struct folio *folio = dax_to_folio(entry);
> > + int order = dax_entry_order(entry);
> > +
> > + /*
> > + * Folio should have been split back to order-0 pages in
> > + * dax_folio_share_put() when they were removed from their
> > + * final mapping.
> > + */
> > + WARN_ON_ONCE(folio_order(folio));
> > +
> > + if (order > 0) {
> > + prep_compound_page(&folio->page, order);
> > + if (order > 1)
> > + INIT_LIST_HEAD(&folio->_deferred_list);
> > + WARN_ON_ONCE(folio_ref_count(folio));
> > + }
> > }
> >
> > /*
> > @@ -388,72 +437,58 @@ static inline unsigned long dax_page_share_put(struct page *page)
> > * dax_holder_operations.
> > */
> > static void dax_associate_entry(void *entry, struct address_space *mapping,
> > - struct vm_area_struct *vma, unsigned long address, bool shared)
> > + struct vm_area_struct *vma, unsigned long address, bool shared)
> > {
> > - unsigned long size = dax_entry_size(entry), pfn, index;
> > - int i = 0;
> > + unsigned long size = dax_entry_size(entry), index;
> > + struct folio *folio = dax_to_folio(entry);
> >
> > if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> > return;
> >
> > index = linear_page_index(vma, address & ~(size - 1));
> > - for_each_mapped_pfn(entry, pfn) {
> > - struct page *page = pfn_to_page(pfn);
> > -
> > - if (shared && page->mapping && page->share) {
> > - if (page->mapping) {
> > - page->mapping = NULL;
> > + if (shared && (folio->mapping || dax_folio_is_shared(folio))) {
> > + if (folio->mapping) {
> > + folio->mapping = NULL;
> >
> > - /*
> > - * Page has already been mapped into one address
> > - * space so set the share count.
> > - */
> > - page->share = 1;
> > - }
> > -
> > - dax_page_share_get(page);
> > - } else {
> > - WARN_ON_ONCE(page->mapping);
> > - page->mapping = mapping;
> > - page->index = index + i++;
> > + /*
> > + * folio has already been mapped into one address
> > + * space so set the share count.
> > + */
> > + folio->share = 1;
> > }
> > +
> > + dax_folio_share_get(entry);
> > + } else {
> > + WARN_ON_ONCE(folio->mapping);
> > + dax_device_folio_init(entry);
> > + folio = dax_to_folio(entry);
> > + folio->mapping = mapping;
> > + folio->index = index;
> > }
> > }
> >
> > static void dax_disassociate_entry(void *entry, struct address_space *mapping,
> > - bool trunc)
> > + bool trunc)
> > {
> > - unsigned long pfn;
> > + struct folio *folio = dax_to_folio(entry);
> >
> > if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
> > return;
> >
> > - for_each_mapped_pfn(entry, pfn) {
> > - struct page *page = pfn_to_page(pfn);
> > -
> > - WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> > - if (dax_page_is_shared(page)) {
> > - /* keep the shared flag if this page is still shared */
> > - if (dax_page_share_put(page) > 0)
> > - continue;
> > - } else
> > - WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> > - page->mapping = NULL;
> > - page->index = 0;
> > - }
> > + dax_folio_share_put(folio);
> > }
> >
> > static struct page *dax_busy_page(void *entry)
> > {
> > - unsigned long pfn;
> > + struct folio *folio = dax_to_folio(entry);
> >
> > - for_each_mapped_pfn(entry, pfn) {
> > - struct page *page = pfn_to_page(pfn);
> > + if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry))
> > + return NULL;
> >
> > - if (page_ref_count(page) > 1)
> > - return page;
> > - }
> > - return NULL;
> > + if (folio_ref_count(folio) - folio_mapcount(folio))
> > + return &folio->page;
> > + else
> > + return NULL;
> > }
> >
> > /**
> > @@ -786,7 +821,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> > EXPORT_SYMBOL_GPL(dax_layout_busy_page);
> >
> > static int __dax_invalidate_entry(struct address_space *mapping,
> > - pgoff_t index, bool trunc)
> > + pgoff_t index, bool trunc)
> > {
> > XA_STATE(xas, &mapping->i_pages, index);
> > int ret = 0;
> > @@ -892,7 +927,7 @@ static int wait_page_idle(struct page *page,
> > void (cb)(struct inode *),
> > struct inode *inode)
> > {
> > - return ___wait_var_event(page, page_ref_count(page) == 1,
> > + return ___wait_var_event(page, page_ref_count(page) == 0,
> > TASK_INTERRUPTIBLE, 0, 0, cb(inode));
> > }
> >
> > @@ -900,7 +935,7 @@ static void wait_page_idle_uninterruptible(struct page *page,
> > void (cb)(struct inode *),
> > struct inode *inode)
> > {
> > - ___wait_var_event(page, page_ref_count(page) == 1,
> > + ___wait_var_event(page, page_ref_count(page) == 0,
> > TASK_UNINTERRUPTIBLE, 0, 0, cb(inode));
> > }
> >
> > @@ -949,7 +984,8 @@ void dax_break_mapping_uninterruptible(struct inode *inode,
> > wait_page_idle_uninterruptible(page, cb, inode);
> > } while (true);
> >
> > - dax_delete_mapping_range(inode->i_mapping, 0, LLONG_MAX);
> > + if (!page)
> > + dax_delete_mapping_range(inode->i_mapping, 0, LLONG_MAX);
> > }
> > EXPORT_SYMBOL_GPL(dax_break_mapping_uninterruptible);
> >
> > @@ -1035,8 +1071,10 @@ static void *dax_insert_entry(struct xa_state *xas, struct vm_fault *vmf,
> > void *old;
> >
> > dax_disassociate_entry(entry, mapping, false);
> > - dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
> > - shared);
> > + if (!(flags & DAX_ZERO_PAGE))
> > + dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
> > + shared);
> > +
> > /*
> > * Only swap our new entry into the page cache if the current
> > * entry is a zero page or an empty entry. If a normal PTE or
> > @@ -1224,9 +1262,7 @@ static int dax_iomap_direct_access(const struct iomap *iomap, loff_t pos,
> > goto out;
> > if (pfn_t_to_pfn(*pfnp) & (PHYS_PFN(size)-1))
> > goto out;
> > - /* For larger pages we need devmap */
> > - if (length > 1 && !pfn_t_devmap(*pfnp))
> > - goto out;
> > +
> > rc = 0;
> >
> > out_check_addr:
> > @@ -1333,7 +1369,7 @@ static vm_fault_t dax_load_hole(struct xa_state *xas, struct vm_fault *vmf,
> >
> > *entry = dax_insert_entry(xas, vmf, iter, *entry, pfn, DAX_ZERO_PAGE);
> >
> > - ret = vmf_insert_mixed(vmf->vma, vaddr, pfn);
> > + ret = vmf_insert_page_mkwrite(vmf, pfn_t_to_page(pfn), false);
> > trace_dax_load_hole(inode, vmf, ret);
> > return ret;
> > }
> > @@ -1804,7 +1840,8 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
> > loff_t pos = (loff_t)xas->xa_index << PAGE_SHIFT;
> > bool write = iter->flags & IOMAP_WRITE;
> > unsigned long entry_flags = pmd ? DAX_PMD : 0;
> > - int err = 0;
> > + struct folio *folio;
> > + int ret, err = 0;
> > pfn_t pfn;
> > void *kaddr;
> >
> > @@ -1836,17 +1873,18 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
> > return dax_fault_return(err);
> > }
> >
> > + folio = dax_to_folio(*entry);
> > if (dax_fault_is_synchronous(iter, vmf->vma))
> > return dax_fault_synchronous_pfnp(pfnp, pfn);
> >
> > - /* insert PMD pfn */
> > + folio_ref_inc(folio);
> > if (pmd)
> > - return vmf_insert_pfn_pmd(vmf, pfn, write);
> > + ret = vmf_insert_folio_pmd(vmf, pfn_folio(pfn_t_to_pfn(pfn)), write);
> > + else
> > + ret = vmf_insert_page_mkwrite(vmf, pfn_t_to_page(pfn), write);
> > + folio_put(folio);
> >
> > - /* insert PTE pfn */
> > - if (write)
> > - return vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
> > - return vmf_insert_mixed(vmf->vma, vmf->address, pfn);
> > + return ret;
> > }
> >
> > static vm_fault_t dax_iomap_pte_fault(struct vm_fault *vmf, pfn_t *pfnp,
> > @@ -2085,6 +2123,7 @@ dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order)
> > {
> > struct address_space *mapping = vmf->vma->vm_file->f_mapping;
> > XA_STATE_ORDER(xas, &mapping->i_pages, vmf->pgoff, order);
> > + struct folio *folio;
> > void *entry;
> > vm_fault_t ret;
> >
> > @@ -2102,14 +2141,17 @@ dax_insert_pfn_mkwrite(struct vm_fault *vmf, pfn_t pfn, unsigned int order)
> > xas_set_mark(&xas, PAGECACHE_TAG_DIRTY);
> > dax_lock_entry(&xas, entry);
> > xas_unlock_irq(&xas);
> > + folio = pfn_folio(pfn_t_to_pfn(pfn));
> > + folio_ref_inc(folio);
> > if (order == 0)
> > - ret = vmf_insert_mixed_mkwrite(vmf->vma, vmf->address, pfn);
> > + ret = vmf_insert_page_mkwrite(vmf, &folio->page, true);
> > #ifdef CONFIG_FS_DAX_PMD
> > else if (order == PMD_ORDER)
> > - ret = vmf_insert_pfn_pmd(vmf, pfn, FAULT_FLAG_WRITE);
> > + ret = vmf_insert_folio_pmd(vmf, folio, FAULT_FLAG_WRITE);
> > #endif
> > else
> > ret = VM_FAULT_FALLBACK;
> > + folio_put(folio);
> > dax_unlock_entry(&xas, entry);
> > trace_dax_insert_pfn_mkwrite(mapping->host, vmf, ret);
> > return ret;
> > diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> > index 82afe78..2c7b24c 100644
> > --- a/fs/fuse/virtio_fs.c
> > +++ b/fs/fuse/virtio_fs.c
> > @@ -1017,8 +1017,7 @@ static long virtio_fs_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
> > if (kaddr)
> > *kaddr = fs->window_kaddr + offset;
> > if (pfn)
> > - *pfn = phys_to_pfn_t(fs->window_phys_addr + offset,
> > - PFN_DEV | PFN_MAP);
> > + *pfn = phys_to_pfn_t(fs->window_phys_addr + offset, 0);
> > return nr_pages > max_nr_pages ? max_nr_pages : nr_pages;
> > }
> >
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index c7ec5ab..7bfb4eb 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -2740,7 +2740,7 @@ xfs_mmaplock_two_inodes_and_break_dax_layout(
> > * for this nested lock case.
> > */
> > page = dax_layout_busy_page(VFS_I(ip2)->i_mapping);
> > - if (page && page_ref_count(page) != 1) {
> > + if (page && page_ref_count(page) != 0) {
>
> You might want to wrap this weird detail for the next filesystem that
> uses it, so that the fine details of fsdax aren't opencoded in xfs:
Good idea. I will introduce this in patch 5 as dax_page_is_idle() and do this
change there. That way I can replace some of the other page_ref_count() checks
in the dax code as well.
- Alistair
>
> static inline bool dax_page_in_use(struct page *page)
> {
> return page && page_ref_count(page) != 0;
> }
>
> page = dax_layout_busy_page(VFS_I(ip2)->i_mapping);
> if (dax_page_in_use(page)) {
> /* unlock and retry... */
> }
>
> --D
>
> > xfs_iunlock(ip2, XFS_MMAPLOCK_EXCL);
> > xfs_iunlock(ip1, XFS_MMAPLOCK_EXCL);
> > goto again;
> > diff --git a/include/linux/dax.h b/include/linux/dax.h
> > index 7c3773f..dbefea1 100644
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -211,8 +211,12 @@ static inline int dax_wait_page_idle(struct page *page,
> > void (cb)(struct inode *),
> > struct inode *inode)
> > {
> > - return ___wait_var_event(page, page_ref_count(page) == 1,
> > + int ret;
> > +
> > + ret = ___wait_var_event(page, !page_ref_count(page),
> > TASK_INTERRUPTIBLE, 0, 0, cb(inode));
> > +
> > + return ret;
> > }
> >
> > #if IS_ENABLED(CONFIG_DAX)
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 01edca9..a734278 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1161,6 +1161,8 @@ int vma_is_stack_for_current(struct vm_area_struct *vma);
> > struct mmu_gather;
> > struct inode;
> >
> > +extern void prep_compound_page(struct page *page, unsigned int order);
> > +
> > /*
> > * compound_order() can be called without holding a reference, which means
> > * that niceties like page_folio() don't work. These callers should be
> > @@ -1482,25 +1484,6 @@ vm_fault_t finish_fault(struct vm_fault *vmf);
> > * back into memory.
> > */
> >
> > -#if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_FS_DAX)
> > -DECLARE_STATIC_KEY_FALSE(devmap_managed_key);
> > -
> > -bool __put_devmap_managed_folio_refs(struct folio *folio, int refs);
> > -static inline bool put_devmap_managed_folio_refs(struct folio *folio, int refs)
> > -{
> > - if (!static_branch_unlikely(&devmap_managed_key))
> > - return false;
> > - if (!folio_is_zone_device(folio))
> > - return false;
> > - return __put_devmap_managed_folio_refs(folio, refs);
> > -}
> > -#else /* CONFIG_ZONE_DEVICE && CONFIG_FS_DAX */
> > -static inline bool put_devmap_managed_folio_refs(struct folio *folio, int refs)
> > -{
> > - return false;
> > -}
> > -#endif /* CONFIG_ZONE_DEVICE && CONFIG_FS_DAX */
> > -
> > /* 127: arbitrary random number, small enough to assemble well */
> > #define folio_ref_zero_or_close_to_overflow(folio) \
> > ((unsigned int) folio_ref_count(folio) + 127u <= 127u)
> > @@ -1615,12 +1598,6 @@ static inline void put_page(struct page *page)
> > {
> > struct folio *folio = page_folio(page);
> >
> > - /*
> > - * For some devmap managed pages we need to catch refcount transition
> > - * from 2 to 1:
> > - */
> > - if (put_devmap_managed_folio_refs(folio, 1))
> > - return;
> > folio_put(folio);
> > }
> >
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 54b59b8..e308cb9 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -295,6 +295,8 @@ typedef struct {
> > * anonymous memory.
> > * @index: Offset within the file, in units of pages. For anonymous memory,
> > * this is the index from the beginning of the mmap.
> > + * @share: number of DAX mappings that reference this folio. See
> > + * dax_associate_entry.
> > * @private: Filesystem per-folio data (see folio_attach_private()).
> > * @swap: Used for swp_entry_t if folio_test_swapcache().
> > * @_mapcount: Do not access this member directly. Use folio_mapcount() to
> > @@ -344,7 +346,10 @@ struct folio {
> > struct dev_pagemap *pgmap;
> > };
> > struct address_space *mapping;
> > - pgoff_t index;
> > + union {
> > + pgoff_t index;
> > + unsigned long share;
> > + };
> > union {
> > void *private;
> > swp_entry_t swap;
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 9b587b5..d6575ed 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -96,8 +96,7 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
> > * belongs to this folio.
> > */
> > if (unlikely(page_folio(page) != folio)) {
> > - if (!put_devmap_managed_folio_refs(folio, refs))
> > - folio_put_refs(folio, refs);
> > + folio_put_refs(folio, refs);
> > goto retry;
> > }
> >
> > @@ -116,8 +115,7 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
> > refs *= GUP_PIN_COUNTING_BIAS;
> > }
> >
> > - if (!put_devmap_managed_folio_refs(folio, refs))
> > - folio_put_refs(folio, refs);
> > + folio_put_refs(folio, refs);
> > }
> >
> > /**
> > @@ -565,8 +563,7 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs,
> > */
> > if (unlikely((flags & FOLL_LONGTERM) &&
> > !folio_is_longterm_pinnable(folio))) {
> > - if (!put_devmap_managed_folio_refs(folio, refs))
> > - folio_put_refs(folio, refs);
> > + folio_put_refs(folio, refs);
> > return NULL;
> > }
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index d1ea76e..0cf1151 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2209,7 +2209,7 @@ int zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
> > tlb->fullmm);
> > arch_check_zapped_pmd(vma, orig_pmd);
> > tlb_remove_pmd_tlb_entry(tlb, pmd, addr);
> > - if (vma_is_special_huge(vma)) {
> > + if (!vma_is_dax(vma) && vma_is_special_huge(vma)) {
> > if (arch_needs_pgtable_deposit())
> > zap_deposited_table(tlb->mm, pmd);
> > spin_unlock(ptl);
> > @@ -2853,13 +2853,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> > */
> > if (arch_needs_pgtable_deposit())
> > zap_deposited_table(mm, pmd);
> > - if (vma_is_special_huge(vma))
> > + if (!vma_is_dax(vma) && vma_is_special_huge(vma))
> > return;
> > if (unlikely(is_pmd_migration_entry(old_pmd))) {
> > swp_entry_t entry;
> >
> > entry = pmd_to_swp_entry(old_pmd);
> > folio = pfn_swap_entry_folio(entry);
> > + } else if (is_huge_zero_pmd(old_pmd)) {
> > + return;
> > } else {
> > page = pmd_page(old_pmd);
> > folio = page_folio(page);
> > diff --git a/mm/internal.h b/mm/internal.h
> > index 3922788..c4df0ad 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -733,8 +733,6 @@ static inline void prep_compound_tail(struct page *head, int tail_idx)
> > set_page_private(p, 0);
> > }
> >
> > -extern void prep_compound_page(struct page *page, unsigned int order);
> > -
> > void post_alloc_hook(struct page *page, unsigned int order, gfp_t gfp_flags);
> > extern bool free_pages_prepare(struct page *page, unsigned int order);
> >
> > diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> > index a7b8ccd..7838bf1 100644
> > --- a/mm/memory-failure.c
> > +++ b/mm/memory-failure.c
> > @@ -419,18 +419,18 @@ static unsigned long dev_pagemap_mapping_shift(struct vm_area_struct *vma,
> > pud = pud_offset(p4d, address);
> > if (!pud_present(*pud))
> > return 0;
> > - if (pud_devmap(*pud))
> > + if (pud_trans_huge(*pud))
> > return PUD_SHIFT;
> > pmd = pmd_offset(pud, address);
> > if (!pmd_present(*pmd))
> > return 0;
> > - if (pmd_devmap(*pmd))
> > + if (pmd_trans_huge(*pmd))
> > return PMD_SHIFT;
> > pte = pte_offset_map(pmd, address);
> > if (!pte)
> > return 0;
> > ptent = ptep_get(pte);
> > - if (pte_present(ptent) && pte_devmap(ptent))
> > + if (pte_present(ptent))
> > ret = PAGE_SHIFT;
> > pte_unmap(pte);
> > return ret;
> > diff --git a/mm/memory.c b/mm/memory.c
> > index c60b819..02e12b0 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3843,13 +3843,15 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
> > if (vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) {
> > /*
> > * VM_MIXEDMAP !pfn_valid() case, or VM_SOFTDIRTY clear on a
> > - * VM_PFNMAP VMA.
> > + * VM_PFNMAP VMA. FS DAX also wants ops->pfn_mkwrite called.
> > *
> > * We should not cow pages in a shared writeable mapping.
> > * Just mark the pages writable and/or call ops->pfn_mkwrite.
> > */
> > - if (!vmf->page)
> > + if (!vmf->page || is_fsdax_page(vmf->page)) {
> > + vmf->page = NULL;
> > return wp_pfn_shared(vmf);
> > + }
> > return wp_page_shared(vmf, folio);
> > }
> >
> > diff --git a/mm/memremap.c b/mm/memremap.c
> > index 68099af..9a8879b 100644
> > --- a/mm/memremap.c
> > +++ b/mm/memremap.c
> > @@ -458,8 +458,13 @@ EXPORT_SYMBOL_GPL(get_dev_pagemap);
> >
> > void free_zone_device_folio(struct folio *folio)
> > {
> > - if (WARN_ON_ONCE(!folio->pgmap->ops ||
> > - !folio->pgmap->ops->page_free))
> > + struct dev_pagemap *pgmap = folio->pgmap;
> > +
> > + if (WARN_ON_ONCE(!pgmap->ops))
> > + return;
> > +
> > + if (WARN_ON_ONCE(pgmap->type != MEMORY_DEVICE_FS_DAX &&
> > + !pgmap->ops->page_free))
> > return;
> >
> > mem_cgroup_uncharge(folio);
> > @@ -484,26 +489,36 @@ void free_zone_device_folio(struct folio *folio)
> > * For other types of ZONE_DEVICE pages, migration is either
> > * handled differently or not done at all, so there is no need
> > * to clear folio->mapping.
> > + *
> > + * FS DAX pages clear the mapping when the folio->share count hits
> > + * zero which indicating the page has been removed from the file
> > + * system mapping.
> > */
> > - folio->mapping = NULL;
> > - folio->pgmap->ops->page_free(folio_page(folio, 0));
> > + if (pgmap->type != MEMORY_DEVICE_FS_DAX)
> > + folio->mapping = NULL;
> >
> > - switch (folio->pgmap->type) {
> > + switch (pgmap->type) {
> > case MEMORY_DEVICE_PRIVATE:
> > case MEMORY_DEVICE_COHERENT:
> > - put_dev_pagemap(folio->pgmap);
> > + pgmap->ops->page_free(folio_page(folio, 0));
> > + put_dev_pagemap(pgmap);
> > break;
> >
> > - case MEMORY_DEVICE_FS_DAX:
> > case MEMORY_DEVICE_GENERIC:
> > /*
> > * Reset the refcount to 1 to prepare for handing out the page
> > * again.
> > */
> > + pgmap->ops->page_free(folio_page(folio, 0));
> > folio_set_count(folio, 1);
> > break;
> >
> > + case MEMORY_DEVICE_FS_DAX:
> > + wake_up_var(&folio->page);
> > + break;
> > +
> > case MEMORY_DEVICE_PCI_P2PDMA:
> > + pgmap->ops->page_free(folio_page(folio, 0));
> > break;
> > }
> > }
> > @@ -519,21 +534,3 @@ void zone_device_page_init(struct page *page)
> > lock_page(page);
> > }
> > EXPORT_SYMBOL_GPL(zone_device_page_init);
> > -
> > -#ifdef CONFIG_FS_DAX
> > -bool __put_devmap_managed_folio_refs(struct folio *folio, int refs)
> > -{
> > - if (folio->pgmap->type != MEMORY_DEVICE_FS_DAX)
> > - return false;
> > -
> > - /*
> > - * fsdax page refcounts are 1-based, rather than 0-based: if
> > - * refcount is 1, then the page is free and the refcount is
> > - * stable because nobody holds a reference on the page.
> > - */
> > - if (folio_ref_sub_return(folio, refs) == 1)
> > - wake_up_var(&folio->_refcount);
> > - return true;
> > -}
> > -EXPORT_SYMBOL(__put_devmap_managed_folio_refs);
> > -#endif /* CONFIG_FS_DAX */
> > diff --git a/mm/mm_init.c b/mm/mm_init.c
> > index cb73402..0c12b29 100644
> > --- a/mm/mm_init.c
> > +++ b/mm/mm_init.c
> > @@ -1017,23 +1017,22 @@ static void __ref __init_zone_device_page(struct page *page, unsigned long pfn,
> > }
> >
> > /*
> > - * ZONE_DEVICE pages other than MEMORY_TYPE_GENERIC and
> > - * MEMORY_TYPE_FS_DAX pages are released directly to the driver page
> > - * allocator which will set the page count to 1 when allocating the
> > - * page.
> > + * ZONE_DEVICE pages other than MEMORY_TYPE_GENERIC are released
> > + * directly to the driver page allocator which will set the page count
> > + * to 1 when allocating the page.
> > *
> > * MEMORY_TYPE_GENERIC and MEMORY_TYPE_FS_DAX pages automatically have
> > * their refcount reset to one whenever they are freed (ie. after
> > * their refcount drops to 0).
> > */
> > switch (pgmap->type) {
> > + case MEMORY_DEVICE_FS_DAX:
> > case MEMORY_DEVICE_PRIVATE:
> > case MEMORY_DEVICE_COHERENT:
> > case MEMORY_DEVICE_PCI_P2PDMA:
> > set_page_count(page, 0);
> > break;
> >
> > - case MEMORY_DEVICE_FS_DAX:
> > case MEMORY_DEVICE_GENERIC:
> > break;
> > }
> > diff --git a/mm/swap.c b/mm/swap.c
> > index 062c856..a587842 100644
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -952,8 +952,6 @@ void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
> > unlock_page_lruvec_irqrestore(lruvec, flags);
> > lruvec = NULL;
> > }
> > - if (put_devmap_managed_folio_refs(folio, nr_refs))
> > - continue;
> > if (folio_ref_sub_and_test(folio, nr_refs))
> > free_zone_device_folio(folio);
> > continue;
> > --
> > git-series 0.9.1
> >
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