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Message-ID: <CAPcyv4h7zVYu7K3j2JNEd7jTHJRvVDwqhhRCBbq6ru4+QGY9Hg@mail.gmail.com>
Date: Tue, 15 Feb 2022 19:09:01 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-xfs <linux-xfs@...r.kernel.org>,
Linux NVDIMM <nvdimm@...ts.linux.dev>,
Linux MM <linux-mm@...ck.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
"Darrick J. Wong" <djwong@...nel.org>, david <david@...morbit.com>,
Christoph Hellwig <hch@...radead.org>,
Jane Chu <jane.chu@...cle.com>
Subject: Re: [PATCH v10 9/9] fsdax: set a CoW flag when associate reflink mappings
On Tue, Feb 15, 2022 at 6:55 PM Shiyang Ruan <ruansy.fnst@...itsu.com> wrote:
>
>
>
> 在 2022/2/16 10:09, Dan Williams 写道:
> > On Thu, Jan 27, 2022 at 4:41 AM Shiyang Ruan <ruansy.fnst@...itsu.com> wrote:
> >>
> >> Introduce a PAGE_MAPPING_DAX_COW flag to support association with CoW file
> >> mappings. In this case, the dax-RMAP already takes the responsibility
> >> to look up for shared files by given dax page. The page->mapping is no
> >> longer to used for rmap but for marking that this dax page is shared.
> >> And to make sure disassociation works fine, we use page->index as
> >> refcount, and clear page->mapping to the initial state when page->index
> >> is decreased to 0.
> >>
> >> With the help of this new flag, it is able to distinguish normal case
> >> and CoW case, and keep the warning in normal case.
> >>
> >> Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
> >> ---
> >> fs/dax.c | 65 ++++++++++++++++++++++++++++++++------
> >> include/linux/page-flags.h | 6 ++++
> >> 2 files changed, 62 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/fs/dax.c b/fs/dax.c
> >> index 250794a5b789..88879c579c1f 100644
> >> --- a/fs/dax.c
> >> +++ b/fs/dax.c
> >> @@ -334,13 +334,46 @@ static unsigned long dax_end_pfn(void *entry)
> >> for (pfn = dax_to_pfn(entry); \
> >> pfn < dax_end_pfn(entry); pfn++)
> >>
> >> +static inline void dax_mapping_set_cow_flag(struct address_space *mapping)
> >> +{
> >> + mapping = (struct address_space *)PAGE_MAPPING_DAX_COW;
> >> +}
> >> +
> >> +static inline bool dax_mapping_is_cow(struct address_space *mapping)
> >> +{
> >> + return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
> >> +}
> >> +
> >> /*
> >> - * TODO: for reflink+dax we need a way to associate a single page with
> >> - * multiple address_space instances at different linear_page_index()
> >> - * offsets.
> >> + * Set or Update the page->mapping with FS_DAX_MAPPING_COW flag.
> >> + * Return true if it is an Update.
> >> + */
> >> +static inline bool dax_mapping_set_cow(struct page *page)
> >> +{
> >> + if (page->mapping) {
> >> + /* flag already set */
> >> + if (dax_mapping_is_cow(page->mapping))
> >> + return false;
> >> +
> >> + /*
> >> + * This page has been mapped even before it is shared, just
> >> + * need to set this FS_DAX_MAPPING_COW flag.
> >> + */
> >> + dax_mapping_set_cow_flag(page->mapping);
> >> + return true;
> >> + }
> >> + /* Newly associate CoW mapping */
> >> + dax_mapping_set_cow_flag(page->mapping);
> >> + return false;
> >> +}
> >> +
> >> +/*
> >> + * When it is called in dax_insert_entry(), the cow flag will indicate that
> >> + * whether this entry is shared by multiple files. If so, set the page->mapping
> >> + * to be FS_DAX_MAPPING_COW, and use page->index as refcount.
> >> */
> >> static void dax_associate_entry(void *entry, struct address_space *mapping,
> >> - struct vm_area_struct *vma, unsigned long address)
> >> + struct vm_area_struct *vma, unsigned long address, bool cow)
> >> {
> >> unsigned long size = dax_entry_size(entry), pfn, index;
> >> int i = 0;
> >> @@ -352,9 +385,17 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
> >> for_each_mapped_pfn(entry, pfn) {
> >> struct page *page = pfn_to_page(pfn);
> >>
> >> - WARN_ON_ONCE(page->mapping);
> >> - page->mapping = mapping;
> >> - page->index = index + i++;
> >> + if (cow) {
> >> + if (dax_mapping_set_cow(page)) {
> >> + /* Was normal, now updated to CoW */
> >> + page->index = 2;
> >> + } else
> >> + page->index++;
> >> + } else {
> >> + WARN_ON_ONCE(page->mapping);
> >> + page->mapping = mapping;
> >> + page->index = index + i++;
> >> + }
> >> }
> >> }
> >>
> >> @@ -370,7 +411,12 @@ static void dax_disassociate_entry(void *entry, struct address_space *mapping,
> >> struct page *page = pfn_to_page(pfn);
> >>
> >> WARN_ON_ONCE(trunc && page_ref_count(page) > 1);
> >> - WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> >> + if (!dax_mapping_is_cow(page->mapping)) {
> >> + /* keep the CoW flag if this page is still shared */
> >> + if (page->index-- > 0)
> >> + continue;
> >> + } else
> >> + WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> >> page->mapping = NULL;
> >> page->index = 0;
> >> }
> >> @@ -810,7 +856,8 @@ static void *dax_insert_entry(struct xa_state *xas,
> >> void *old;
> >>
> >> dax_disassociate_entry(entry, mapping, false);
> >> - dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address);
> >> + dax_associate_entry(new_entry, mapping, vmf->vma, vmf->address,
> >> + false);
> >
> > Where is the caller that passes 'true'? Also when that caller arrives
> > introduce a separate dax_associate_cow_entry() as that's easier to
> > read than dax_associate_entry(..., true) in case someone does not
> > remember what that boolean flag means.
>
> This flag is supposed to be used when CoW support is introduced.
Ok, so should this patch wait and be a part of that series? It's
otherwise confusing to introduce a new capability in a patch set and
not take advantage of it until a separate / later patch set.
> When
> it is a CoW operation, which is decided by iomap & srcmap's flag, this
> flag will be set true.
>
> I think I should describe it in detail in the commit message.
That could help, or move it to the COW support series.
> > However, it's not clear to me that this approach is a good idea given
> > that the filesystem is the source of truth for how many address_spaces
> > this page mapping might be duplicated. What about a iomap_page_ops for
> > fsdax to ask the filesystem when it is ok to clear the mapping
> > association for a page?
>
> I'll think how to implement it in this way.
>
>
> --
> Thanks,
> Ruan.
>
> >
> >> /*
> >> * 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
> >> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> >> index 1c3b6e5c8bfd..6370d279795a 100644
> >> --- a/include/linux/page-flags.h
> >> +++ b/include/linux/page-flags.h
> >> @@ -572,6 +572,12 @@ __PAGEFLAG(Reported, reported, PF_NO_COMPOUND)
> >> #define PAGE_MAPPING_KSM (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
> >> #define PAGE_MAPPING_FLAGS (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
> >>
> >> +/*
> >> + * Different with flags above, this flag is used only for fsdax mode. It
> >> + * indicates that this page->mapping is now under reflink case.
> >> + */
> >> +#define PAGE_MAPPING_DAX_COW 0x1
> >> +
> >> static __always_inline int PageMappingFlags(struct page *page)
> >> {
> >> return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) != 0;
> >> --
> >> 2.34.1
> >>
> >>
> >>
>
>
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