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Date: Sun, 4 Dec 2022 23:01:49 -0800
From: "Darrick J. Wong" <djwong@...nel.org>
To: Shiyang Ruan <ruansy.fnst@...itsu.com>
Cc: Dan Williams <dan.j.williams@...el.com>, david@...morbit.com,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-xfs@...r.kernel.org, nvdimm@...ts.linux.dev,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2.1 1/8] fsdax: introduce page->share for fsdax in
reflink mode
On Mon, Dec 05, 2022 at 01:56:24PM +0800, Shiyang Ruan wrote:
>
>
> 在 2022/12/3 10:07, Dan Williams 写道:
> > Shiyang Ruan wrote:
> > > fsdax page is used not only when CoW, but also mapread. To make the it
> > > easily understood, use 'share' to indicate that the dax page is shared
> > > by more than one extent. And add helper functions to use it.
> > >
> > > Also, the flag needs to be renamed to PAGE_MAPPING_DAX_SHARED.
> > >
> > > Signed-off-by: Shiyang Ruan <ruansy.fnst@...itsu.com>
> > > ---
> > > fs/dax.c | 38 ++++++++++++++++++++++----------------
> > > include/linux/mm_types.h | 5 ++++-
> > > include/linux/page-flags.h | 2 +-
> > > 3 files changed, 27 insertions(+), 18 deletions(-)
> > >
> > > diff --git a/fs/dax.c b/fs/dax.c
> > > index 1c6867810cbd..edbacb273ab5 100644
> > > --- a/fs/dax.c
> > > +++ b/fs/dax.c
> > > @@ -334,35 +334,41 @@ static unsigned long dax_end_pfn(void *entry)
> > > for (pfn = dax_to_pfn(entry); \
> > > pfn < dax_end_pfn(entry); pfn++)
> > > -static inline bool dax_mapping_is_cow(struct address_space *mapping)
> > > +static inline bool dax_page_is_shared(struct page *page)
> > > {
> > > - return (unsigned long)mapping == PAGE_MAPPING_DAX_COW;
> > > + return (unsigned long)page->mapping == PAGE_MAPPING_DAX_SHARED;
> > > }
> > > /*
> > > - * Set the page->mapping with FS_DAX_MAPPING_COW flag, increase the refcount.
> > > + * Set the page->mapping with PAGE_MAPPING_DAX_SHARED flag, increase the
> > > + * refcount.
> > > */
> > > -static inline void dax_mapping_set_cow(struct page *page)
> > > +static inline void dax_page_bump_sharing(struct page *page)
> >
> > Similar to page_ref naming I would call this page_share_get() and the
> > corresponding function page_share_put().
> >
> > > {
> > > - if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_COW) {
> > > + if ((uintptr_t)page->mapping != PAGE_MAPPING_DAX_SHARED) {
> > > /*
> > > * Reset the index if the page was already mapped
> > > * regularly before.
> > > */
> > > if (page->mapping)
> > > - page->index = 1;
> > > - page->mapping = (void *)PAGE_MAPPING_DAX_COW;
> > > + page->share = 1;
> > > + page->mapping = (void *)PAGE_MAPPING_DAX_SHARED;
> >
> > Small nit, You could save a cast here by defining
> > PAGE_MAPPING_DAX_SHARED as "((void *) 1)".
>
> Ok.
It's sort of a pity you can't pass around a pointer to a privately
defined const struct in dax.c. But yeah, you might as well include the
cast in the macro definition.
> >
> > > }
> > > - page->index++;
> > > + page->share++;
> > > +}
> > > +
> > > +static inline unsigned long dax_page_drop_sharing(struct page *page)
> > > +{
> > > + return --page->share;
> > > }
> > > /*
> > > - * When it is called in dax_insert_entry(), the cow flag will indicate that
> > > + * When it is called in dax_insert_entry(), the shared flag will indicate that
> > > * whether this entry is shared by multiple files. If so, set the page->mapping
> > > - * FS_DAX_MAPPING_COW, and use page->index as refcount.
> > > + * PAGE_MAPPING_DAX_SHARED, and use page->share as refcount.
> > > */
> > > static void dax_associate_entry(void *entry, struct address_space *mapping,
> > > - struct vm_area_struct *vma, unsigned long address, bool cow)
> > > + struct vm_area_struct *vma, unsigned long address, bool shared)
> > > {
> > > unsigned long size = dax_entry_size(entry), pfn, index;
> > > int i = 0;
> > > @@ -374,8 +380,8 @@ static void dax_associate_entry(void *entry, struct address_space *mapping,
> > > for_each_mapped_pfn(entry, pfn) {
> > > struct page *page = pfn_to_page(pfn);
> > > - if (cow) {
> > > - dax_mapping_set_cow(page);
> > > + if (shared) {
> > > + dax_page_bump_sharing(page);
> > > } else {
> > > WARN_ON_ONCE(page->mapping);
> > > page->mapping = mapping;
> > > @@ -396,9 +402,9 @@ 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);
> > > - if (dax_mapping_is_cow(page->mapping)) {
> > > - /* keep the CoW flag if this page is still shared */
> > > - if (page->index-- > 0)
> > > + if (dax_page_is_shared(page)) {
> > > + /* keep the shared flag if this page is still shared */
> > > + if (dax_page_drop_sharing(page) > 0)
> > > continue;
> >
> > I think part of what makes this hard to read is trying to preserve the
> > same code paths for shared pages and typical pages.
> >
> > page_share_put() should, in addition to decrementing the share, clear
> > out page->mapping value.
>
> In order to be consistent, how about naming the 3 helper functions like
> this:
>
> bool dax_page_is_shared(struct page *page);
> void dax_page_share_get(struct page *page);
> unsigned long dax_page_share_put(struct page *page);
_sharing_get/_sharing_put ?
Either way sounds fine to me.
--D
>
> --
> Thanks,
> Ruan.
>
> >
> > > } else
> > > WARN_ON_ONCE(page->mapping && page->mapping != mapping);
> > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > > index 500e536796ca..f46cac3657ad 100644
> > > --- a/include/linux/mm_types.h
> > > +++ b/include/linux/mm_types.h
> > > @@ -103,7 +103,10 @@ struct page {
> > > };
> > > /* See page-flags.h for PAGE_MAPPING_FLAGS */
> > > struct address_space *mapping;
> > > - pgoff_t index; /* Our offset within mapping. */
> > > + union {
> > > + pgoff_t index; /* Our offset within mapping. */
> > > + unsigned long share; /* share count for fsdax */
> > > + };
> > > /**
> > > * @private: Mapping-private opaque data.
> > > * Usually used for buffer_heads if PagePrivate.
> > > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > > index 0b0ae5084e60..c8a3aa02278d 100644
> > > --- a/include/linux/page-flags.h
> > > +++ b/include/linux/page-flags.h
> > > @@ -641,7 +641,7 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
> > > * 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
> > > +#define PAGE_MAPPING_DAX_SHARED 0x1
> > > static __always_inline bool folio_mapping_flags(struct folio *folio)
> > > {
> > > --
> > > 2.38.1
> > >
> >
> >
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