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Date: Fri, 2 Dec 2022 18:07:46 -0800
From: Dan Williams <dan.j.williams@...el.com>
To: Shiyang Ruan <ruansy.fnst@...itsu.com>,
<linux-kernel@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
<nvdimm@...ts.linux.dev>, <linux-fsdevel@...r.kernel.org>
CC: <djwong@...nel.org>, <david@...morbit.com>,
<dan.j.williams@...el.com>, <akpm@...ux-foundation.org>
Subject: RE: [PATCH v2.1 1/8] fsdax: introduce page->share for fsdax in
reflink mode
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)".
> }
> - 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.
> } 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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