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Date: Mon, 5 Dec 2022 13:56:24 +0800
From: Shiyang Ruan <ruansy.fnst@...itsu.com>
To: Dan Williams <dan.j.williams@...el.com>
CC: <djwong@...nel.org>, <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
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.
>
>> }
>> - 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);
--
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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