Re: [PATCH v2 4/4] fs: stable_page_flags(): use snapshot_page()

[Shivank Garg <shivankg@amd.com>]

On 7/8/2025 12:20 AM, Luiz Capitulino wrote:
> A race condition is possible in stable_page_flags() where user-space is
> reading /proc/kpageflags concurrently to a folio split. This may lead to
> oopses or BUG_ON()s being triggered.
> 
> To fix this, this commit uses snapshot_page() in stable_page_flags() so
> that stable_page_flags() works with a stable page and folio snapshots
> instead.
> 
> Note that stable_page_flags() makes use of some functions that require
> the original page or folio pointer to work properly (eg.
> is_free_budy_page() and folio_test_idle()). Since those functions can't
> be used on the page snapshot, we replace their usage with flags that
> were set by snapshot_page() for this purpose.
> 
> Signed-off-by: Luiz Capitulino <luizcap@redhat.com>
> ---
>  fs/proc/page.c | 29 +++++++++++++----------------
>  1 file changed, 13 insertions(+), 16 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 936f8bbe5a6f..cb58f066eb31 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -147,6 +147,7 @@ static inline u64 kpf_copy_bit(u64 kflags, int ubit, int kbit)
>  u64 stable_page_flags(const struct page *page)
>  {
>  	const struct folio *folio;
> +	struct page_snapshot ps;
>  	unsigned long k;
>  	unsigned long mapping;
>  	bool is_anon;
> @@ -158,7 +159,9 @@ u64 stable_page_flags(const struct page *page)
>  	 */
>  	if (!page)
>  		return 1 << KPF_NOPAGE;
> -	folio = page_folio(page);
> +
> +	snapshot_page(&ps, page);
> +	folio = &ps.folio_snapshot;
>  
>  	k = folio->flags;
>  	mapping = (unsigned long)folio->mapping;
> @@ -167,7 +170,7 @@ u64 stable_page_flags(const struct page *page)
>  	/*
>  	 * pseudo flags for the well known (anonymous) memory mapped pages
>  	 */
> -	if (page_mapped(page))
> +	if (folio_mapped(folio))
>  		u |= 1 << KPF_MMAP;
>  	if (is_anon) {
>  		u |= 1 << KPF_ANON;
> @@ -179,7 +182,7 @@ u64 stable_page_flags(const struct page *page)
>  	 * compound pages: export both head/tail info
>  	 * they together define a compound page's start/end pos and order
>  	 */
> -	if (page == &folio->page)
> +	if (ps.idx == 0)
>  		u |= kpf_copy_bit(k, KPF_COMPOUND_HEAD, PG_head);
>  	else
>  		u |= 1 << KPF_COMPOUND_TAIL;
> @@ -189,25 +192,19 @@ u64 stable_page_flags(const struct page *page)
>  	         folio_test_large_rmappable(folio)) {
>  		/* Note: we indicate any THPs here, not just PMD-sized ones */
>  		u |= 1 << KPF_THP;
> -	} else if (is_huge_zero_folio(folio)) {
> +	} else if (is_huge_zero_pfn(ps.pfn)) {
>  		u |= 1 << KPF_ZERO_PAGE;
>  		u |= 1 << KPF_THP;
> -	} else if (is_zero_folio(folio)) {
> +	} else if (is_zero_pfn(ps.pfn)) {
>  		u |= 1 << KPF_ZERO_PAGE;
>  	}
>  
> -	/*
> -	 * Caveats on high order pages: PG_buddy and PG_slab will only be set
> -	 * on the head page.
> -	 */
> -	if (PageBuddy(page))
> -		u |= 1 << KPF_BUDDY;
> -	else if (page_count(page) == 0 && is_free_buddy_page(page))
> +	if (ps.flags & PAGE_SNAPSHOT_PG_FREE)
>  		u |= 1 << KPF_BUDDY;
>  
> -	if (PageOffline(page))
> +	if (folio_test_offline(folio))
>  		u |= 1 << KPF_OFFLINE;
> -	if (PageTable(page))
> +	if (folio_test_pgtable(folio))
>  		u |= 1 << KPF_PGTABLE;
>  	if (folio_test_slab(folio))
>  		u |= 1 << KPF_SLAB;
> @@ -215,7 +212,7 @@ u64 stable_page_flags(const struct page *page)
>  #if defined(CONFIG_PAGE_IDLE_FLAG) && defined(CONFIG_64BIT)
>  	u |= kpf_copy_bit(k, KPF_IDLE,          PG_idle);
>  #else
> -	if (folio_test_idle(folio))
> +	if (ps.flags & PAGE_SNAPSHOT_PG_IDLE)
>  		u |= 1 << KPF_IDLE;
>  #endif
>  
> @@ -241,7 +238,7 @@ u64 stable_page_flags(const struct page *page)
>  	if (u & (1 << KPF_HUGE))
>  		u |= kpf_copy_bit(k, KPF_HWPOISON,	PG_hwpoison);
>  	else
> -		u |= kpf_copy_bit(page->flags, KPF_HWPOISON,	PG_hwpoison);
> +		u |= kpf_copy_bit(ps.page_snapshot.flags, KPF_HWPOISON, PG_hwpoison);
>  #endif
>  
>  	u |= kpf_copy_bit(k, KPF_RESERVED,	PG_reserved);
LGTM

Reviewed-by: Shivank Garg <shivankg@amd.com>