Re: [PATCH v3 mm-new] fs/proc/page: avoid anon folio checks on typed pages except hugetlb

[David Hildenbrand <david@redhat.com>]

On 15.07.25 14:07, Harry Yoo wrote:
> folio_test_anon() and folio_test_ksm() may return false positives when
> invoked on typed pages (except hugetlb folios), because lower bits of
> folio->mapping may be set even when they do not indicate
> FOLIO_MAPPING_* flags.
> 
> This leads to tools/mm/page-types reporting pages with
> KPF_SLAB, KPF_ANON and KPF_KSM (with flags, page-counts, MB omitted):
>    $ sudo ./page-types | grep slab
>    _______S___________________________________   slab
>    _______S____a________x_____________________   slab,anonymous,ksm
> 
> Currently, and going forward (The New York interpretation),
> typed pages except hugetlb do not have FOLIO_MAPPING_* flags.
> In the future, they won't even cast to folios.
> 
> For now, avoid checking FOLIO_MAPPING_* flags on pages if they are
> typed pages that are not hugetlb folios.
> 
> Update the comment in FOLIO_MAPPING_* flags accordingly.
> 
> Fixes: 130d4df57390 ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head")
> Suggested-by: Vlastimil Babka <vbabka@suse.cz>
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
> 
> v2->v3: Did not introduce folio_has_mapcount() per David's suggestion
> v2: https://lore.kernel.org/linux-mm/20250707120740.4413-1-harry.yoo@oracle.com
> 
>   fs/proc/page.c             | 19 +++++++++++--------
>   include/linux/page-flags.h | 15 +++++++++------
>   2 files changed, 20 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/proc/page.c b/fs/proc/page.c
> index 0cdc78c0d23f..07582dbdef45 100644
> --- a/fs/proc/page.c
> +++ b/fs/proc/page.c
> @@ -148,18 +148,21 @@ u64 stable_page_flags(const struct page *page)
>   	folio = page_folio(page);
>   
>   	k = folio->flags;
> -	mapping = (unsigned long)folio->mapping;
> -	is_anon = mapping & FOLIO_MAPPING_ANON;
>   
>   	/*
>   	 * pseudo flags for the well known (anonymous) memory mapped pages
>   	 */
> -	if (page_mapped(page))
> -		u |= 1 << KPF_MMAP;
> -	if (is_anon) {
> -		u |= 1 << KPF_ANON;
> -		if (mapping & FOLIO_MAPPING_KSM)
> -			u |= 1 << KPF_KSM;
> +	if (!page_has_type(&folio->page) || folio_test_hugetlb(folio)) {
> +		mapping = (unsigned long)folio->mapping;
> +		is_anon = mapping & FOLIO_MAPPING_ANON;
> +
> +		if (page_mapped(page))
> +			u |= 1 << KPF_MMAP;

Note: Luiz switches to folio_mapped() in his patch that is in mm-new.

> +		if (is_anon) {
> +			u |= 1 << KPF_ANON;
> +			if (mapping & FOLIO_MAPPING_KSM)
> +				u |= 1 << KPF_KSM;

Can we just switch to folio_test_anon() and folio_test_ksm() ?

I don't really see a reason to not do that. Willy converted these
checks from page -> mapping, but we should really just use the folio_test_* functions
I think.

And looking at it, I think Willy introduced an issue in:

commit dee3d0bef2b00772be430425832ead6aa9d707f9
Author: Matthew Wilcox (Oracle) <willy@infradead.org>
Date:   Tue Mar 26 17:10:32 2024 +0000

     proc: rewrite stable_page_flags()


We replaced

	if (PageKsm(page))

essentially by

	if (mapping & PAGE_MAPPING_KSM)

But

	#define PAGE_MAPPING_KSM       (PAGE_MAPPING_ANON | PAGE_MAPPING_ANON_KSM)

So wouldn't we just indicate *all* anon pages as ... KSM pages?

> +		}
>   	}
>   
>   	/*
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 8e4d6eda8a8d..26ae4c7cf8dd 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -706,12 +706,15 @@ PAGEFLAG_FALSE(VmemmapSelfHosted, vmemmap_self_hosted)
>    * address_space which maps the folio from disk; whereas "folio_mapped"
>    * refers to user virtual address space into which the folio is mapped.
>    *
> - * For slab pages, since slab reuses the bits in struct page to store its
> - * internal states, the folio->mapping does not exist as such, nor do
> - * these flags below.  So in order to avoid testing non-existent bits,
> - * please make sure that folio_test_slab(folio) actually evaluates to
> - * false before calling the following functions (e.g., folio_test_anon).
> - * See mm/slab.h.
> + * For certain typed pages like slabs, since they reuse bits in struct page
> + * to store internal states, folio->mapping does not point to a valid
> + * mapping, nor do these flags exist. To avoid testing non-existent bits,
> + * make sure

I would write that explicitly:

that you are dealing with an actual folio: either page has no type or the type indicates a folio (hugetlb).


-- 
Cheers,

David / dhildenb