linux-kernel - Re: [PATCH v5 4/6] mm: Introduce a pageflag for partially mapped folios

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Message-ID: <e53b04ad-1827-43a2-a1ab-864c7efecf6e@redhat.com>
Date: Wed, 11 Dec 2024 16:03:02 +0100
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: hannes@...xchg.org, riel@...riel.com, shakeel.butt@...ux.dev,
 roman.gushchin@...ux.dev, yuzhao@...gle.com, npache@...hat.com,
 baohua@...nel.org, ryan.roberts@....com, rppt@...nel.org,
 willy@...radead.org, cerasuolodomenico@...il.com, ryncsn@...il.com,
 corbet@....net, linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 kernel-team@...a.com
Subject: Re: [PATCH v5 4/6] mm: Introduce a pageflag for partially mapped
 folios

On 30.08.24 12:03, Usama Arif wrote:
> Currently folio->_deferred_list is used to keep track of
> partially_mapped folios that are going to be split under memory
> pressure. In the next patch, all THPs that are faulted in and collapsed
> by khugepaged are also going to be tracked using _deferred_list.
> 
> This patch introduces a pageflag to be able to distinguish between
> partially mapped folios and others in the deferred_list at split time in
> deferred_split_scan. Its needed as __folio_remove_rmap decrements
> _mapcount, _large_mapcount and _entire_mapcount, hence it won't be
> possible to distinguish between partially mapped folios and others in
> deferred_split_scan.
> 
> Eventhough it introduces an extra flag to track if the folio is
> partially mapped, there is no functional change intended with this
> patch and the flag is not useful in this patch itself, it will
> become useful in the next patch when _deferred_list has non partially
> mapped folios.
> 
> Signed-off-by: Usama Arif <usamaarif642@...il.com>
> ---
>   include/linux/huge_mm.h    |  4 ++--
>   include/linux/page-flags.h | 13 +++++++++++-
>   mm/huge_memory.c           | 41 ++++++++++++++++++++++++++++----------
>   mm/memcontrol.c            |  3 ++-
>   mm/migrate.c               |  3 ++-
>   mm/page_alloc.c            |  5 +++--
>   mm/rmap.c                  |  5 +++--
>   mm/vmscan.c                |  3 ++-
>   8 files changed, 56 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 4da102b74a8c..0b0539f4ee1a 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -333,7 +333,7 @@ static inline int split_huge_page(struct page *page)
>   {
>   	return split_huge_page_to_list_to_order(page, NULL, 0);
>   }
> -void deferred_split_folio(struct folio *folio);
> +void deferred_split_folio(struct folio *folio, bool partially_mapped);
>   
>   void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
>   		unsigned long address, bool freeze, struct folio *folio);
> @@ -502,7 +502,7 @@ static inline int split_huge_page(struct page *page)
>   {
>   	return 0;
>   }
> -static inline void deferred_split_folio(struct folio *folio) {}
> +static inline void deferred_split_folio(struct folio *folio, bool partially_mapped) {}
>   #define split_huge_pmd(__vma, __pmd, __address)	\
>   	do { } while (0)
>   
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 2175ebceb41c..1b3a76710487 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -186,6 +186,7 @@ enum pageflags {
>   	/* At least one page in this folio has the hwpoison flag set */
>   	PG_has_hwpoisoned = PG_active,
>   	PG_large_rmappable = PG_workingset, /* anon or file-backed */
> +	PG_partially_mapped = PG_reclaim, /* was identified to be partially mapped */
>   };
>   
>   #define PAGEFLAGS_MASK		((1UL << NR_PAGEFLAGS) - 1)
> @@ -859,8 +860,18 @@ static inline void ClearPageCompound(struct page *page)
>   	ClearPageHead(page);
>   }
>   FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
> +FOLIO_TEST_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
> +/*
> + * PG_partially_mapped is protected by deferred_split split_queue_lock,
> + * so its safe to use non-atomic set/clear.

Just stumbled over that. In my understanding, this assumption is wrong.

I don't think anything prevents other PF_ANY (PG_anon_exclusive, 
PG_PG_hwpoison) / PF_SECOND (PF_has_hwpoisoned) flags from getting 
modified concurrently I'm afraid.

-- 
Cheers,

David / dhildenb