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Message-ID: <CAEvNRgGrpv5h04s+btubhUFHo=d6mBFbr2BVrMt=bWuWOztdJQ@mail.gmail.com>
Date: Thu, 15 Jan 2026 13:40:28 -0800
From: Ackerley Tng <ackerleytng@...gle.com>
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Subject: Re: [PATCH v9 02/13] mm/gup: drop secretmem optimization from gup_fast_folio_allowed
"Kalyazin, Nikita" <kalyazin@...zon.co.uk> writes:
> From: Patrick Roy <patrick.roy@...ux.dev>
>
> This drops an optimization in gup_fast_folio_allowed() where
> secretmem_mapping() was only called if CONFIG_SECRETMEM=y. secretmem is
> enabled by default since commit b758fe6df50d ("mm/secretmem: make it on
> by default"), so the secretmem check did not actually end up elided in
> most cases anymore anyway.
>
> This is in preparation of the generalization of handling mappings where
> direct map entries of folios are set to not present. Currently,
> mappings that match this description are secretmem mappings
> (memfd_secret()). Later, some guest_memfd configurations will also fall
> into this category.
>
> Signed-off-by: Patrick Roy <patrick.roy@...ux.dev>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> Signed-off-by: Nikita Kalyazin <kalyazin@...zon.com>
> ---
> mm/gup.c | 11 +----------
> 1 file changed, 1 insertion(+), 10 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 95d948c8e86c..9cad53acbc99 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2739,7 +2739,6 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
> {
> bool reject_file_backed = false;
> struct address_space *mapping;
> - bool check_secretmem = false;
> unsigned long mapping_flags;
>
> /*
> @@ -2751,14 +2750,6 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
Copying some lines the diff didn't contain:
/*
* If we aren't pinning then no problematic write can occur. A long term
* pin is the most egregious case so this is the one we disallow.
*/
if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) ==
(FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
If we're pinning, can we already return true here? IIUC this function
is passed a folio that is file-backed, and the check if (!mapping) is
just there to catch the case where the mapping got truncated.
Or should we wait for the check where the mapping got truncated? If so,
then maybe we can move this "are we pinning" check to after this check
and remove the reject_file_backed variable?
/*
* The mapping may have been truncated, in any case we cannot determine
* if this mapping is safe - fall back to slow path to determine how to
* proceed.
*/
if (!mapping)
return false;
> reject_file_backed = true;
>
> /* We hold a folio reference, so we can safely access folio fields. */
> -
> - /* secretmem folios are always order-0 folios. */
> - if (IS_ENABLED(CONFIG_SECRETMEM) && !folio_test_large(folio))
> - check_secretmem = true;
> -
> - if (!reject_file_backed && !check_secretmem)
> - return true;
> -
> if (WARN_ON_ONCE(folio_test_slab(folio)))
> return false;
>
> @@ -2800,7 +2791,7 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
> * At this point, we know the mapping is non-null and points to an
> * address_space object.
> */
> - if (check_secretmem && secretmem_mapping(mapping))
> + if (secretmem_mapping(mapping))
> return false;
> /* The only remaining allowed file system is shmem. */
> return !reject_file_backed || shmem_mapping(mapping);
> --
> 2.50.1
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