Re: [PATCH] mm/gup: fix GUP-fast fallback for NULL-mapping order-0 folios

[David Hildenbrand (Arm)]

On 4/9/26 03:46, John Hubbard wrote:
> Since commit f002882ca369 ("mm: merge folio_is_secretmem() and
> folio_fast_pin_allowed() into gup_fast_folio_allowed()"),
> gup_fast_folio_allowed() falls back to the slow path for any order-0
> folio with a NULL mapping when CONFIG_SECRETMEM=y. This causes a
> performance regression for drivers that allocate pages with alloc_page()
> and insert them into VMAs via vm_insert_page(). These pages legitimately
> have a NULL folio->mapping, but they cannot be secretmem pages.
> 
> Secretmem pages are always added to the secretmem inode's page cache via
> filemap_add_folio(), which sets folio->mapping to the inode's i_mapping.
> A folio with a NULL mapping can never be a secretmem folio. The
> NULL-mapping check was intended to handle truncated file-backed pages (a
> reject_file_backed concern), not secretmem detection.
> 
> When only check_secretmem is true (and reject_file_backed is false), a
> NULL mapping is sufficient to prove the folio is not secretmem, so the
> fast path can proceed.

Hm, what if secretmem folio just got truncated? I hate to rely on some
handling in the caller to detect truncation differently during GUP-fast,
but this function returning "true".

Zi is working on a way to distinguish folios from non-folio things: that
we can identify whatever was added through vm_insert_page().

Because that's really the key problem here: vm_insert_page() pages are
not actually folios, they just look like a folio today, but looking at
fields like ->mapping does not make any sense.

> 
> Tested-by: Sourab Gupta <sougupta@xxxxxxxxxx>
> Fixes: f002882ca369 ("mm: merge folio_is_secretmem() and folio_fast_pin_allowed() into gup_fast_folio_allowed()")
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
>  mm/gup.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 8e7dc2c6ee73..3ea661e67eea 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2784,12 +2784,17 @@ static bool gup_fast_folio_allowed(struct folio *folio, unsigned int flags)
>  	mapping = READ_ONCE(folio->mapping);
>  
>  	/*
> -	 * 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 the mapping is NULL (truncated, or never set), we cannot
> +	 * determine whether the folio is file-backed, so a long-term writable
> +	 * pin must fall back to the slow path.
> +	 *
> +	 * Otherwise, a NULL mapping proves this is not a secretmem folio
> +	 * (secretmem folios always have a valid mapping to the secretmem
> +	 * inode's address_space), so in that case, we can continue with the
> +	 * fast path.
>  	 */
>  	if (!mapping)
> -		return false;
> +		return !reject_file_backed;
>  
>  	/* Anonymous folios pose no problem. */
>  	mapping_flags = (unsigned long)mapping & FOLIO_MAPPING_FLAGS;
> 
> base-commit: 7f87a5ea75f011d2c9bc8ac0167e5e2d1adb1594


-- 
Cheers,

David