ACK/Cmnt: [SRU][S/R/N][PATCH 0/3] Resolve get_user_pages() performance regression for 0-order NULL-mapping folios

[Andrei Gherzan]

On 26/09/10 08:58AM, Jacob Martin via kernel-team wrote:
> BugLink: https://bugs.launchpad.net/bugs/2162917
> 
> [Impact]
> 
> There is a performance regression seen for calls to get_user_pages() on
> single-page NULL-mapping folios, introduced by commit f002882ca369 ("mm: merge
> folio_is_secretmem() and folio_fast_pin_allowed() into
> gup_fast_folio_allowed()").
> 
> When either CONFIG_SECRETMEM=y, or when the folio is a long-term writable pin,
> the current implementation requires that the mapping be checked as either a
> secretmem mapping or a file-backed one. If it is either, or if the mapping is
> NULL, the slow path is forced.
> 
> However, when the mapping is NULL, the folio cannot be a secretmem mapping.
> Thus, as long as the folio is NOT a long-term writable pin, the fast path can
> still be used.
> 
> This performance regression was initially observed during GPU Direct Storage
> (GDS) workloads.
> 
> [Fix]
> 
> The upstream commit c494788faffe ("mm/gup: fix GUP-fast fallback for
> NULL-mapping order-0 folios") resolves the performance regression by changing
> gup_fast_folio_allowed() to allow the fast path for the case described above,
> where the folio is NOT a long-term writable pin and its mapping field is NULL.
> 
> [Test Plan]
> 
> Build and boot tested.
> 
> The performance regression and subsequent fix can be verified with a GDS
> workload. The original bug report also describes a test kernel module that uses
> the `alloc_page` + `vm_insert_page` + `pin_user_pages_fast(..., FOLL_WRITE,
> ...)` functions to emulate GDS behavior, and using `get_user_pages_fast_only()`
> on that to validate that the fast path is now allowed in this scenario.
> 
> [Where problems could occur]
> 
> The fix affects the get_user_pages*() path of the mm subsystem, which is used
> heavily. The fix has been reviewed and applied in upstream Linux.

Looks good but there are some unexpected empty newlines in git footers.

Acked-by: Andrei Gherzan <andrei.gherzan at canonical.com>

-- 
Andrei Gherzan
gpg: rsa4096/D4D94F67AD0E9640
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