Re: [PATCH] mm/gup: stop leaking pinned pages in low memory conditions

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 16 Oct 2024 13:22:42 -0700 John Hubbard <jhubbard@nvidia.com> wrote:

> If a driver tries to call any of the pin_user_pages*(FOLL_LONGTERM)
> family of functions, and requests "too many" pages, then the call will
> erroneously leave pages pinned. This is visible in user space as an
> actual memory leak.
> 
> Repro is trivial: just make enough pin_user_pages(FOLL_LONGTERM) calls
> to exhaust memory.
> 
> The root cause of the problem is this sequence, within
> __gup_longterm_locked():
> 
>     __get_user_pages_locked()
>     rc = check_and_migrate_movable_pages()
> 
> ...which gets retried in a loop. The loop error handling is incomplete,
> clearly due to a somewhat unusual and complicated tri-state error API.
> But anyway, if -ENOMEM, or in fact, any unexpected error is returned
> from check_and_migrate_movable_pages(), then __gup_longterm_locked()
> happily returns the error, while leaving the pages pinned.
> 
> In the failed case, which is an app that requests (via a device driver)
> 30720000000 bytes to be pinned, and then exits, I see this:
> 
>     $ grep foll /proc/vmstat
>         nr_foll_pin_acquired 7502048
>         nr_foll_pin_released 2048
> 
> And after applying this patch, it returns to balanced pins:
> 
>     $ grep foll /proc/vmstat
>         nr_foll_pin_acquired 7502048
>         nr_foll_pin_released 7502048
> 
> Fix this by unpinning the pages that __get_user_pages_locked() has
> pinned, in such error cases.

Thanks.

> Fixes: 24a95998e9ba ("mm/gup.c: simplify and fix check_and_migrate_movable_pages() return codes")

I'll add this to the -stable backport pile, although this seems a bit
marginal?