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

[David Hildenbrand <david@redhat.com>]

On 16.10.24 22:22, John Hubbard 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.
> 
> Fixes: 24a95998e9ba ("mm/gup.c: simplify and fix check_and_migrate_movable_pages() return codes")
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Shigeru Yoshida <syoshida@redhat.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
> ---
>   mm/gup.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index a82890b46a36..24acf53c8294 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2492,6 +2492,17 @@ static long __gup_longterm_locked(struct mm_struct *mm,
>   
>   		/* FOLL_LONGTERM implies FOLL_PIN */
>   		rc = check_and_migrate_movable_pages(nr_pinned_pages, pages);
> +
> +		/*
> +		 * The __get_user_pages_locked() call happens before we know
> +		 * that whether it's possible to successfully complete the whole
> +		 * operation. To compensate for this, if we get an unexpected
> +		 * error (such as -ENOMEM) then we must unpin everything, before
> +		 * erroring out.
> +		 */
> +		if (rc != -EAGAIN && rc != 0)
> +			unpin_user_pages(pages, nr_pinned_pages);
> +
>   	} while (rc == -EAGAIN);

Wouldn't it be cleaner to simply have here after the loop (possibly even 
after the memalloc_pin_restore())

if (rc)
	unpin_user_pages(pages, nr_pinned_pages);

But maybe I am missing something.

>   	memalloc_pin_restore(flags);
>   	return rc ? rc : nr_pinned_pages;


-- 
Cheers,

David / dhildenb