linux-kernel - Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

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Date:   Wed, 25 Jan 2023 10:06:33 -0800
From:   Yang Shi <shy828301@...il.com>
To:     "Zach O'Keefe" <zokeefe@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 1/2] mm/MADV_COLLAPSE: set EAGAIN on unexpected page refcount

On Tue, Jan 24, 2023 at 5:58 PM Zach O'Keefe <zokeefe@...gle.com> wrote:
>
> During collapse, in a few places we check to see if a given small page
> has any unaccounted references.  If the refcount on the page doesn't
> match our expectations, it must be there is an unknown user concurrently
> interested in the page, and so it's not safe to move the contents
> elsewhere. However, the unaccounted pins are likely an ephemeral state.
>
> In such a situation, make MADV_COLLAPSE set EAGAIN errno, indicating that
> collapse may succeed on retry.

The page may be DMA pinned (for example, pin_user_pages()), it is not
worth retrying for such pages. But it may also not be worth optimizing
for this case at this point.

So the patch looks good to me. Reviewed-by: Yang Shi <shy828301@...il.com>

>
> Fixes: 7d8faaf15545 ("mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse")
> Reported-by: Hugh Dickins <hughd@...gle.com>
> Signed-off-by: Zach O'Keefe <zokeefe@...gle.com>
>
> ---
>  mm/khugepaged.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index e23619bfecc4..fa38cae240b9 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -2712,6 +2712,7 @@ static int madvise_collapse_errno(enum scan_result r)
>         case SCAN_CGROUP_CHARGE_FAIL:
>                 return -EBUSY;
>         /* Resource temporary unavailable - trying again might succeed */
> +       case SCAN_PAGE_COUNT:
>         case SCAN_PAGE_LOCK:
>         case SCAN_PAGE_LRU:
>         case SCAN_DEL_PAGE_LRU:
> --
> 2.39.1.405.gd4c25cc71f-goog
>