linux-kernel - Re: [PATCH] mm/huge_memory.c: __split_huge

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Date:   Wed, 30 May 2018 10:59:00 +0300
From:   Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:     Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm/huge_memory.c: __split_huge_page() use atomic
 ClearPageDirty()

On 30.05.2018 04:50, Hugh Dickins wrote:
> Swapping load on huge=always tmpfs (with khugepaged tuned up to be very
> eager, but I'm not sure that is relevant) soon hung uninterruptibly,
> waiting for page lock in shmem_getpage_gfp()'s find_lock_entry(), most
> often when "cp -a" was trying to write to a smallish file.  Debug showed
> that the page in question was not locked, and page->mapping NULL by now,
> but page->index consistent with having been in a huge page before.
> 
> Reproduced in minutes on a 4.15 kernel, even with 4.17's 605ca5ede764
> ("mm/huge_memory.c: reorder operations in __split_huge_page_tail()")
> added in; but took hours to reproduce on a 4.17 kernel (no idea why).
> 
> The culprit proved to be the __ClearPageDirty() on tails beyond i_size
> in __split_huge_page(): the non-atomic __bitoperation may have been safe
> when 4.8's baa355fd3314 ("thp: file pages support for split_huge_page()")
> introduced it, but liable to erase PageWaiters after 4.10's 62906027091f
> ("mm: add PageWaiters indicating tasks are waiting for a page bit").
> 
> Fixes: 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> ---
> 
> It's not a 4.17-rc regression that this fixes, so no great need to slip
> this into 4.17 at the last moment - though it makes a good companion to
> Konstantin's 605ca5ede764. I think they both should go to stable, but
> since Konstantin's already went into rc1 without that tag, we shall
> have to recommend Konstantin's to GregKH out-of-band.

Good catch.

This is the same issue, so all 4.10+ needs them both.
Preserving known regressions in core pieces like lock_page() is a bad idea.

> 
>   mm/huge_memory.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- 4.17-rc7/mm/huge_memory.c	2018-04-26 10:48:36.019288258 -0700
> +++ linux/mm/huge_memory.c	2018-05-29 18:14:52.095512715 -0700
> @@ -2431,7 +2431,7 @@ static void __split_huge_page(struct pag
>   		__split_huge_page_tail(head, i, lruvec, list);
>   		/* Some pages can be beyond i_size: drop them from page cache */
>   		if (head[i].index >= end) {
> -			__ClearPageDirty(head + i);
> +			ClearPageDirty(head + i);
>   			__delete_from_page_cache(head + i, NULL);
>   			if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
>   				shmem_uncharge(head->mapping->host, 1);
>