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

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Date:   Wed, 30 May 2018 13:27:50 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        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 Wed, May 30, 2018 at 01:50:22AM +0000, 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>

Thanks for catching this.

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov