Re: [PATCH] mm/huge_memory.c: __split_huge_page() use atomic ClearPageDirty()

[Kirill A. Shutemov]

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@xxxxxxxxxx>

Thanks for catching this.

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov