Re: [PATCH v3 2/2] mm/huge_memory.c: reorder operations in __split_huge_page_tail()

[Kirill A. Shutemov]

On Mon, Feb 12, 2018 at 04:58:53PM +0300, Konstantin Khlebnikov wrote:
> THP split makes non-atomic change of tail page flags. This is almost ok
> because tail pages are locked and isolated but this breaks recent changes
> in page locking: non-atomic operation could clear bit PG_waiters.
> 
> As a result concurrent sequence get_page_unless_zero() -> lock_page()
> might block forever. Especially if this page was truncated later.
> 
> Fix is trivial: clone flags before unfreezing page reference counter.
> 
> This race exists since commit 62906027091f ("mm: add PageWaiters indicating
> tasks are waiting for a page bit") while unsave unfreeze itself was added
> in commit 8df651c7059e ("thp: cleanup split_huge_page()").
> 
> clear_compound_head() also must be called before unfreezing page reference
> because after successful get_page_unless_zero() might follow put_page()
> which needs correct compound_head().
> 
> And replace page_ref_inc()/page_ref_add() with page_ref_unfreeze() which
> is made especially for that and has semantic of smp_store_release().
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

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

-- 
 Kirill A. Shutemov