Re: [PATCH] mm/huge_memory.c: split should clone page flags before unfreezing pageref

[Konstantin Khlebnikov]

On 11.02.2018 14:07, Kirill A. Shutemov wrote:
> On Sun, Feb 11, 2018 at 01:35:17PM +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()").
>
> Hm. Don't we have to have barrier between setting flags and updating
> the refcounter in this case? Atomics don't generally have this semantics,
> so you can see new refcount before new flags even after the change.

Ok.

I see another problem here - clear_compound_head() is placed after unfreeze.

This opens race window with get/put_page after speculative get page.
I think successful get_page_unless_zero() must stabilize compound_head() for tails as well as for heads.