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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
