On Sat, Apr 24, 2010 at 11:02:00AM +0900, KAMEZAWA Hiroyuki wrote: > On Fri, 23 Apr 2010 16:58:01 +0100 > Mel Gorman <mel@xxxxxxxxx> wrote: > > > > I had considered this idea as well as it is vaguely similar to how zones get > > > resized with a seqlock. I was hoping that the existing locking on anon_vma > > > would be usable by backing off until uncontended but maybe not so lets > > > check out this approach. > > > > > > > A possible combination of the two approaches is as follows. It uses the > > anon_vma lock mostly except where the anon_vma differs between the page > > and the VMAs being walked in which case it uses the seq counter. I've > > had it running a few hours now without problems but I'll leave it > > running at least 24 hours. > > > ok, I'll try this, too. > > > > ==== CUT HERE ==== > > mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information by protecting against vma_adjust with a combination of locks and seq counter > > > > vma_adjust() is updating anon VMA information without any locks taken. > > In constract, file-backed mappings use the i_mmap_lock. This lack of > > locking can result in races with page migration. During rmap_walk(), > > vma_address() can return -EFAULT for an address that will soon be valid. > > This leaves a dangling migration PTE behind which can later cause a > > BUG_ON to trigger when the page is faulted in. > > > > With the recent anon_vma changes, there is no single anon_vma->lock that > > can be taken that is safe for rmap_walk() to guard against changes by > > vma_adjust(). Instead, a lock can be taken on one VMA while changes > > happen to another. > > > > What this patch does is protect against updates with a combination of > > locks and seq counters. First, the vma->anon_vma lock is taken by > > vma_adjust() and the sequence counter starts. The lock is released and > > the sequence ended when the VMA updates are complete. > > > > The lock serialses rmap_walk_anon when the page and VMA share the same > > anon_vma. Where the anon_vmas do not match, the seq counter is checked. > > If a change is noticed, rmap_walk_anon drops its locks and starts again > > from scratch as the VMA list may have changed. The dangling migration > > PTE bug was not triggered after several hours of stress testing with > > this patch applied. > > > > [kamezawa.hiroyu@xxxxxxxxxxxxxx: Use of a seq counter] > > Signed-off-by: Mel Gorman <mel@xxxxxxxxx> > > I think this patch is nice! > It looks nice but it still broke after 28 hours of running. The seq-counter is still insufficient to catch all changes that are made to the list. I'm beginning to wonder if a) this really can be fully safely locked with the anon_vma changes and b) if it has to be a spinlock to catch the majority of cases but still a lazy cleanup if there happens to be a race. It's unsatisfactory and I'm expecting I'll either have some insight to the new anon_vma changes that allow it to be locked or Rik knows how to restore the original behaviour which as Andrea pointed out was safe. > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> > > > -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>