On Mon, Apr 26, 2010 at 8:49 AM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > On Sat, 24 Apr 2010 11:43:24 +0100 > Mel Gorman <mel@xxxxxxxxx> wrote: > >> 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. >> > Ouch. Hmm, how about the race in fork() I pointed out ? I thought it's possible. Mel's test would take a long time to trigger BUG. So I think we could solve one of problems. Remained one is about fork race, I think. Mel. Could you retry your test with below Kame's patch? http://lkml.org/lkml/2010/4/23/58 -- Kind regards, Minchan Kim -- 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