Messages in this thread |  | | Date | Fri, 1 Nov 2013 19:44:29 +0800 | From | Yuanhan Liu <> | Subject | Re: [PATCH 1/4] mm/rmap: per anon_vma lock |
| |
On Fri, Nov 01, 2013 at 11:15:14AM +0100, Peter Zijlstra wrote: > On Fri, Nov 01, 2013 at 06:07:07PM +0800, Yuanhan Liu wrote: > > > I also want to point out that lately we've seen several changes sent > > > out that relax locking with no accompanying explanation of why the > > > relaxed locking would be safe. Please don't do that - having a lot of > > > performance data is worthless if you can't explain why the new locking > > > is safe. > > > > Agreed. > > > > > And I'm not asking to prove a negative ('lack of any possible > > > races') there, but at least in this case one could dig out why the > > > root anon vma locking was introduced and if they think that this > > > reason doesn't apply anymore, explain why... > > > > It was introduced by commit 2b575eb6(And, BTW, I'm sorry that this commit log > > about bb4aa39676f is wrong) > > > > commit 2b575eb64f7a9c701fb4bfdb12388ac547f6c2b6 > > Author: Peter Zijlstra <a.p.zijlstra@chello.nl> > > Date: Tue May 24 17:12:11 2011 -0700 > > > > mm: convert anon_vma->lock to a mutex > > > > Straightforward conversion of anon_vma->lock to a mutex. > > > > As you can see, Peter didn't tell why before. Honestly speaking, that > > was my originaly concern as well. I tried to find some possible races; > > I guess I may miss something. > > Bullshit; I didn't change the locking. I only changed the lock primitive > from a spinlock to a mutex. The anon_vma->root->lock is completely > unrelated to this change.
Oops, sorry for that. Just made a *horrible* mistake: it was commit 012f18004da33ba672e3c60838cc4898126174d3.
commit 012f18004da33ba672e3c60838cc4898126174d3 Author: Rik van Riel <riel@redhat.com> Date: Mon Aug 9 17:18:40 2010 -0700
mm: always lock the root (oldest) anon_vma
Always (and only) lock the root (oldest) anon_vma whenever we do something in an anon_vma. The recently introduced anon_vma scalability is due to the rmap code scanning only the VMAs that need to be scanned. Many common operations still took the anon_vma lock on the root anon_vma, so always taking that lock is not expected to introduce any scalability issues.
However, always taking the same lock does mean we only need to take one lock, which means rmap_walk on pages from any anon_vma in the vma is excluded from occurring during an munmap, expand_stack or other operation that needs to exclude rmap_walk and similar functions.
Also add the proper locking to vma_adjust.
Signed-off-by: Rik van Riel <riel@redhat.com> Tested-by: Larry Woodman <lwoodman@redhat.com> Acked-by: Larry Woodman <lwoodman@redhat.com> Reviewed-by: Minchan Kim <minchan.kim@gmail.com> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Acked-by: Mel Gorman <mel@csn.ul.ie> Acked-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
|  |