Re: [PATCH 1/4] mm/rmap: per anon_vma lock

[Peter Zijlstra]

On Fri, Nov 01, 2013 at 07:44:29PM +0800, Yuanhan Liu wrote:
> commit 012f18004da33ba672e3c60838cc4898126174d3
> Author: Rik van Riel <riel@xxxxxxxxxx>
> 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@xxxxxxxxxx>
>     Tested-by: Larry Woodman <lwoodman@xxxxxxxxxx>
>     Acked-by: Larry Woodman <lwoodman@xxxxxxxxxx>
>     Reviewed-by: Minchan Kim <minchan.kim@xxxxxxxxx>
>     Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
>     Acked-by: Mel Gorman <mel@xxxxxxxxx>
>     Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>     Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>     Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

Right that commit did. I'm still not sure why you change both the
locking proper and the locking primitive used in one patch set.

Also, changing the locking proper requires a very detailed explanation
on why it is correct; we've had far too many 'fun' issues with the
anon_vma locking in the past.
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