Re: [PATCH] mm: make faultaround produce old ptes

[Michal Hocko <mhocko@kernel.org>]

On Wed 18-05-16 11:33:48, Kirill A. Shutemov wrote:
> On Wed, May 18, 2016 at 10:22:28AM +0200, Michal Hocko wrote:
> > On Wed 18-05-16 11:04:33, Kirill A. Shutemov wrote:
> > > On Wed, May 18, 2016 at 09:25:50AM +0200, Michal Hocko wrote:
> > > > On Tue 17-05-16 15:32:46, Kirill A. Shutemov wrote:
> > > > > Currently, faultaround code produces young pte. This can screw up vmscan
> > > > > behaviour[1], as it makes vmscan think that these pages are hot and not
> > > > > push them out on first round.
> > > > > 
> > > > > Let modify faultaround to produce old pte, so they can easily be
> > > > > reclaimed under memory pressure.
> > > > 
> > > > Could you be more specific about what was the original issue that led to
> > > > this patch? I can understand that marking all those pages new might be
> > > > too optimistic but when does it matter actually? Sparsely access file
> > > > mmap?
> > > 
> > > Yes, sparse file access. Faultaround gets more pages mapped and all of
> > > them are young. Under memory pressure, this makes vmscan to swap out anon
> > > pages instead or drop other page cache pages which otherwise stay
> > > resident.
> > 
> > I am wondering whether it would make more sense to do the fault around
> > only when chances are that the memory will be used. E.g. ~VM_RAND_READ
> > resp VM_SEQ_READ rather than unconditionally.
> 
> I'm not sure about this.
> 
> The idea of faularound is that we already have almost everything in our
> hands to map additional pages for very little cost: all required locks
> has been taken and radix-tree look up will be done anyway.

Fair enough. Meddling pte young bits sounds like a better approach then.

Feel free to add
Acked-by: Michal Hocko <mhocko@suse.com>

-- 
Michal Hocko
SUSE Labs

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