linux-kernel - Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20121025221031.GA29910@otc-wbsnb-06>
Date:	Fri, 26 Oct 2012 01:10:31 +0300
From:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	Andi Kleen <ak@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 10/10] thp: implement refcounting for huge zero page

On Thu, Oct 25, 2012 at 02:37:07PM -0700, Andrew Morton wrote:
> On Fri, 26 Oct 2012 00:22:51 +0300
> "Kirill A. Shutemov" <kirill@...temov.name> wrote:
> 
> > On Thu, Oct 25, 2012 at 02:05:24PM -0700, Andrew Morton wrote:
> > > hm.  It's odd that the kernel didn't try to shrink slabs in this case. 
> > > Why didn't it??
> > 
> > nr_to_scan == 0 asks for the fast path. shrinker callback can shink, if
> > it thinks it's good idea.
> 
> What nr_objects does your shrinker return in that case?

HPAGE_PMD_NR if hzp is freeable, otherwise 0.

> > > > I also tried another scenario: usemem -n16 100M -r 1000. It creates real
> > > > memory pressure - no easy reclaimable memory. This time callback called
> > > > with nr_to_scan > 0 and we freed hzp. Under pressure we fails to allocate
> > > > hzp and code goes to fallback path as it supposed to.
> > > > 
> > > > Do I need to check any other scenario?
> > > 
> > > I'm thinking that if we do hit problems in this area, we could avoid
> > > freeing the hugepage unless the scan_control.priority is high enough. 
> > > That would involve adding a magic number or a tunable to set the
> > > threshold.
> > 
> > What about ratelimit on alloc path to force fallback if we allocate
> > to often? Is it good idea?
> 
> mmm...  ratelimit via walltime is always a bad idea.  We could
> ratelimit by "number of times the shrinker was called", and maybe that
> would work OK, unsure.
> 
> It *is* appropriate to use sc->priority to be more reluctant to release
> expensive-to-reestablish objects.  But there is already actually a
> mechanism in the shrinker code to handle this: the shrink_control.seeks
> field.  That was originally added to provide an estimate of "how
> expensive will it be to recreate this object if we were to reclaim it".
> So perhaps we could generalise that a bit, and state that the zero
> hugepage is an expensive thing.

I've proposed DEFAULT_SEEKS * 4 already.

> I don't think the shrink_control.seeks facility had ever been used much,
> so it's possible that it is presently mistuned or not working very
> well.

Yeah, non-default .seeks is only in kvm mmu_shrinker and in few places in
staging/android/.

-- 
 Kirill A. Shutemov

Download attachment "signature.asc" of type "application/pgp-signature" (837 bytes)