From: Hugh Dickins <hughd@google.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: Hugh Dickins <hughd@google.com>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Oscar Salvador <OSalvador@suse.com>,
Pavel Tatashin <pasha.tatashin@oracle.com>,
David Hildenbrand <david@redhat.com>,
LKML <linux-kernel@vger.kernel.org>,
"Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [RFC PATCH 3/3] mm, fault_around: do not take a reference to a locked page
Date: Wed, 21 Nov 2018 18:27:11 -0800 (PST) [thread overview]
Message-ID: <alpine.LSU.2.11.1811211757070.5557@eggly.anvils> (raw)
In-Reply-To: <20181121071132.GD12932@dhcp22.suse.cz>
On Wed, 21 Nov 2018, Michal Hocko wrote:
> On Tue 20-11-18 17:47:21, Hugh Dickins wrote:
> > On Tue, 20 Nov 2018, Michal Hocko wrote:
> >
> > > From: Michal Hocko <mhocko@suse.com>
> > >
> > > filemap_map_pages takes a speculative reference to each page in the
> > > range before it tries to lock that page. While this is correct it
> > > also can influence page migration which will bail out when seeing
> > > an elevated reference count. The faultaround code would bail on
> > > seeing a locked page so we can pro-actively check the PageLocked
> > > bit before page_cache_get_speculative and prevent from pointless
> > > reference count churn.
> > >
> > > Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
> > > Suggested-by: Jan Kara <jack@suse.cz>
> > > Signed-off-by: Michal Hocko <mhocko@suse.com>
> >
> > Acked-by: Hugh Dickins <hughd@google.com>
>
> Thanks!
>
> > though I think this patch is more useful to the avoid atomic ops,
> > and unnecessary dirtying of the cacheline, than to avoid the very
> > transient elevation of refcount, which will not affect page migration
> > very much.
>
> Are you sure it would really be transient? In other words is it possible
> that the fault around can block migration repeatedly under refault heavy
> workload? I just couldn't convince myself, to be honest.
I don't deny that it is possible: I expect that, using fork() (which does
not copy the ptes in a shared file vma), you can construct a test case
where each child faults one or another page near a page of no interest,
and that page of no interest is a target of migration perpetually
frustrated by filemap_map_pages()'s briefly raised refcount.
But I suggest that's a third-order effect: well worth fixing because
it's easily and uncontroversially dealt with, as you have; but not of
great importance.
The first-order effect is migration conspiring to defeat itself: that's
what my put_and_wait_on_page_locked() patch, in other thread, is about.
The second order effect is when a page that is really wanted is waited
on - the target of a fault, for which page refcount is raised maybe
long before it finally gets into the page table (whereupon it becomes
visible to try_to_unmap(), and its mapcount matches refcount so that
migration can fully account for the page). One class of that can be
well dealt with by using put_and_wait_on_page_locked_killable() in
lock_page_or_retry(), but I was keeping that as a future instalment.
But I shouldn't denigrate the transient case by referring so lightly
to migrate_pages()' 10 attempts: each of those failed attempts can
be very expensive, unmapping and TLB flushing (including IPIs) and
remapping. It may well be that 2 or 3 would be a more cost-effective
number of attempts, at least when the page is mapped.
Hugh
next prev parent reply other threads:[~2018-11-22 2:27 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-20 13:43 [RFC PATCH 0/3] few memory offlining enhancements Michal Hocko
2018-11-20 13:43 ` [RFC PATCH 1/3] mm, memory_hotplug: try to migrate full section worth of pages Michal Hocko
2018-11-20 14:18 ` David Hildenbrand
2018-11-20 14:25 ` Michal Hocko
2018-11-20 14:27 ` David Hildenbrand
2018-11-20 14:33 ` Pavel Tatashin
2018-11-20 14:51 ` osalvador
2018-11-20 15:00 ` Michal Hocko
2018-11-20 13:43 ` [RFC PATCH 2/3] mm, memory_hotplug: deobfuscate migration part of offlining Michal Hocko
2018-11-20 14:26 ` David Hildenbrand
2018-11-20 14:34 ` Michal Hocko
2018-11-20 14:34 ` David Hildenbrand
2018-11-20 15:13 ` osalvador
2018-11-20 15:18 ` Michal Hocko
2018-11-20 13:43 ` [RFC PATCH 3/3] mm, fault_around: do not take a reference to a locked page Michal Hocko
2018-11-20 14:07 ` Kirill A. Shutemov
2018-11-20 14:12 ` Michal Hocko
2018-11-20 14:17 ` Kirill A. Shutemov
2018-11-20 14:25 ` Michal Hocko
2018-11-21 4:51 ` William Kucharski
2018-11-21 7:07 ` Michal Hocko
2018-11-20 14:33 ` David Hildenbrand
2018-11-21 1:47 ` Hugh Dickins
2018-11-21 7:11 ` Michal Hocko
2018-11-22 2:27 ` Hugh Dickins [this message]
2018-11-22 9:05 ` Michal Hocko
2018-11-23 20:22 ` Hugh Dickins
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