Re: [PATCH v4 05/10] mm/gup: migrate pinned pages out of movable zone

From: Pavel Tatashin <pasha.tatashin@soleen.com>
To: Michal Hocko <mhocko@suse.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	David Hildenbrand <david@redhat.com>,
	Oscar Salvador <osalvador@suse.de>,
	Dan Williams <dan.j.williams@intel.com>,
	Sasha Levin <sashal@kernel.org>,
	Tyler Hicks <tyhicks@linux.microsoft.com>,
	Joonsoo Kim <iamjoonsoo.kim@lge.com>,
	mike.kravetz@oracle.com, Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>, Jason Gunthorpe <jgg@ziepe.ca>,
	Peter Zijlstra <peterz@infradead.org>,
	Mel Gorman <mgorman@suse.de>,
	Matthew Wilcox <willy@infradead.org>,
	David Rientjes <rientjes@google.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Linux Doc Mailing List <linux-doc@vger.kernel.org>,
	Ira Weiny <ira.weiny@intel.com>,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH v4 05/10] mm/gup: migrate pinned pages out of movable zone
Date: Wed, 13 Jan 2021 14:14:11 -0500	[thread overview]
Message-ID: <CA+CK2bBLZu3X5P3qkaTV7To4qx2W79Y1MQdpJM3134LtkzkhXA@mail.gmail.com> (raw)
In-Reply-To: <20201218130825.GY32193@dhcp22.suse.cz>

On Fri, Dec 18, 2020 at 8:08 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Fri 18-12-20 07:24:53, Pavel Tatashin wrote:
> > On Fri, Dec 18, 2020 at 4:43 AM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Thu 17-12-20 13:52:38, Pavel Tatashin wrote:
> > > > +      * 1. Pinned pages: (long-term) pinning of movable pages is avoided
> > > > +      *    when pages are pinned and faulted, but it is still possible that
> > > > +      *    address space already has pages in ZONE_MOVABLE at the time when
> > > > +      *    pages are pinned (i.e. user has touches that memory before
> > > > +      *    pinning). In such case we try to migrate them to a different zone,
> > > > +      *    but if migration fails the pages can still end-up pinned in
> > > > +      *    ZONE_MOVABLE. In such case, memory offlining might retry a long
> > > > +      *    time and will only succeed once user application unpins pages.
> > >
> > > I still dislike this. Pinning can fail so there shouldn't be any reasons
> > > to break MOVABLE constrain for something that can be handled. If
> > > anything there should be a very good reasoning behind this decision
> > > documented.
> >
> > This is basically current behaviour, after patch 8, we can never pin
> > pages in the movable zone, so I will update this comment in that
> > patch.
>
> Then it would be much easier for review to state that the existing
> behavior is unchanged and do not update this comment just to remove it
> in a later patch. Because this patch should be straightforward change of
> the condition which pages to migrate (+some renaming which should be
> reasonably easy to follow).

Makes sense, I will update this comment correctly right away when the
behaviour changes.