From: Michal Hocko <mhocko@suse.com>
To: Pavel Tatashin <pasha.tatashin@soleen.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 08/10] mm/gup: limit number of gup migration failures, honor failures
Date: Fri, 18 Dec 2020 14:14:49 +0100 [thread overview]
Message-ID: <20201218131449.GZ32193@dhcp22.suse.cz> (raw)
In-Reply-To: <CA+CK2bCn++2Sk4-Eunibj6f+JoOL77uJQXGU2+dScHQ3RgC7_Q@mail.gmail.com>
On Fri 18-12-20 07:43:15, Pavel Tatashin wrote:
> On Fri, Dec 18, 2020 at 5:46 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Thu 17-12-20 13:52:41, Pavel Tatashin wrote:
> > [...]
> > > +#define PINNABLE_MIGRATE_MAX 10
> > > +#define PINNABLE_ISOLATE_MAX 100
> >
> > Why would we need to limit the isolation retries. Those should always be
> > temporary failure unless I am missing something.
>
> Actually, during development, I was retrying isolate errors
> infinitely, but during testing found a hung where when FOLL_TOUCH
> without FOLL_WRITE is passed (fault in kernel without write flag), the
> zero page is faulted. The isolation of the zero page was failing every
> time, therefore the process was hanging.
Why would you migrate zero page in the first place? Simply instantiate
it.
> Since then, I fixed this problem by adding FOLL_WRITE unconditionally
> to FOLL_LONGTERM, but I was worried about other possible bugs that
> would cause hangs, so decided to limit isolation errors. If you think
> it its not necessary, I can unlimit isolate retires.
It should have a really good reason to exist. Worries about some corner
cases is definitely not a reason to put some awkward retry mechanism.
My historical experience is that these things are extremely hard to get
rid of later.
> > I am not sure about the
> > PINNABLE_MIGRATE_MAX either. Why do we want to limit that? migrate_pages
> > already implements its retry logic why do you want to count retries on
> > top of that? I do agree that the existing logic is suboptimal because
>
> True, but again, just recently, I worked on a race bug where pages can
> end up in per-cpu list after lru_add_drain_all() but before isolation,
> so I think retry is necessary.
There are ways to make sure pages are not ending on pcp list. Have a
look at how hotplug does that.
> > the migration failure might be ephemeral or permanent but that should be
> > IMHO addressed at migrate_pages (resp. unmap_and_move) and simply report
> > failures that are permanent - e.g. any potential pre-existing long term
> > pin - if that is possible at all. If not what would cause permanent
> > migration failure? OOM?
>
> Yes, OOM is the main cause for migration failures.
Then you can treat ENOMEM as a permanent failure.
> And also a few
> cases described in movable zone comment, where it is possible during
> boot some pages can be allocated by memblock in movable zone due to
> lack of memory resources (even if those resources were added later),
Do you have any examples? I find it hard to follow that somebody would
be pinning early boot allocations.
> hardware page poisoning is another rare example.
Could you elaborate please?
--
Michal Hocko
SUSE Labs
next prev parent reply other threads:[~2020-12-18 13:15 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-17 18:52 [PATCH v4 00/10] prohibit pinning pages in ZONE_MOVABLE Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 01/10] mm/gup: don't pin migrated cma pages in movable zone Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 02/10] mm cma: rename PF_MEMALLOC_NOCMA to PF_MEMALLOC_PIN Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 03/10] mm: apply per-task gfp constraints in fast path Pavel Tatashin
2020-12-18 9:36 ` Michal Hocko
2020-12-18 12:23 ` Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 04/10] mm: honor PF_MEMALLOC_PIN for all movable pages Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 05/10] mm/gup: migrate pinned pages out of movable zone Pavel Tatashin
2020-12-18 9:43 ` Michal Hocko
2020-12-18 12:24 ` Pavel Tatashin
2020-12-18 13:08 ` Michal Hocko
2021-01-13 19:14 ` Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 06/10] memory-hotplug.rst: add a note about ZONE_MOVABLE and page pinning Pavel Tatashin
2020-12-18 9:44 ` Michal Hocko
2020-12-17 18:52 ` [PATCH v4 07/10] mm/gup: change index type to long as it counts pages Pavel Tatashin
2020-12-18 9:50 ` Michal Hocko
2020-12-18 12:32 ` Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 08/10] mm/gup: limit number of gup migration failures, honor failures Pavel Tatashin
2020-12-17 20:50 ` Jason Gunthorpe
2020-12-17 22:02 ` Pavel Tatashin
2020-12-18 14:19 ` Jason Gunthorpe
2021-01-13 19:43 ` Pavel Tatashin
2021-01-13 19:55 ` Jason Gunthorpe
2021-01-13 20:05 ` Pavel Tatashin
2021-01-13 23:40 ` Jason Gunthorpe
2021-01-15 18:10 ` Pavel Tatashin
2021-01-15 18:40 ` Jason Gunthorpe
2020-12-18 10:46 ` Michal Hocko
2020-12-18 12:43 ` Pavel Tatashin
2020-12-18 13:04 ` David Hildenbrand
2020-12-18 13:14 ` Michal Hocko [this message]
2021-01-13 19:49 ` Pavel Tatashin
2020-12-17 18:52 ` [PATCH v4 09/10] selftests/vm: test flag is broken Pavel Tatashin
2020-12-18 9:06 ` John Hubbard
2020-12-18 9:11 ` John Hubbard
2020-12-17 18:52 ` [PATCH v4 10/10] selftests/vm: test faulting in kernel, and verify pinnable pages Pavel Tatashin
2020-12-19 5:57 ` John Hubbard
2020-12-19 15:22 ` Pavel Tatashin
2020-12-19 23:51 ` John Hubbard
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