From: Jason Gunthorpe <jgg@ziepe.ca>
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>, Michal Hocko <mhocko@suse.com>,
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>,
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 10:19:27 -0400 [thread overview]
Message-ID: <20201218141927.GM5487@ziepe.ca> (raw)
In-Reply-To: <CA+CK2bA4F+SipkReJzFjCSC-8kZdK4yrwCQZM+TvCTrqV2CGHg@mail.gmail.com>
On Thu, Dec 17, 2020 at 05:02:03PM -0500, Pavel Tatashin wrote:
> Hi Jason,
>
> Thank you for your comments. My replies below.
>
> On Thu, Dec 17, 2020 at 3:50 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> >
> > On Thu, Dec 17, 2020 at 01:52:41PM -0500, Pavel Tatashin wrote:
> > > +/*
> > > + * Verify that there are no unpinnable (movable) pages, if so return true.
> > > + * Otherwise an unpinnable pages is found return false, and unpin all pages.
> > > + */
> > > +static bool check_and_unpin_pages(unsigned long nr_pages, struct page **pages,
> > > + unsigned int gup_flags)
> > > +{
> > > + unsigned long i, step;
> > > +
> > > + for (i = 0; i < nr_pages; i += step) {
> > > + struct page *head = compound_head(pages[i]);
> > > +
> > > + step = compound_nr(head) - (pages[i] - head);
> >
> > You can't assume that all of a compound head is in the pages array,
> > this assumption would only work inside the page walkers if the page
> > was found in a PMD or something.
>
> I am not sure I understand your comment. The compound head is not
> taken from the pages array, and not assumed to be in it. It is exactly
> the same logic as that we currently have:
> https://soleen.com/source/xref/linux/mm/gup.c?r=a00cda3f#1565
Oh, that existing logic is wrong too :( Another bug.
You can't skip pages in the pages[] array under the assumption they
are contiguous. ie the i+=step is wrong.
> >
> > > + if (gup_flags & FOLL_PIN) {
> > > + unpin_user_pages(pages, nr_pages);
> >
> > So we throw everything away? Why? That isn't how the old algorithm worked
>
> It is exactly like the old algorithm worked: if there are pages to be
> migrated (not pinnable pages) we unpinned everything.
> See here:
> https://soleen.com/source/xref/linux/mm/gup.c?r=a00cda3f#1603
Hmm, OK, but I'm not sure that is great either
> cleaner, and handle errors. We must unpin everything because if we
> fail, no pages should stay pinned, and also if we migrated some pages,
> the pages array must be updated, so we need to call
> __get_user_pages_locked() pin and repopulated pages array.
However the page can't be unpinned until it is put on the LRU (and I'm
hoping that the LRU is enough of a 'lock' to make that safe, no idea)
> > I don't like this at all. It shouldn't be so flakey
> >
> > Can you do migration without the LRU?
>
> I do not think it is possible, we must isolate pages before migration.
I don't like this at all :( Lots of stuff relies on GUP, introducing a
random flakiness like this not good.
Jason
next prev parent reply other threads:[~2020-12-18 14:20 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 [this message]
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
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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