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b=6WPq0YN/kQAeKdz9xJzOKpnzZ7b7swbIE4kb/XtoBphWmIq0lL4LqJxL5MO7cefY2PyRw5 yeAvKphjDPq+9QL+r2sHTqVbKqlnrk41AsZDDyQH67c+85SP5+vIK9Z/yjaE9zFeCbRwgg I5ruZwCrtRK1SnfNnPWVctd0j52+YA8= X-Stat-Signature: arn6ufr1o3xuaimjhes68xba3ys5xy7n X-Rspamd-Queue-Id: 8D51B1C0122 Authentication-Results: imf18.hostedemail.com; dkim=pass header.d=Nvidia.com header.s=selector2 header.b=r+rmr6tS; spf=pass (imf18.hostedemail.com: domain of apopple@nvidia.com designates 40.107.237.40 as permitted sender) smtp.mailfrom=apopple@nvidia.com; dmarc=pass (policy=reject) header.from=nvidia.com; arc=pass ("microsoft.com:s=arcselector9901:i=1") X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1659571972-51028 X-Bogosity: Ham, tests=bogofilter, spamicity=0.002677, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Pasha Tatashin writes: > On Mon, Aug 1, 2022 at 8:32 PM Alistair Popple wrote: >> >> When pinning pages with FOLL_LONGTERM check_and_migrate_movable_pages() >> is called to migrate pages out of zones which should not contain any >> longterm pinned pages. >> >> When migration succeeds all pages will have been unpinned so pinning >> needs to be retried. This is indicated by returning zero. When all pages >> are in the correct zone the number of pinned pages is returned. >> >> However migration can also fail, in which case pages are unpinned and >> -ENOMEM is returned. However if the failure was due to not being unable >> to isolate a page zero is returned. This leads to indefinite looping in >> __gup_longterm_locked(). > > Hi Alistair, > > During prohibiting pinning movable zone development, there was a > discussion where we figured that isolation errors should be transient > [1]. What isolation errors are you seeing that lead to infinite loop? > Why do they happen? Thanks for the pointer Pasha. There were reports of qemu running into the same zero page problem you reported there, see https://lore.kernel.org/linux-mm/165490039431.944052.12458624139225785964.stgit@omen/ This doesn't directly fix that problem as we need to allow pinning of the zero page, but it does prevent the infinite loop. I was going to re-spin this patch to retry instead of instant failure however reading that thread it seems the infinite loop is desired behaviour. So will re-spin this to leave that in-place. - Alistair > Pasha > > [1] https://lore.kernel.org/linux-mm/20201218104655.GW32193@dhcp22.suse.cz