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Date: Fri, 18 Dec 2020 14:04:48 +0100 From: David Hildenbrand <david@...hat.com> To: Pavel Tatashin <pasha.tatashin@...een.com> Cc: Michal Hocko <mhocko@...e.com>, LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>, Andrew Morton <akpm@...ux-foundation.org>, Vlastimil Babka <vbabka@...e.cz>, David Hildenbrand <david@...hat.com>, Oscar Salvador <osalvador@...e.de>, Dan Williams <dan.j.williams@...el.com>, Sasha Levin <sashal@...nel.org>, Tyler Hicks <tyhicks@...ux.microsoft.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, mike.kravetz@...cle.com, Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...hat.com>, Jason Gunthorpe <jgg@...pe.ca>, Peter Zijlstra <peterz@...radead.org>, Mel Gorman <mgorman@...e.de>, Matthew Wilcox <willy@...radead.org>, David Rientjes <rientjes@...gle.com>, John Hubbard <jhubbard@...dia.com>, Linux Doc Mailing List <linux-doc@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, linux-kselftest@...r.kernel.org Subject: Re: [PATCH v4 08/10] mm/gup: limit number of gup migration failures, honor failures > Am 18.12.2020 um 13:43 schrieb Pavel Tatashin <pasha.tatashin@...een.com>: > > On Fri, Dec 18, 2020 at 5:46 AM Michal Hocko <mhocko@...e.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. > > 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. > >> 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. > >> 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. 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), > hardware page poisoning is another rare example. > How is concurrent migration handled? Like memory offlining, compaction, alloc_contig_range() while trying to pin? >> -- >> Michal Hocko >> SUSE Labs >
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