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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429059713c4sm132070245e9.12.2024.08.09.06.47.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 09 Aug 2024 06:47:50 -0700 (PDT) Message-ID: <761ba58e-9d6f-4a14-a513-dcc098c2aa94@redhat.com> Date: Fri, 9 Aug 2024 15:47:49 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] mm: Retry migration earlier upon refcount mismatch To: Dev Jain , akpm@linux-foundation.org, shuah@kernel.org, willy@infradead.org Cc: ryan.roberts@arm.com, anshuman.khandual@arm.com, catalin.marinas@arm.com, cl@gentwo.org, vbabka@suse.cz, mhocko@suse.com, apopple@nvidia.com, osalvador@suse.de, baolin.wang@linux.alibaba.com, dave.hansen@linux.intel.com, will@kernel.org, baohua@kernel.org, ioworker0@gmail.com, gshan@redhat.com, mark.rutland@arm.com, kirill.shutemov@linux.intel.com, hughd@google.com, aneesh.kumar@kernel.org, yang@os.amperecomputing.com, peterx@redhat.com, broonie@kernel.org, mgorman@techsingularity.net, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org References: <20240809103129.365029-1-dev.jain@arm.com> <20240809103129.365029-2-dev.jain@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: ij64aqk1b7cfb7udsbmdtagmbf77nai5 X-Rspamd-Queue-Id: EDEB11C0012 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1723211276-550368 X-HE-Meta: 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 0SWhBsPc 0ZM6CpxgUikiWqXForkmK8gk4Kqcc8m4Us8ji6SPPSK/X1nq30zYVRPF4OVQcKi0LePGkOIFYhUIAvDsd+Iik0NWpcn8+5bTGQ3T6dzMIFaPeWaRclspUUnbFyf7H+NVHpoyqMV727eg0GV4V+ReuxCRyK4zhnrtISVQ1WQBifLpVhkKHaMgEETFqONWy6MUeoIYnptvcIhK8yp6wzcNGKvswYAMevkR72yabJtmcsRfHa3GgfwPm0Op9IrbDtK+FOrWGolWOQuhSsWr+O4h3K3n57A4xNs5kKD2oeSzVzMrejomUnCciG838QSKlF7XpvAUaTQAODvS4XudMFeIlQhw8hKQUhtoiLj/yMiPPL/XAgZAY1bh/bK3EmyR8nPlfO6D0v8NArzq4TXbnPmjUFNZeA6u6bbAvF0/ULzAq0KFmDvdKw6cUwkJoNA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09.08.24 12:31, Dev Jain wrote: > As already being done in __migrate_folio(), wherein we backoff if the > folio refcount is wrong, make this check during the unmapping phase, upon > the failure of which, the original state of the PTEs will be restored and > the folio lock will be dropped via migrate_folio_undo_src(), any racing > thread will make progress and migration will be retried. > > Signed-off-by: Dev Jain > --- > mm/migrate.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/mm/migrate.c b/mm/migrate.c > index e7296c0fb5d5..477acf996951 100644 > --- a/mm/migrate.c > +++ b/mm/migrate.c > @@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t get_new_folio, > } > > if (!folio_mapped(src)) { > + /* > + * Someone may have changed the refcount and maybe sleeping > + * on the folio lock. In case of refcount mismatch, bail out, > + * let the system make progress and retry. > + */ > + struct address_space *mapping = folio_mapping(src); > + > + if (folio_ref_count(src) != folio_expected_refs(mapping, src)) > + goto out; This really seems to be the latest point where we can "easily" back off and unlock the source folio -- in this function :) I wonder if we should be smarter in the migrate_pages_batch() loop when we start the actual migrations via migrate_folio_move(): if we detect that a folio has unexpected references *and* it has waiters (PG_waiters), back off then and retry the folio later. If it only has unexpected references, just keep retrying: no waiters -> nobody is waiting for the lock to make progress. For example, when migrate_folio_move() fails with -EAGAIN, check if there are waiters (PG_waiter?) and undo+unlock to try again later. But I'm not really a migration expert, so only my 2 cents :) > __migrate_folio_record(dst, old_page_state, anon_vma); > return MIGRATEPAGE_UNMAP; > } -- Cheers, David / dhildenb