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[2003:d8:2f09:3f00:d228:bd67:7baa:d604]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429c751b021sm40350645e9.21.2024.08.10.11.52.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 10 Aug 2024 11:52:47 -0700 (PDT) Message-ID: <367b0403-7477-4857-9e7c-5a749c723432@redhat.com> Date: Sat, 10 Aug 2024 20:52:44 +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> <761ba58e-9d6f-4a14-a513-dcc098c2aa94@redhat.com> <5a4ae1d3-d753-4261-97a8-926e44d4217a@arm.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 1C3F280011 X-Stat-Signature: xezzi9m5agd7zw8whycyuy8hx5t9ds33 X-Rspam-User: X-HE-Tag: 1723315971-549584 X-HE-Meta: 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 yALOpNY2 aVCC4elkcUM/AhQQzLxwCaqM8rBdzuUNc+Axv6u+gQCibqQ54bN6jtV8EH5mYZtWa0qS9JRHKW94k6Q6xQTGk5IvGNpZRhxkohrssVzyds8iBSupkPZxUHz1AGRh9qoK+IE90dQsRV5LGZEYTZ1wuFW+ezMEidm34mUOPWdsGySWyY0LWEzcC5DVBZZ82gVR+9icaTByApJ+TvqtfmzT4gbjoOHvVbdqO2hh+h5xzxZLV2jl8sAhOiEYdyUl+Vp58DmBUWfnS/8nVcCrBOPmvrGvsdkVQsCbHo3Ncv6zmMPwDlCRScFH+XGmQHXVmb3P01EQ22Wzf7+WiepM0Hn6ct5zzRZOK64tQ5W3+vPZwRlH17JMjHnUlaTn8PzR3PewVCfVIQNZ2W4LHiBwaW1xq8rHGau3PpxO1R6vDoTxsApntM6NQ09Got93dUz0LaedAclWbsvzp2ZtwSjk= 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 10.08.24 20:42, Dev Jain wrote: > > On 8/9/24 19:17, David Hildenbrand wrote: >> 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. > > > The patch currently retries migration irrespective of the reason of > refcount change. > > If you are suggesting that, break the retrying according to two conditions: That's not what I am suggesting ... > > >> This really seems to be the latest point where we can "easily" back >> off and unlock the source folio -- in this function :) >> For example, when migrate_folio_move() fails with -EAGAIN, check if >> there are waiters (PG_waiter?) and undo+unlock to try again later. > > > Currently, on -EAGAIN, migrate_folio_move() returns without undoing src > and dst; even if we were to fall ... I am wondering if we should detect here if there are waiters and undo src+dst. Does that make sense? -- Cheers, David / dhildenb