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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 9F301180012 X-Stat-Signature: 66uequs47nxkudgnn7thhb8itgr74muf X-Rspam-User: X-HE-Tag: 1769784360-233135 X-HE-Meta: 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 L9qitUWw 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 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: If hmm_range_fault() fails a folio_trylock() in do_swap_page, trying to acquire the lock of a device-private folio for migration, to ram, the function will spin until it succeeds grabbing the lock. However, if the process holding the lock is depending on a work item to be completed, which is scheduled on the same CPU as the spinning hmm_range_fault(), that work item might be starved and we end up in a livelock / starvation situation which is never resolved. This can happen, for example if the process holding the device-private folio lock is stuck in migrate_device_unmap()->lru_add_drain_all() The lru_add_drain_all() function requires a short work-item to be run on all online cpus to complete. A prerequisite for this to happen is: a) Both zone device and system memory folios are considered in migrate_device_unmap(), so that there is a reason to call lru_add_drain_all() for a system memory folio while a folio lock is held on a zone device folio. b) The zone device folio has an initial mapcount > 1 which causes at least one migration PTE entry insertion to be deferred to try_to_migrate(), which can happen after the call to lru_add_drain_all(). c) No or voluntary only preemption. This all seems pretty unlikely to happen, but indeed is hit by the "xe_exec_system_allocator" igt test. Resolve this using a cond_resched() after each iteration in hmm_range_fault(). Future code improvements might consider moving the lru_add_drain_all() call in migrate_device_unmap() out of the folio locked region. Also, hmm_range_fault() can be a very long-running function so a cond_resched() at the end of each iteration can be motivated even in the absence of an -EBUSY. Fixes: d28c2c9a4877 ("mm/hmm: make full use of walk_page_range()") Cc: Ralph Campbell Cc: Christoph Hellwig Cc: Jason Gunthorpe Cc: Jason Gunthorpe Cc: Leon Romanovsky Cc: Andrew Morton Cc: Matthew Brost Cc: linux-mm@kvack.org Cc: # v5.5+ Cc: Signed-off-by: Thomas Hellström --- mm/hmm.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/mm/hmm.c b/mm/hmm.c index 4ec74c18bef6..160c9e4e5a92 100644 --- a/mm/hmm.c +++ b/mm/hmm.c @@ -674,6 +674,13 @@ int hmm_range_fault(struct hmm_range *range) return -EBUSY; ret = walk_page_range(mm, hmm_vma_walk.last, range->end, &hmm_walk_ops, &hmm_vma_walk); + /* + * Conditionally reschedule to let other work items get + * a chance to unlock device-private pages whose locks + * we're spinning on. + */ + cond_resched(); + /* * When -EBUSY is returned the loop restarts with * hmm_vma_walk.last set to an address that has not been stored -- 2.52.0