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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1r5TNW-005jSi-VU; Tue, 21 Nov 2023 16:13:35 +0000 Date: Tue, 21 Nov 2023 16:13:34 +0000 From: Matthew Wilcox To: Charan Teja Kalla Cc: akpm@linux-foundation.org, david@redhat.com, hannes@cmpxchg.org, kirill.shutemov@linux.intel.com, shakeelb@google.com, n-horiguchi@ah.jp.nec.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] [RFC] mm: migrate: rcu stalls because of invalid swap cache entries Message-ID: References: <1700569840-17327-1-git-send-email-quic_charante@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1700569840-17327-1-git-send-email-quic_charante@quicinc.com> X-Rspam-User: X-Stat-Signature: d7iuc4xexztwy8cf87ohsc6c6ct4bndx X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: BC884140029 X-HE-Tag: 1700583229-591593 X-HE-Meta: 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 6Ua4hZJ5 mnpvf9hzzxu4/VpIhUog4BZ8MkvfvRnalLjZoDnbBkzOIaeM5M2a4ps9M3UvXKbAieR+4Tcv9cjrQasV8SMpDPUiaI6y0OvS9xv+Vx3qyXKq6pB+pklDWm/JVQMSm3K2Mda8PHmOhRzLBHcw= 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 Tue, Nov 21, 2023 at 06:00:40PM +0530, Charan Teja Kalla wrote: > The below race on a folio between reclaim and migration exposed a bug > of not populating the swap cache with proper folio resulting into the > rcu stalls: Thank you for figuring out this race and describing it so well. It explains a few things I've seen, at least potentially. What would you think to this? I think a better fix would be to fix the swap cache to user multi-order entries, but I would like to see this backportable! diff --git a/mm/migrate.c b/mm/migrate.c index d9d2b9432e81..2d67ca47d2e2 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -405,6 +405,7 @@ int folio_migrate_mapping(struct address_space *mapping, int dirty; int expected_count = folio_expected_refs(mapping, folio) + extra_count; long nr = folio_nr_pages(folio); + long entries, i; if (!mapping) { /* Anonymous page without mapping */ @@ -442,8 +443,10 @@ int folio_migrate_mapping(struct address_space *mapping, folio_set_swapcache(newfolio); newfolio->private = folio_get_private(folio); } + entries = nr; } else { VM_BUG_ON_FOLIO(folio_test_swapcache(folio), folio); + entries = 1; } /* Move dirty while page refs frozen and newpage not yet exposed */ @@ -453,7 +456,11 @@ int folio_migrate_mapping(struct address_space *mapping, folio_set_dirty(newfolio); } - xas_store(&xas, newfolio); + /* Swap cache still stores N entries instead of a high-order entry */ + for (i = 0; i < entries; i++) { + xas_store(&xas, newfolio); + xas_next(&xas); + } /* * Drop cache reference from old page by unfreezing