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From: Chengming Zhou In-Reply-To: <20240314164941.580454-1-hannes@cmpxchg.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 53C0B20018 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: hcpkhbtwfb1f6ytjgf43ga1r91kitzpc X-HE-Tag: 1710472603-323122 X-HE-Meta: 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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: On 2024/3/15 00:49, Johannes Weiner wrote: > When cachestat against shmem races with swapping and invalidation, the > shadow entry might not exist: swapout IO is still in progress and > we're before __remove_mapping; or swapin/invalidation/swapoff has > removed the shadow from swapcache after we saw a shmem swap entry. > > This will send a NULL to workingset_test_recent(). The latter purely > operates on pointer bits, so it won't crash - node 0, memcg ID 0, > eviction timestamp 0, etc. are all valid inputs - but it's a bogus > test. In theory that could result in a false "recently evicted" count. Good catch! > > Such a false positive wouldn't be the end of the world. But for code > clarity and (future) robustness, be explicit about this case. > > Fixes: cf264e1329fb ("cachestat: implement cachestat syscall") > Reported-by: Jann Horn > Signed-off-by: Johannes Weiner > --- > mm/filemap.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/mm/filemap.c b/mm/filemap.c > index 222adac7c9c5..a07c27df7eab 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -4199,6 +4199,9 @@ static void filemap_cachestat(struct address_space *mapping, > swp_entry_t swp = radix_to_swp_entry(folio); > IIUC, we should first check if it's a real swap entry using non_swap_entry(), right? Since there maybe other types of entries in shmem. And need to get_swap_device() to prevent concurrent swapoff here, get_shadow_from_swap_cache() won't do it for us. Thanks. > shadow = get_shadow_from_swap_cache(swp); > + /* can race with swapping & invalidation */ > + if (!shadow) > + goto resched; > } > #endif > if (workingset_test_recent(shadow, true, &workingset))