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From: Chengming Zhou In-Reply-To: <20240315093010.GB581298@cmpxchg.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Stat-Signature: eyjj8ecxuqt3yb5c8jai3yxq8hwqpoju X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 4F98A8000A X-Rspam-User: X-HE-Tag: 1710496035-987408 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 17:30, Johannes Weiner wrote: > On Fri, Mar 15, 2024 at 11:16:35AM +0800, Chengming Zhou wrote: >> 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. > > Good point, it could be a poisoned entry. I'll add the > non_swap_entry() check on swp. > >> And need to get_swap_device() to prevent concurrent swapoff here, >> get_shadow_from_swap_cache() won't do it for us. > > We're holding rcu_read_lock() for the xarray iteration, so if we see > the swap entry in the shmem mapping, it means we beat shmem_unuse() > and swapoff hasn't run synchronize_rcu() yet. Ah, you are right, so it's safe. > > So it's safe. But I think it could use a comment. Maybe the > documentation of get_swap_device() should mention this option too?