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Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1w4WQJ-0000000BaLK-1rfD; Mon, 23 Mar 2026 03:57:51 +0000 Date: Mon, 23 Mar 2026 03:57:51 +0000 From: Matthew Wilcox To: Abhishek Kumar Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, syzbot+606f94dfeaaa45124c90@syzkaller.appspotmail.com, Axel Rasmussen , Yuanchu Xie , Wei Xu Subject: Re: [PATCH] mm: fix data race in __filemap_remove_folio / folio_mapping Message-ID: References: <20260322190319.85301-1-abhishek_sts8.ref@yahoo.com> <20260322190319.85301-1-abhishek_sts8@yahoo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260322190319.85301-1-abhishek_sts8@yahoo.com> X-Stat-Signature: qruq3qgsghdeqwaysufk6cdj5zobfpwt X-Rspam-User: X-Rspamd-Queue-Id: 4A16F18000B X-Rspamd-Server: rspam12 X-HE-Tag: 1774238274-54046 X-HE-Meta: U2FsdGVkX1/JZPDnqvVKPw2kTlhh6TzuE8t2NgtsvbA2IEbOGCDnXdNouoYgyND9ZcIxv/ijFKiujJLaQCsLuGBdQPuA5+gEoetjg2WlPNXvUXrUFP5SrS9zBTIiFpjHPrZE6Q/rA0tiB9ZGIaJ+qAazCPCkpfLz6x6BK5VM1Oq5HiWEDjeCoHA8IfSsE+ROEFqUgeZWyaXluFwESlZKzIwZ2ZJukyzxPoPOqt3uHrfnX6X3Rab2keNkvpgO03egDIw7b9kiNBLID5mzEjEu8T2yHzxzfW+Zdo+KFeb5rO7xvAbBCGVVkfrdM5l4X11bWRYOZT1x6kUhkcaBiQgsRCBIb75Qh7levaa/XO75ETkZJoO1864O66FOQ9FsZ/mBFHBOa64vU5kvtQP7xh4wjNLAyGhvqKSqwvdkKxnKIQBPAtoqPnYiWq5b4ZdQY7vGBit7mFjEDLoAUiiA6fc2d7iTpAJpInyDXLmjTzrzPcjZ6JJbr5g6fCeMiqJ4TunU75pbfirrcu+pSh5sTdoBN4bMae18SlDQxtvBY/zkJ5wvhCUaOkhYHBaHBrxmFiN2QTTzqRBm2I/vrVP6i2rAZFoKEzYoG73D2I6ZJY2p151KN0Qd55RDWiJOZENJgR/Mgi0dqlFu1l1UsbGQVujOfJ0Qzz0isqPu+8aeZpSMz2Kst6BJWU6859H4YBPV0ThByELd3R1Ohug2pJ8bs+WCh0sVbwC5HKmKbdQB5iFHVrE7gBZlc62mL1XxBhWufrsU6nCxb2Cj4YNqzyY4xlXGvF7PPV9dDETds4jlViqGO1Blm6h6WdWyL72B0cgzc40mu4Juqxxx2pxZKVSNAh/zXiHxpqnJ0Otkk4poelsOjaD0UA20GiYq1EQ0+FdC7pQEB3MPgPDPmOY8BblOhKDhPtYsueKr3A/san0BCB00zAepNS0Q3HuJkMEcMubM1QOlmcGGO42ChylZhWhxs5V htKBq6sX +cXF/bpViiq054s2GnpsJL9KxsS9hRZH6hOAjsfzPN6hqjX+sL5cfPx+xIM3gH2WMTDFXJsMuP1BoMYXWnPubMqo33Cfe7KPQ5Phd Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Mar 23, 2026 at 12:33:19AM +0530, Abhishek Kumar wrote: > KCSAN reports a data race between page_cache_delete() and > folio_mapping(): > > page_cache_delete() performs a plain store to folio->mapping: > folio->mapping = NULL; > > folio_mapping() performs a plain load from folio->mapping: > mapping = folio->mapping; > > page_cache_delete() is called from the truncation path under the i_pages > xarray lock, That's not relevant. The important lock for maintaining folio->mapping is the folio lock (see the VM_BUG_ON_FOLIO line in page_cache_delete()). At a minimum, this changelog needs to be fixed because there's already too much confusion around the locking rules. > while folio_mapping() is called from the reclaim path > (evict_folios -> folio_evictable -> folio_mapping) under only > rcu_read_lock() without the xarray lock. Umm. First up, this is MGLRU-only code, right? Adding the so-called maintainers. Second ... I'm really unsure how we want to handle this generally. This could be quite the game of whack-a-mole; we have many, many places in the kernel which dereference folio->mapping without holding a lock. Perhaps they are all fine; but 12 of the 455 references to folio->mapping currently have READ_ONCE attached. That's a lot of code to audit. > The race is benign since the reclaim path tolerates stale values -- > reading a stale non-NULL mapping simply results in a suboptimal eviction > decision. However, the plain accesses risk store/load tearing and allow > the compiler to perform harmful optimizations (merging, elision, or > fission of the accesses). I think the bigger problem is reloading. As I understand it, this code: struct address_space *m = folio->mapping; if (m && m->flags) could end up loading 'm' twice, once before the setting to NULL and once after. That's more plausible than deciding to load byte-by-byte, or whatever else these "merging, elision, or fission" words mean. > Fix this by using WRITE_ONCE() in page_cache_delete() and READ_ONCE() > in folio_mapping() to prevent compiler misbehavior and silence the KCSAN > report. Just to be clear, I don't object to the patch itself, I'm just scared of the consequences. And the locking comment above needs to be fixed. But please wait a few days for discussion to play out.