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From: "Hui Zhu" Message-ID: <40a6017455ce039225fc331940be6cb27d13bc3e@linux.dev> TLS-Required: No Subject: Re: [PATCH] mm: avoid KCSAN false positive in page_to_nid() To: "Lorenzo Stoakes" , "David Hildenbrand (Arm)" Cc: "Andrew Morton" , "Liam R. Howlett" , "Vlastimil Babka" , "Mike Rapoport" , "Suren Baghdasaryan" , "Michal Hocko" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Hui Zhu" In-Reply-To: References: <20260623074157.578113-1-hui.zhu@linux.dev> <97f2c4ad-c302-424d-a7c8-fdfa0fc0d811@kernel.org> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B445340007 X-Stat-Signature: umukg99weniqnwxtzchnhxx7unb1fsd6 X-HE-Tag: 1782264808-207010 X-HE-Meta: U2FsdGVkX1/0AB0r1ZL6autZPLg3D9nkRz8ranSQ2fSoN0hfbmcOVU6i69xhXm1PHOggc4Qb8aG/eNxyWL9APYNNzcMWb5B3lgOlwNnwT2+TNLlnUPWMPq+DNJnJuRc80cRlJVMl5BiG11jluBsjwWI2z63ko2fLKpqJ9/1JXx2nv2eOd7iMU+EHJYG9Ekn8z0VsJzypVKqC13bve3uuG93w+jir5CP+SO6yWeD4b1Wy5kRbguNZVSJRtTVaESyhDybK7y5cb/Jw+tEUZgNx/wk85DLX52UqnlzviPaig78UOnmobUzkr/URIG5USdQDtenpu1fvDqtYEfMkb9NTA48wE15p/gd0UkbTvx8XhQRWTmFCG/w9R4I9QyE1E0FOEx9PjqpFX2JNHxRFTf+RoV/m1rFfGz/bYxUbtcYQtwQZSvkG/niBKpHy9iCr2dbLxBMqeJBq0/3dTWMyK6c1/Rmh1M2LYC1RGP+IFZR0M5opGHJGCbIsqBiMGxyXpOUh7o14UQ4HSQKVFd9hONgo8YN/irbUI2AIDmMOu3qlFwvKAHFnhXk5Y+7TJANI8BY4r6GDg03Rfy2wxkX7g+30NNt159ouOyIDt5FmAWExQjuqTmfReI628y2aUz2RmUudouMtNIhGAuVXxQZ5WkBpzyf3Fq9DDdZKBAPoud2djc06zgjA/OL4C3sjKYFI1Hf8S98kfcDEp5doAQ2SC60yoB/hx7QaE9WDZKNzBgRNyoAu096pO3GSlf6jgrPx8f8P7COwrws6HviS6dhXnbeWIKAq4JepHEr+uWV7F1uu++bUnfiZvJ4gLE+6ZCq3SzReE3JdY6Y/7DXTbEPG/vp0oF0na+wB+loURRJEoxqNxzoFztQG0l4QdkVj+0jUjWriwSJIirMKPEmUaPbZnmG53vy3IuB5oUlyfNiUWRmgXDSu4fo6BqrkZmlfCmBV6YSWBLm6IjQIWDduxredEwl xV/arbdL pxkh+/3VqXkpWj/TQ5YNuRN+rpgzY6h8sM5FjahGX/nlqS2fX61PgnpY55rz3fnhqcjDJ66lgXwn0ZKjJVEhnVvx6ZKya+Qfom4EluS8x5HkzZCi2tbp3aCZ2IA0DvdhsNfe8wAfXvGAbnxEDiXfilCys44Kq5s2MdQ1/igZu265qajiJR6BNw01krDOF60zjjFQMVDjn31tCeZcUgL1HO+0sayHo0Chk2BDzQHXEbUoW9FdcgFTbax4OvozX7g1Eg2i9sW3XYPFMhMDaxUdfGEwm34XBkLqm2WWpz8W5aFjntOlH9mzDUTba6A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >=20 >=20On Tue, Jun 23, 2026 at 09:59:45AM +0200, David Hildenbrand (Arm) wro= te: >=20 >=20>=20 >=20> On 6/23/26 09:41, Hui Zhu wrote: > > From: Hui Zhu > >=20 >=20> KCSAN reports a data race between page_to_nid() reading page->flag= s > > and folio_trylock()/folio_lock() doing test_and_set_bit_lock(PG_lock= ed, > > ...) on the same word from another CPU, e.g.: > >=20 >=20> BUG: KCSAN: data-race in __lruvec_stat_mod_folio / shmem_get_folio= _gfp > >=20 >=20> The node id occupies a fixed, high bit-range of page->flags that i= s > > set once when the page is initialized and never modified afterwards, > > so it can never overlap with the low PG_locked/PG_waiters bits touch= ed > > by the folio lock path. The race is therefore harmless: page_to_nid(= ) > > always returns a consistent value regardless of how the read > > interleaves with the lock bit ops. > >=20 >=20> Wrap the flags read with data_race() to tell KCSAN this race is > > intentional and benign, consistent with how page->page_type is > > already annotated for similar packed-field accesses. > >=20 >=20> Signed-off-by: Hui Zhu > >=20 >=20How did you discover this? >=20 >=20A syzbot report? If so please include Reported-by, Closes tags. This wasn't reported by syzbot - it was found by our internal fuzzing CI on a KCSAN-enabled build of our downstream tree (based on v6.6.121), triggered by a multi-threaded mlockall() fuzz testcase. There's no public bug id for it, so Reported-by/Closes don't apply here. Best, Hui >=20 >=20>=20 >=20> --- > > include/linux/mm.h | 9 ++++++++- > > 1 file changed, 8 insertions(+), 1 deletion(-) > >=20 >=20> diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 485df9c2dbdd..122d3b39369f 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -2296,7 +2296,14 @@ static inline int memdesc_nid(memdesc_flags_t= mdf) > >=20 >=20> static inline int page_to_nid(const struct page *page) > > { > > - return memdesc_nid(PF_POISONED_CHECK(page)->flags); > > + /* > > + * The node id occupies a fixed high bit-range of page->flags > > + * that is set once at page init and never changed afterwards. > > + * It cannot overlap with the low PG_locked/PG_waiters bits > > + * that folio_lock()/folio_unlock() concurrently update, so > > + * this data race is benign. > > + */ > >=20 >=20> Do we really need this excessive comment? > >=20 >=20Agreed, just delete it. For a trivial benign data race it's a bit muc= h, and the > commit message can cover it off for those who are curious. >=20 >=20>=20 >=20> + return memdesc_nid(data_race(PF_POISONED_CHECK(page)->flags)); > >=20 >=20> In memdesc_zonenum() we use ASSERT_EXCLUSIVE_BITS. > >=20 >=20> Can we do the same here inside memdesc_nid? > >=20 >=20Also agreed >=20 >=20>=20 >=20> diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 69daeeab7fe8f..76d3bb54be844 100644 > > --- a/include/linux/mm.h > > +++ b/include/linux/mm.h > > @@ -2290,6 +2290,7 @@ int memdesc_nid(memdesc_flags_t mdf); > > #else > > static inline int memdesc_nid(memdesc_flags_t mdf) > > { > > + ASSERT_EXCLUSIVE_BITS(mdf.f, NODES_MASK << NODES_PGSHIFT); > > return (mdf.f >> NODES_PGSHIFT) & NODES_MASK; > > } > > #endif > >=20 >=20> -- > > Cheers, > >=20 >=20> David > >=20 >=20Thanks, Lorenzo >