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Received: from willy by casper.infradead.org with local (Exim 4.98.2 #2 (Red Hat Linux)) id 1vZiPk-00000000SDO-25lI; Sun, 28 Dec 2025 04:29:56 +0000 Date: Sun, 28 Dec 2025 04:29:56 +0000 From: Matthew Wilcox To: Barry Song <21cnbao@gmail.com> Cc: akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Baolin Wang , syzbot+178fff6149127421c2cc@syzkaller.appspotmail.com Subject: Re: [PATCH] mm/shmem: fix uninitialized folio in shmem_symlink Message-ID: References: <20251224094027.65842-1-21cnbao@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspamd-Server: rspam02 X-Stat-Signature: mn3rxkbq9746ia9ebjje5r1ewx3ee5nq X-Rspam-User: X-Rspamd-Queue-Id: 4E9B3140007 X-HE-Tag: 1766896201-374935 X-HE-Meta: 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 EZBh8vPd 8Amb7DZcsr1RGmy6iAIq9T9JJP8DIhx7OEleJoKccdHr6DOirI4dICd40t4FaTeIUQv273wEv64oZuIa0AZlTt4e8xNF/Z/L9ryg1 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 Thu, Dec 25, 2025 at 05:04:38PM +1300, Barry Song wrote: > On Thu, Dec 25, 2025 at 6:01 AM Matthew Wilcox wrote: > > > > On Wed, Dec 24, 2025 at 10:40:27PM +1300, Barry Song wrote: > > > From: Barry Song > > > > > > Uninitialized folio allocated in shmem_symlink() may be accessed > > > during swap-out, causing KMSAN BUG: > > > > This would be an unfortunate way to fix it. The vast majority of > > symlinks are short, and we'll never access past the \0 in normal > > operation, so we'll be dirtying a lot of cachelines essentially to (1) > > shut up an automated tool and (2) optimise a corner case. > > > > How about this instead which delays zeroing to swapout? > > Matthew, thank you very much for your review, even during Christmas. > I would like to wish you a happy holiday! Heh, thanks. My mailserver is actually in a different timezone from me, so it was still the 24th when I sent it ;-) > I am not quite sure, as shm symlinks do not seem very common. Since > allocating a folio requires a symname longer than 128 bytes (where > 128 == SHORT_SYMLINK_LEN), such cases appear even rarer. Fair enough. I hadn't noticed the SHORT_SYMLINK_LEN case when I wrote this. That does change things somewhat, but still for a 160 byte symlink, we have 5 cachelines of data and 59 cachelines of zeroes. > BTW, do we need to migrate the owner_2 flag in folio_migrate_flags()? > If so, I am not quite sure it is worth changing the hotpath to > accommodate this. It already happens, it's just camouflaged. owner_2 is the same bit as mappedtodisk.