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Howlett" , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, syzkaller-bugs@googlegroups.com, syzbot Subject: Re: [syzbot] [mm?] WARNING: suspicious RCU usage in mas_walk (2) Message-ID: References: <000000000000607ff905ffc8e477@google.com> <0000000000000aeb7f06015e5cbd@google.com> <20230727164757.e2di75xjybxncohn@revolver> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: 5FE5C8001C X-Stat-Signature: 34pc4cbak36jah5uxmrs4gu1h3kkbcif X-Rspam-User: X-HE-Tag: 1690488535-934062 X-HE-Meta: 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 6UijqgaK hHN2jcVokC/cv7dA05BtwVS6pvUVtfGyYbdKjco3H/8qd7RSUJwSklkvoO0hx6D3MbHnPHjeWjSgnTcMoR25Sqz6+K/Vzom9puV7SvbfsZfUqb6gxo9tWMTbuFAITY0MTWc16qkW+7zFOmIwaSQ3yTFlPqL45VKBh8Xhv9CHTeAHBcCOzWMDf2BzpqK9X22SL/bqVTBaT8ryAStEhZcBeh184WckwhbJmP70urjn8WL0CYtbuhYm8YzENom6NIuzV/gGHYa239eb55sIWsEUzUA9eNI6ivd4575BLvG5j2/g/XQjja+E7JnwZ5uHDwsrvkw0+rSbfhY8YqsleZIZHn5R72+IlSQoi7RKM 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: On Thu, Jul 27, 2023 at 09:20:33PM +0200, Jann Horn wrote: > One kinda straightforward option would be to pass the vmf (or NULL if > it's not in fault context) to anon_vma_prepare(), teach it to bail if > it runs under the mm lock, and propagate a VM_FAULT_RETRY all the way > up? It can already fail due to OOM, so the bailout paths exist, though > you'd have to work a bit to plumb the right error code up. > > And if you're feeling adventurous, you could try to build a way to > opportunistically upgrade from vma lock to mmap lock, to avoid having > to bail out all the way back up and then dive back in when that > happens. Something that does mmap_read_trylock(); on failure, bail out > with VM_FAULT_RETRY; on success, drop the VMA lock and change > vmf->flags to note the changed locking context. I think that's all a little more adventurous than I'd be comfortable with right now ;-) I just sent the fix patch that syzbot tested to Andrew for integration.