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Howlett" , akpm@linux-foundation.org, michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, peterz@infradead.org, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 3DE8A4002D X-Stat-Signature: pmse69jmjbdse36qn9ugph4cwbx8x4a8 X-HE-Tag: 1674493668-360850 X-HE-Meta: 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 pWDXfgJQ j08b9iS3KiQa0Ney7h4K9ps+WxgcSfTt6MxMsRE2LZcfWQE9Xvgw1wpFazdBoSd6CJ66LInHo7qgGohP+Xu0XZdCacZ3MG9ssKBoPcUf7ExNKTWgXE4XimB8iMLcMqC7NZOJYiQU3QmIhcqcfeYHzUEugW8XwFIWMeawUoEwNpSL2MG/yz/sxQndR5+ULAK6qmQSKY1OKWkwEXvH8tHawLvO3zPOUcpSzc8hg18rUqpSgd6Xz0j87LX/c9f1R8A/+GZuEfcoscOvQp90fmeUau6g33qM2V4Na54UiGp8A3yTdOVtkJBYEFC5IVbdNIcSQ9WK5qN2PBhrGlLXFkuYPxKy0wQFXcgYaQ6liSKD6wUL+xee5F4xrposoIU6pM8qJJPSBm4HGBVH8J1Nm+byWh5hQcSb+WIWeA+Gw 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 Mon, Jan 23, 2023 at 8:55 AM Michal Hocko wrote: > > On Mon 23-01-23 08:22:53, Suren Baghdasaryan wrote: > > On Mon, Jan 23, 2023 at 1:56 AM Michal Hocko wrote: > > > > > > On Fri 20-01-23 09:50:01, Suren Baghdasaryan wrote: > > > > On Fri, Jan 20, 2023 at 9:32 AM Matthew Wilcox wrote: > > > [...] > > > > > The page fault handler (or whatever other reader -- ptrace, proc, etc) > > > > > should have a refcount on the mm_struct, so we can't be in this path > > > > > trying to free VMAs. Right? > > > > > > > > Hmm. That sounds right. I checked process_mrelease() as well, which > > > > operated on mm with only mmgrab()+mmap_read_lock() but it only unmaps > > > > VMAs without freeing them, so we are still good. Michal, do you agree > > > > this is ok? > > > > > > Don't we need RCU procetions for the vma life time assurance? Jann has > > > already shown how rwsem is not safe wrt to unlock and free without RCU. > > > > Jann's case requires a thread freeing the VMA to be blocked on vma > > write lock waiting for the vma real lock to be released by a page > > fault handler. However exit_mmap() means mm->mm_users==0, which in > > turn suggests that there are no racing page fault handlers and no new > > page fault handlers will appear. Is that a correct assumption? If so, > > then races with page fault handlers can't happen while in exit_mmap(). > > Any other path (other than page fault handlers), accesses vma->lock > > under protection of mmap_lock (for read or write, does not matter). > > One exception is when we operate on an isolated VMA, then we don't > > need mmap_lock protection, but exit_mmap() does not deal with isolated > > VMAs, so out of scope here. exit_mmap() frees vm_area_structs under > > protection of mmap_lock in write mode, so races with anything other > > than page fault handler should be safe as they are today. > > I do not see you talking about #PF (RCU + vma read lock protected) with > munmap. It is my understanding that the latter will synchronize over per > vma lock (along with mmap_lock exclusive locking). But then we are back > to the lifetime guarantees, or do I miss anything. munmap() or any VMA-freeing operation other than exit_mmap() will free using call_rcu(), as implemented today. The suggestion is to free VMAs directly, without RCU grace period only when done from exit_mmap(). That' because VMA freeing flood has been seen so far only in the case of exit_mmap() and we assume other cases are not that heavy to cause call_rcu() flood to cause regressions. That assumption might prove false but we can deal with that once we know it needs fixing. > -- > Michal Hocko > SUSE Labs