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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 Subject: Re: [PATCH 39/41] kernel/fork: throttle call_rcu() calls in vm_area_free Message-ID: References: <20230120170815.yuylbs27r6xcjpq5@revolver> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 823302001C X-Stat-Signature: kqreqbpaptmkfsim5bbtkycjeg78ar8u X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1674492951-247237 X-HE-Meta: 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 1LF0p6mn d1IMC9Jsv8ajcmRusE+KW6k2h785Lh9MXZWD0Q5+YqTf6RQpui4LjrRpLAojCR+cVI8PW6e3+Q/2Wh2KmQX5PceOYwpnwI9RIiabUDtYb3SSDNXvY36FqnT9pzUxvCAd2dahKJwCMNtfasa9oL7EjYMXSBOo2WCmdq0l2slfPptM4YdDkepKjvQ63551wejHxk3+UdEnGxI4UOxLJHD6OBuCjE2KLO4s/x1wzfiiyvbfzNdm9vawI58XjrUx4KqdJprd56UIGsqlOYqBPaQaJaJLuOUSNYYQIwv+6d0+rUYbqkDmNleM8wXCzREorvrGwRn8uefsLqHJIL90= 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 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. -- Michal Hocko SUSE Labs