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Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1ponJj-00CPTZ-KO; Tue, 18 Apr 2023 15:32:27 +0000 Date: Tue, 18 Apr 2023 16:32:27 +0100 From: Matthew Wilcox To: Peter Xu Cc: Suren Baghdasaryan , akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@suse.com, josef@toxicpanda.com, jack@suse.cz, ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com, michel@lespinasse.org, liam.howlett@oracle.com, jglisse@google.com, vbabka@suse.cz, minchan@google.com, dave@stgolabs.net, punit.agrawal@bytedance.com, lstoakes@gmail.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v2 1/1] mm: do not increment pgfault stats when page fault handler retries Message-ID: References: <20230415000818.1955007-1-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 52D69100015 X-Stat-Signature: 8yf3neee5e6ihuaxeoubiobrt73zkd14 X-HE-Tag: 1681831968-825109 X-HE-Meta: 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 /7OlgNWp c0D+R1zmUL6d2FH0quU6nMkoW6JkNADqn0T4LMK9FBVzpQKDZdnwKVkPX2Rq6VyeNpsvH/3MTzF+XMBjfM9/0XHvc/q4QuGNnzgO31Qxf1PsEiglhkH9sU054aQAFTm4fDIsyGDn8p2hiP9LxNOMxizZ9/kMSKmMGVe2pRBahNf5yTPAhT/FxMs4FcQ== 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 Tue, Apr 18, 2023 at 11:19:40AM -0400, Peter Xu wrote: > On Mon, Apr 17, 2023 at 03:47:57PM -0700, Suren Baghdasaryan wrote: > > On Mon, Apr 17, 2023 at 2:26 PM Peter Xu wrote: > > > > > > On Fri, Apr 14, 2023 at 05:08:18PM -0700, Suren Baghdasaryan wrote: > > > > @@ -5223,8 +5230,8 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address, > > > > if (task_in_memcg_oom(current) && !(ret & VM_FAULT_OOM)) > > > > mem_cgroup_oom_synchronize(false); > > > > } > > > > - > > > > - mm_account_fault(regs, address, flags, ret); > > > > +out: > > > > + mm_account_fault(mm, regs, address, flags, ret); > > > > > > Ah, one more question.. can this cached mm race with a destroying mm (just > > > like the vma race we wanted to avoid)? Still a question only applies to > > > COMPLETE case when mmap read lock can be released. Thanks, > > > > I believe that is impossible because whoever is calling the page fault > > handler has stabilized the mm by getting a refcount. > > Do you have a hint on where that refcount is taken? ... when we called clone()? A thread by definition has a reference to its own mm. > Btw, it's definitely not a question sololy for this patch but a more common > question to the page fault path. It's just that when I wanted to look for > any refcount boost (which I also expect to have somewhere) I didn't really > see that in current path (e.g. do_user_addr_fault() for x86_64). > > I also had a quick look on do_exit() but I also didn't see where do we > e.g. wait for all the threads to stop before recycles a mm. > > I had a feeling that I must have missed something, but just want to make > sure it's the case. > > -- > Peter Xu >