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AJvYcCX9Y7wz92XfxLXBHPBXHvlCG3x58DdQt009o7/qOyU/ujZBiEHBfZDML0bFzSxhuma4UPF2ipVR3w==@kvack.org X-Gm-Message-State: AOJu0YzSvzQWKMfEfGBaj28nt6UEOusbSzBOdtuc9rTTyGxAreNdCFJI e0Orqc/tO5KTkYUDcYzNvT7PRf5+CXt1t0HuOLYRo+/XXIuEfeG+HcjQVImZWCU9GiezMFaDtTW JzHyYYgZvh6GZiGciXykwEr0DkqUlrSNZN7bn X-Gm-Gg: ASbGncveqA7w/NlOLsUZTN2EPpg4oDckmfw3hjcWEwxOFeO7U3eeOUvCYU9NPhXgG+I JdsUw02DNnZpSbA+wr1nEUpHporVv3zZtCT91KQ== X-Google-Smtp-Source: AGHT+IEsMRlH6v6Lz/in+BfakjEnu5qz3Akw3EqKVImVcmfzugXt6fQ734q4f8TcVBrh0DnHRYCrLtDIxLKhu7sX2rU= X-Received: by 2002:ac8:5f87:0:b0:466:97d6:b245 with SMTP id d75a77b69052e-46a3ba6786dmr4014381cf.22.1734624875665; Thu, 19 Dec 2024 08:14:35 -0800 (PST) MIME-Version: 1.0 References: <20241218174428.GQ2354@noisy.programming.kicks-ass.net> <20241219091334.GC26551@noisy.programming.kicks-ass.net> In-Reply-To: <20241219091334.GC26551@noisy.programming.kicks-ass.net> From: Suren Baghdasaryan Date: Thu, 19 Dec 2024 08:14:24 -0800 Message-ID: Subject: Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count To: Peter Zijlstra Cc: "Liam R. Howlett" , akpm@linux-foundation.org, willy@infradead.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mjguzik@gmail.com, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Stat-Signature: ctmxa6a8yxrub6azn8uf3ie65hgc6b7q X-Rspam-User: X-Rspamd-Queue-Id: ED789140010 X-Rspamd-Server: rspam08 X-HE-Tag: 1734624854-318332 X-HE-Meta: U2FsdGVkX1/200+p76lLBaUdVQCCqC/dpWmN2uHjRXileY77xcA7KNezfwbMlujq3MQKqqiT4tbqBCF9B45jnOXjfcH10BxxqZcHW8D8VopzEnjIiSkGvaSPdp8ZTaIGnbv+Hg2Od7igs1Wt0Fr1Duo6qna1PfJkEVNzRBWHq8IhKFUEyEj3/DGFePdsMOmw2vWZxbOLqMBe/7STShfx2LECkxuDfGMnLtBhZ5h8203N63aohsrzwCtrMNVVFkcT8d8wRuPHYYXcw4Yg7w7Et4MaE6w/tqiPfBToqzAFpkNMgtZxcgXI/rYhgP9De/VnE7KOCfIALkwdQKTQn35huknPlJJNDTgR3qef3JhPdl5ueQsQkeci6myj3X1cHaRsnUGLloTAlR458jrA1FziGCAAAb43XOIP/+ZEBbw6KbdbUkazcehC/RkkhiONO/Gcn6tTU4kUSq6nmLbbKTf9Y2DR3HMdd9p7juSIdmwQBlufl3zbQZpzzHwSm9oC1PPbWQOHHgCYpvXB0znki8sgkXjnOlOnVHzoKRurFOpfa+GH/3PQLem+Z596ZJYPHDLcRmdwcQdlwSK8UQBMhxnAuZGu7URIClL98dQpN/MSl00xrfUvIpCR94s9B8m4JoTbX8ZmXX7Qwcr2OdohpANVr6HRXqXg8noUPxxigczzqogwrCoEmSddWJdC22EDY2TGymmtpKvDahjP4lV8JUiuh+QZsbb7sWYwPPy0S9PvBtYqhH1BgHru1OZ3eoBFR6JR2aAlNjrbZ7HT+NEcLfJp74SGV0YEUGQhqDt3EKC38VDxRSrptFuVGNuxaDkrBZ8HcaY2+7Wh6Urij/ZH+AkEQx/w/ZJ7h+31nbGlDbxmXNwGUpg5AeSxOg1azrR8BOhWuZCtV1BUjtze8r/CmFcSRsXiybjLJ44F8iZTYTOMghBXG1zDydkl42Z7By0kX66yPB8JQLEJ9h755TSjj+o bbLJi7Ef 2RbYGWfHkdMrY2TSpcclOU3mhKaIoNsLmufVhJrQlwFrFFZspm7Fr9o6hOT/GmIMi8YPZJs4E86Qko7/yxaqYp9EGXpCrx+MgT1zt3gR4lwCp3Zk1OxtHwpqmqQ1uWDu98seALJs3hio82K8Cf3IV8h92En+TzPJZfHk7OeI/s/JYnlB7gWaElDPVMaGoYGLlVdvv6s0RREzKtBAEvYiohoEyQrYdY6aiKXn7SBrMMNhFAAmlIhmkGtZFDJPUnQnrMxtPOztIO7Uzx+fn/32QVtf5lSGfHBaLZEmaecGjcHDNdYo= X-Bogosity: Ham, tests=bogofilter, spamicity=0.107093, 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 19, 2024 at 1:13=E2=80=AFAM Peter Zijlstra wrote: > > On Wed, Dec 18, 2024 at 01:53:17PM -0800, Suren Baghdasaryan wrote: > > > Ah, ok I see now. I completely misunderstood what for_each_vma_range() > > was doing. > > > > Then I think vma_start_write() should remain inside > > vms_gather_munmap_vmas() and all vmas in mas_detach should be > > No, it must not. You really are not modifying anything yet (except the > split, which we've already noted mark write themselves). > > > write-locked, even the ones we are not modifying. Otherwise what would > > prevent the race I mentioned before? > > > > __mmap_region > > __mmap_prepare > > vms_gather_munmap_vmas // adds vmas to be unmapped into mas_det= ach, > > // some locked > > by __split_vma(), some not locked > > > > lock_vma_under_rcu() > > vma =3D mas_walk // finds > > unlocked vma also in mas_detach > > vma_start_read(vma) // > > succeeds since vma is not locked > > // vma->detached, vm_start, > > vm_end checks pass > > // vma is successfully read-locked > > > > vms_clean_up_area(mas_detach) > > vms_clear_ptes > > // steps on a cleared PTE > > So here we have the added complexity that the vma is not unhooked at > all. Is there anything that would prevent a concurrent gup_fast() from > doing the same -- touch a cleared PTE? > > AFAICT two threads, one doing overlapping mmap() and the other doing > gup_fast() can result in exactly this scenario. > > If we don't care about the GUP case, when I'm thinking we should not > care about the lockless RCU case either. > > > __mmap_new_vma > > vma_set_range // installs new vma in the range > > __mmap_complete > > vms_complete_munmap_vmas // vmas are write-locked and detached > > but it's too late > > But at this point that old vma really is unhooked, and the > vma_write_start() here will ensure readers are gone and it will clear > PTEs *again*. So, to summarize, you want vma_start_write() and vma_mark_detached() to be done when we are removing the vma from the tree, right? Something like: vma_start_write() vma_iter_store() vma_mark_detached() And the race I described is not a real problem since the vma is still in the tree, so gup_fast() does exactly that and will simply reinstall the ptes. > >