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Fri, 23 Jan 2026 10:02:04 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id I1SsFxxHc2k3HAAAD6G6ig (envelope-from ); Fri, 23 Jan 2026 10:02:04 +0000 Message-ID: <76e112da-a8b6-4004-9bce-2dcc29dea7fc@suse.cz> Date: Fri, 23 Jan 2026 11:02:04 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RESEND v3 07/10] mm/vma: introduce helper struct + thread through exclusive lock fns Content-Language: en-US To: Lorenzo Stoakes , Andrew Morton Cc: David Hildenbrand , "Liam R . Howlett" , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shakeel Butt , Jann Horn , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-devel@lists.linux.dev, Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , Waiman Long , Sebastian Andrzej Siewior , Clark Williams , Steven Rostedt References: <4f95671feac6b6d4cea3c53426c875f3fd8a8855.1769086312.git.lorenzo.stoakes@oracle.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJnyBr8BQka0IFQAAoJECJPp+fMgqZkqmMQ AIbGN95ptUMUvo6aAdhxaOCHXp1DfIBuIOK/zpx8ylY4pOwu3GRe4dQ8u4XS9gaZ96Gj4bC+ jwWcSmn+TjtKW3rH1dRKopvC07tSJIGGVyw7ieV/5cbFffA8NL0ILowzVg8w1ipnz1VTkWDr 2zcfslxJsJ6vhXw5/npcY0ldeC1E8f6UUoa4eyoskd70vO0wOAoGd02ZkJoox3F5ODM0kjHu Y97VLOa3GG66lh+ZEelVZEujHfKceCw9G3PMvEzyLFbXvSOigZQMdKzQ8D/OChwqig8wFBmV QCPS4yDdmZP3oeDHRjJ9jvMUKoYODiNKsl2F+xXwyRM2qoKRqFlhCn4usVd1+wmv9iLV8nPs 2Db1ZIa49fJet3Sk3PN4bV1rAPuWvtbuTBN39Q/6MgkLTYHb84HyFKw14Rqe5YorrBLbF3rl M51Dpf6Egu1yTJDHCTEwePWug4XI11FT8lK0LNnHNpbhTCYRjX73iWOnFraJNcURld1jL1nV r/LRD+/e2gNtSTPK0Qkon6HcOBZnxRoqtazTU6YQRmGlT0v+rukj/cn5sToYibWLn+RoV1CE Qj6tApOiHBkpEsCzHGu+iDQ1WT0Idtdynst738f/uCeCMkdRu4WMZjteQaqvARFwCy3P/jpK uvzMtves5HvZw33ZwOtMCgbpce00DaET4y/UzsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZ8gcVAUJFhTonwAKCRAiT6fnzIKmZLY8D/9uo3Ut9yi2YCuASWxr7QQZ lJCViArjymbxYB5NdOeC50/0gnhK4pgdHlE2MdwF6o34x7TPFGpjNFvycZqccSQPJ/gibwNA zx3q9vJT4Vw+YbiyS53iSBLXMweeVV1Jd9IjAoL+EqB0cbxoFXvnjkvP1foiiF5r73jCd4PR rD+GoX5BZ7AZmFYmuJYBm28STM2NA6LhT0X+2su16f/HtummENKcMwom0hNu3MBNPUOrujtW khQrWcJNAAsy4yMoJ2Lw51T/5X5Hc7jQ9da9fyqu+phqlVtn70qpPvgWy4HRhr25fCAEXZDp xG4RNmTm+pqorHOqhBkI7wA7P/nyPo7ZEc3L+ZkQ37u0nlOyrjbNUniPGxPxv1imVq8IyycG AN5FaFxtiELK22gvudghLJaDiRBhn8/AhXc642/Z/yIpizE2xG4KU4AXzb6C+o7LX/WmmsWP Ly6jamSg6tvrdo4/e87lUedEqCtrp2o1xpn5zongf6cQkaLZKQcBQnPmgHO5OG8+50u88D9I rywqgzTUhHFKKF6/9L/lYtrNcHU8Z6Y4Ju/MLUiNYkmtrGIMnkjKCiRqlRrZE/v5YFHbayRD dJKXobXTtCBYpLJM4ZYRpGZXne/FAtWNe4KbNJJqxMvrTOrnIatPj8NhBVI0RSJRsbilh6TE m6M14QORSWTLRg== In-Reply-To: <4f95671feac6b6d4cea3c53426c875f3fd8a8855.1769086312.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspam-User: X-Rspamd-Queue-Id: A365B40009 X-Rspamd-Server: rspam07 X-Stat-Signature: rrk83iebaxzwohon34pbpfmgh4cemn5a X-HE-Tag: 1769162526-722359 X-HE-Meta: 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 dPAIUJ44 56gZ0eMXOvVaJDDJxGkOgDYUoOuEPn9kdxlEqdHpzTlnuVr8PPEDNaSsMNFtDUuB8Mwo2tc5QL5o1yvLAzk1wVuAdE1llvf1/cgkycPMndycbZA9FPeLS02TmIWiyZ1omrn27MUD7KlDhGbDP9blv9vaBLClwru0nEbVzTO33Em17bTH7Rer6N/Jfy+5dCVMalxgI5JoK/7954gWQ1wUtKSsJnVP87fZvPPVMlAZp2F8J2gQpJ2sK9pNG9aCoippczjv6CewfDEbaDyPTR4HcgaDuYyDFAkuio0Ze/b+cqg1IMJQZ2CQzWdQ6YJpNEOPUtrNWAz7AoWEzulLKfh1dlIuYg7cO1rCYqAmmrqIBEGiERVgdGyhJPu6MqUsapVnph5o4 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: List-Subscribe: List-Unsubscribe: On 1/22/26 14:01, Lorenzo Stoakes wrote: > It is confusing to have __vma_enter_exclusive_locked() return 0, 1 or an > error (but only when waiting for readers in TASK_KILLABLE state), and > having the return value be stored in a stack variable called 'locked' is > further confusion. > > More generally, we are doing a lock of rather finnicky things during the > acquisition of a state in which readers are excluded and moving out of this > state, including tracking whether we are detached or not or whether an > error occurred. > > We are implementing logic in __vma_enter_exclusive_locked() that > effectively acts as if 'if one caller calls us do X, if another then do Y', > which is very confusing from a control flow perspective. > > Introducing the shared helper object state helps us avoid this, as we can > now handle the 'an error arose but we're detached' condition correctly in > both callers - a warning if not detaching, and treating the situation as if > no error arose in the case of a VMA detaching. > > This also acts to help document what's going on and allows us to add some > more logical debug asserts. > > Also update vma_mark_detached() to add a guard clause for the likely > 'already detached' state (given we hold the mmap write lock), and add a > comment about ephemeral VMA read lock reference count increments to clarify > why we are entering/exiting an exclusive locked state here. > > No functional change intended. > > Signed-off-by: Lorenzo Stoakes > --- > mm/mmap_lock.c | 144 +++++++++++++++++++++++++++++++------------------ > 1 file changed, 91 insertions(+), 53 deletions(-) > > diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c > index f73221174a8b..75166a43ffa4 100644 > --- a/mm/mmap_lock.c > +++ b/mm/mmap_lock.c > @@ -46,20 +46,40 @@ EXPORT_SYMBOL(__mmap_lock_do_trace_released); > #ifdef CONFIG_MMU > #ifdef CONFIG_PER_VMA_LOCK > > +/* State shared across __vma_[enter, exit]_exclusive_locked(). */ > +struct vma_exclude_readers_state { > + /* Input parameters. */ > + struct vm_area_struct *vma; > + int state; /* TASK_KILLABLE or TASK_UNINTERRUPTIBLE. */ > + bool detaching; > + > + bool detached; > + bool exclusive; /* Are we exclusively locked? */ > +}; > + > /* > * Now that all readers have been evicted, mark the VMA as being out of the > * 'exclude readers' state. > * > * Returns true if the VMA is now detached, otherwise false. > */ > -static bool __must_check __vma_exit_exclusive_locked(struct vm_area_struct *vma) > +static void __vma_exit_exclusive_locked(struct vma_exclude_readers_state *ves) > { > - bool detached; > + struct vm_area_struct *vma = ves->vma; > + > + VM_WARN_ON_ONCE(ves->detached); > + VM_WARN_ON_ONCE(!ves->exclusive); I think this will triger when called on wait failure from __vma_enter_exclusive_locked(). Given the other things Suren raised about the field, I wonder if it's worth keeping it? > - detached = refcount_sub_and_test(VM_REFCNT_EXCLUDE_READERS_FLAG, > - &vma->vm_refcnt); > + ves->detached = refcount_sub_and_test(VM_REFCNT_EXCLUDE_READERS_FLAG, > + &vma->vm_refcnt); > __vma_lockdep_release_exclusive(vma); > - return detached; > +} > + > @@ -151,7 +176,12 @@ EXPORT_SYMBOL_GPL(__vma_start_write); > > void vma_mark_detached(struct vm_area_struct *vma) > { > - bool detached; > + struct vma_exclude_readers_state ves = { > + .vma = vma, > + .state = TASK_UNINTERRUPTIBLE, > + .detaching = true, > + }; > + int err; > > vma_assert_write_locked(vma); > vma_assert_attached(vma); > @@ -160,18 +190,26 @@ void vma_mark_detached(struct vm_area_struct *vma) > * See the comment describing the vm_area_struct->vm_refcnt field for > * details of possible refcnt values. > */ > - detached = __vma_refcount_put(vma, NULL); > - if (unlikely(!detached)) { > - /* Wait until vma is detached with no readers. */ > - if (__vma_enter_exclusive_locked(vma, true, TASK_UNINTERRUPTIBLE)) { > - /* > - * Once this is complete, no readers can increment the > - * reference count, and the VMA is marked detached. > - */ > - detached = __vma_exit_exclusive_locked(vma); > - WARN_ON_ONCE(!detached); > - } > + if (likely(__vma_refcount_put(vma, NULL))) > + return; Seems to me it would be worthwhile splitting this function to an static-inline-in-header vma_mark_detached() that does only the asserts and __vma_refcount_put(), and keeping the function here as __vma_mark_detached() (or maybe differently named since the detaching kinda already happened with the refcount put... __vma_mark_detached_finish()?) handling the rare case __vma_refcount_put() returns false. > + > + /* > + * Wait until the VMA is detached with no readers. Since we hold the VMA > + * write lock, the only read locks that might be present are those from > + * threads trying to acquire the read lock and incrementing the > + * reference count before realising the write lock is held and > + * decrementing it. > + */ > + err = __vma_enter_exclusive_locked(&ves); > + if (!err && !ves.detached) { > + /* > + * Once this is complete, no readers can increment the > + * reference count, and the VMA is marked detached. > + */ > + __vma_exit_exclusive_locked(&ves); > } > + /* If an error arose but we were detached anyway, we don't care. */ > + WARN_ON_ONCE(!ves.detached); > } > > /* > -- > 2.52.0