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Tue, 20 Jan 2026 13:53:30 +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 iGHtL9qIb2kHPAAAD6G6ig (envelope-from ); Tue, 20 Jan 2026 13:53:30 +0000 Message-ID: <30f843d9-03cf-4c7c-8a29-8e11b12e47e4@suse.cz> Date: Tue, 20 Jan 2026 14:53:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 1/2] mm/vma: use lockdep where we can, reduce duplication 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: 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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: AE2CB20008 X-Stat-Signature: o9bjkdpidezgfchncoyabs1sinpa96jg X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1768917212-873719 X-HE-Meta: U2FsdGVkX1/YuBBiNrh3it7HXi6y/8c/WQorqfb+wIBiNqZg8jhnikdW+M7UL8URDAjTxpbdC+gsGo9fzy00EN6jMQYyUW9nMRp7lurPC3M8w4S8/8AOaCkeDGDG3Bt4KJnZ4v8plRNC8BL5zmU/EcRd1O+XFh1KbVyac0/q8PL59OoI+ls+kLWexlJ//sarxyNwPkTKFMvFxJR/VC9UQUg9CcSwVCjwmx94pz2HvaFU3XQdPl3KgC10+BSNkw9UkJkhbceNxy5DjyNMFItxdzGaYA7C5bQ4Gq6CQWPCI9j4R11mF8CdfeJNnTdm7m1zgEG0KHh6eVkHecFtBnybzYhIG/L/KIqDRagU7yy5kQgkICgemN2LPG8+aQOZygHxybD4+uQj2GCL/JKtLHU/fiZOH06YfJrxs2HRQQ0nqTJCwK7nsygq422qmjOSyAsc+qLuSDp+DCLTdoOMMZaWHSsDfYeWeJl77sl1h0pbVCaY+fe6jljQRqA6VPSsTUAm17ccjItaYeeSo7yEBx3Z/4G/ZoKgfvUh7VP0AxkIOXjE0kB+uFRDfdID9AUw2du+wcGtrn/9DH7f/YQeVqjw9G74GjdJu5Zirt9/bhXB6lqwz6bb6KF1sujoYmVJ30PWdo1HgSiGwr1P4yT5JJPDY9Ud7mcTzctNq4aP4kg0EW1NzggNu9b3RgpMHGm5LmaMeqlNWdpZs8tl8I6FFRGmSRPxlh6o6jA7TzUyLbW9eFhXzPmdi6F21wcmNUW+WGylfLgInomqVo1sBwf5MPsrM9LPyovSCXFBBDsbkV310OBJpCHb9ifu6BiFpYqh4kZvg5BKiU0w8CY9cikJxR4GQqQCtlDVRYjxm7OJ19Vc6iyhap7n2u9s+p2Zwv7vULhk0H99pNw3GtJwGv8sv0HSm4u6vrVCNAK6FNzZtGs1UL6vRPh8saiBu5b/U1sCsainHRondrhdcgQsV58W8tN F8+x+ENi 65Cvk1qk6kOp6H3i44BbbHQtDzlc7LCeLbtXfo2b76ooDk+r9N3XEBH5fXup1rFoVNkwH7nWsYhsvR/JGiDenhHJvjcxqzjsarN4oGzCfkrLxZCbKx8mavIfknp8PyZSDWZ1iupPNnbZ8b2kKnUu4sO7KV5ofv1PdBk5fvIalS0cuGG7R4tGplrYZncy3CEZsgCD9ItiXPP3dCDN48cQhSnWtAdy+r8NNgSf9nkdm4Ux511MYdu5Dv/OmOdjEzZRxuAeKh0M+RYAJ9PAIw2iTh1FEJyVEuOFRci0hJM+zGnescjMX0MdH9WIV6HkqSajcEaHA2JEybrWE113AVnUmjz9d8RKTLfMn4U13QWqX9lXUlHRvjfIMp7adHbxQ/riQWDZB 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 1/19/26 21:59, Lorenzo Stoakes wrote: > We introduce vma_is_read_locked(), which must deal with the case in which > VMA write lock sets refcnt to VMA_LOCK_OFFSET or VMA_LOCK_OFFSET + > 1. Luckily is_vma_writer_only() already exists which we can use to check > this. So I think there's a bit of a caveat in that - is_vma_writer_only() may be a false positive if there is a temporary reader of a detached vma (per comments in vma_mark_detached() and vma_mark_detached()) - hence vma_is_read_locked() may be a false negative - hence vma_assert_locked() might assume wrongly that we should not assert being a reader, so we vma_assert_write_locked() instead, and fail Howevever the above should mean it could be only us who is the temporary reader. And we are not going to use vma_assert_locked() during the temporary reader part (in vma_start_read()). So it's probably fine, but maybe worth some comments to prevent people getting suspicious and reconstructing this? But I think perhaps also vma_assert_locked() could, with lockdep enabled (similarly to vma_assert_stabilised() in patch 2), use the "lock_is_held(&vma->vmlock_dep_map)" condition (without immediately asserting it) for the primary reader vs writer decision, and not rely on vma_is_read_locked()? Because lockdep has the precise information. It would likely make things more ugly, or require more refactoring, but hopefully worthwhile? > We then try to make vma_assert_locked() use lockdep as far as we can. > > Unfortunately the VMA lock implementation does not even try to track VMA > write locks using lockdep, so we cannot track the lock this way. > > This is less egregious than it might seem as VMA write locks are predicated > on mmap write locks, which we do lockdep assert. > > vma_assert_write_locked() already asserts the mmap write lock is taken so > we get that checked implicitly. > However for read locks we do indeed use lockdup, via rwsem_acquire_read() > called in vma_start_read() and rwsem_release_read() called in > vma_refcount_put() called in turn by vma_end_read(). > > Therefore we perform a lockdep assertion if the VMA is known to be > read-locked. > > If it is write-locked, we assert the mmap lock instead, with a lockdep > check if lockdep is enabled. > > If lockdep is not enabled, we just check that locks are in place. > > Signed-off-by: Lorenzo Stoakes > --- > include/linux/mmap_lock.h | 34 ++++++++++++++++++++++++++++++---- > 1 file changed, 30 insertions(+), 4 deletions(-) > > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h > index b50416fbba20..6979222882f1 100644 > --- a/include/linux/mmap_lock.h > +++ b/include/linux/mmap_lock.h > @@ -236,6 +236,13 @@ int vma_start_write_killable(struct vm_area_struct *vma) > return __vma_start_write(vma, mm_lock_seq, TASK_KILLABLE); > } > > +static inline bool vma_is_read_locked(const struct vm_area_struct *vma) > +{ > + const unsigned int refcnt = refcount_read(&vma->vm_refcnt); > + > + return refcnt > 1 && !is_vma_writer_only(refcnt); > +} > + > static inline void vma_assert_write_locked(struct vm_area_struct *vma) > { > unsigned int mm_lock_seq; > @@ -243,12 +250,31 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma) > VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma); > } > > +/** > + * vma_assert_locked() - Assert that @vma is either read or write locked and > + * that we have ownership of that lock (if lockdep is enabled). > + * @vma: The VMA we assert. > + * > + * If lockdep is enabled, we ensure ownership of the VMA lock. Otherwise we > + * assert that we are VMA write-locked, which implicitly asserts that we hold > + * the mmap write lock. > + */ > static inline void vma_assert_locked(struct vm_area_struct *vma) > { > - unsigned int mm_lock_seq; > - > - VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 && > - !__is_vma_write_locked(vma, &mm_lock_seq), vma); > + /* > + * VMA locks currently only utilise lockdep for read locks, as > + * vma_end_write_all() releases an unknown number of VMA write locks and > + * we don't currently walk the maple tree to identify which locks are > + * released even under CONFIG_LOCKDEP. > + * > + * However, VMA write locks are predicated on an mmap write lock, which > + * we DO track under lockdep, and which vma_assert_write_locked() > + * asserts. > + */ > + if (vma_is_read_locked(vma)) > + lockdep_assert(lock_is_held(&vma->vmlock_dep_map)); > + else > + vma_assert_write_locked(vma); > } > > static inline bool vma_is_attached(struct vm_area_struct *vma)