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[98.225.44.182]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2cf8f38757esm40198575ad.75.2026.07.23.15.22.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 23 Jul 2026 15:22:28 -0700 (PDT) Date: Thu, 23 Jul 2026 15:22:25 -0700 From: Stanislav Kinsburskii To: Andrew Morton Cc: Jason Gunthorpe , Leon Romanovsky , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Jonathan Corbet , Shuah Khan , Shuah Khan , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Long Li , Lyude Paul , Danilo Krummrich , Maarten Lankhorst , Maxime Ripard , Thomas Zimmermann , David Airlie , Simona Vetter , Min Ma , Lizhi Hou , Oded Gabbay , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-hyperv@vger.kernel.org, dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org, linux-rdma@vger.kernel.org Subject: Re: [PATCH v11 0/8] mm/hmm: Add mmap lock-drop support for userfaultfd-backed mappings Message-ID: References: <20260723-hmm-v10-v11-0-c55b003a4b61@gmail.com> <20260723142242.5d3f87208d47819d873ae458@linux-foundation.org> Precedence: bulk X-Mailing-List: linux-hyperv@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260723142242.5d3f87208d47819d873ae458@linux-foundation.org> On Thu, Jul 23, 2026 at 02:22:42PM -0700, Andrew Morton wrote: > On Thu, 23 Jul 2026 10:36:32 -0700 Stanislav Kinsburskii wrote: > > > This series extends the HMM framework to support userfaultfd-backed memory > > by allowing the mmap read lock to be dropped during hmm_range_fault(). > > > > Thanks, I've updated mm.git to this version. > > AI review suggests there may be some problems. Sorry, I don't recall if > these were considered in previous versions of the patchset: > > https://sashiko.dev/#/patchset/20260723-hmm-v10-v11-0-c55b003a4b61@gmail.com > These look like new findings, perhaps from a newer model or review prompt. I do not think they apply here. The unlockable path sets FAULT_FLAG_ALLOW_RETRY only when HMM provides a non-NULL lock state pointer. The legacy hmm_range_fault() path passes NULL and therefore does not opt in to lock-dropping faults. Fault handlers are not supposed to return VM_FAULT_RETRY/VM_FAULT_COMPLETED with the mmap lock dropped unless FAULT_FLAG_ALLOW_RETRY allows that, so the NULL locked state is not expected to be dereferenced on the legacy path. For the FAULT_FLAG_TRIED concern, HMM is not trying to emulate GUP's exact single-address retry loop. If a fault drops mmap_lock, HMM has to restart the range walk because the VMA/page table state may have changed. Resetting the timeout on that path is intentional: a lock-dropping fault made progress, and the timeout is meant to bound mmu-notifier retry churn rather than the time spent servicing faults. Also, the newly converted HMM users are not using this path for ordinary file-backed page-cache population, and process-context callers can still be interrupted by fatal signals. All the existent kernel thread callers are capped by a timeout. So I do not think this requires a code change. Thanks, Stanislav > > > > Changes in v11: > > - Reject unstable address spaces in hmm_range_fault_unlocked_timeout() > > after taking mmap_lock and before walking page tables. > > - Compute the remaining HMM timeout budget before the time_after_eq() > > check in drm_gpusvm_get_pages() to make sure it can't result in zero > > and lead to infinite HMM range faulting loop. > > Here's how v11 altered mm.git: > > > drivers/gpu/drm/drm_gpusvm.c | 4 ++-- > mm/hmm.c | 6 ++++++ > 2 files changed, 8 insertions(+), 2 deletions(-) > > --- a/drivers/gpu/drm/drm_gpusvm.c~b > +++ a/drivers/gpu/drm/drm_gpusvm.c > @@ -1422,11 +1422,11 @@ int drm_gpusvm_get_pages(struct drm_gpus > struct dma_iova_state *state = &svm_pages->state; > > retry: > + remaining = timeout - jiffies; > + > if (time_after_eq(jiffies, timeout)) > return -EBUSY; > > - remaining = timeout - jiffies; > - > hmm_range.notifier_seq = mmu_interval_read_begin(notifier); > if (drm_gpusvm_pages_valid_unlocked(gpusvm, svm_pages)) > goto set_seqno; > --- a/mm/hmm.c~b > +++ a/mm/hmm.c > @@ -17,6 +17,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -806,6 +807,11 @@ int hmm_range_fault_unlocked_timeout(str > if (ret) > return ret; > > + if (check_stable_address_space(mm)) { > + mmap_read_unlock(mm); > + return -EFAULT; > + } > + > if (timeout && time_after(jiffies, deadline)) { > mmap_read_unlock(mm); > return -EBUSY; > _ >