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a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784647157; bh=NcQHotnRc1YltaL0d91NNFzXPnSIvU2fa84uq1Kqf80=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=cxXu/JglamTfxZku20/hvwti8kyhkeyVDle5r6Gs3gp+SkJnbyhtcGv/c18jtV/cw 7gSgQmx19CA7r6u805JO5I8cZ3xBRPmkyR/TaKUjbH5t+Gm9nKMDsP+xj1jwAvi8dW owS3DMGZ59TLPI3kIqnJaRSYe1gjvqDyjXDBUA50bTQChhd5HKjqPm/P51z59oZg6/ DbaXmu+vpmoqXHG96HzQT4+JM+fRaZglAuD4Kv4OKXvbdx+2jLMN67pqWyuu0y85qm dUyOAjy7uV0NxLmjNVxyy+EME6N/YRuWZvdtvW6P4ZjgS0iJJkfPgii0UTXWZb8cqx way087G+3sExg== Message-ID: <0b9be5b3-93aa-407f-b83d-409bec4b55e1@kernel.org> Date: Tue, 21 Jul 2026 17:19:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v9 1/8] mm/hmm: move page fault handling out of walk callbacks To: Stanislav Kinsburskii , airlied@gmail.com, akhilesh@ee.iitb.ac.in, akpm@linux-foundation.org, corbet@lwn.net, dakr@kernel.org, decui@microsoft.com, haiyangz@microsoft.com, jgg@ziepe.ca, kees@kernel.org, kys@microsoft.com, leon@kernel.org, liam@infradead.org, lizhi.hou@amd.com, ljs@kernel.org, longli@microsoft.com, lyude@redhat.com, maarten.lankhorst@linux.intel.com, mamin506@gmail.com, mhocko@suse.com, mripard@kernel.org, nouveau@lists.freedesktop.org, ogabbay@kernel.org, oleg@redhat.com, rppt@kernel.org, shuah@kernel.org, simona@ffwll.ch, skhan@linuxfoundation.org, surenb@google.com, tzimmermann@suse.de, vbabka@kernel.org, wei.liu@kernel.org References: <178413903133.1155966.3904063656020521607.stgit@skinsburskii> <178413935809.1155966.6398279131483632373.stgit@skinsburskii> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Message-ID-Hash: U4VXVBTTYEVDCQ5ZPJKCJR7CQHDP44MM X-Message-ID-Hash: U4VXVBTTYEVDCQ5ZPJKCJR7CQHDP44MM X-MailFrom: david@kernel.org X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; emergency; loop; banned-address; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header CC: dri-devel@lists.freedesktop.org, intel-xe@lists.freedesktop.org, linux-mm@kvack.org, linux-doc@vger.kernel.org, linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-rdma@vger.kernel.org X-Mailman-Version: 3.3.8 Precedence: list List-Id: Nouveau development list Archived-At: Archived-At: List-Archive: List-Archive: List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: On 7/15/26 20:15, Stanislav Kinsburskii wrote: > hmm_range_fault() currently triggers page faults from inside the page-table > walk callbacks: hmm_vma_walk_pmd(), hmm_vma_walk_pud(), > hmm_vma_walk_hugetlb_entry() and the pte-level helper all call > hmm_vma_fault(), which in turn calls handle_mm_fault() while the walker > still holds nested locks. The pte spinlock is dropped explicitly by each > caller, and the hugetlb path manually drops and retakes > hugetlb_vma_lock_read around the fault to dodge a deadlock against the walk > framework's unconditional unlock. > > This layering does not extend cleanly to fault handlers that may release > mmap_lock (VM_FAULT_RETRY, VM_FAULT_COMPLETED). If the lock is dropped > while walk_page_range() is mid-traversal, the VMA can be freed before the > walk framework's matching hugetlb_vma_unlock_read(), turning that unlock > into a use-after-free. > > Split the responsibilities the way get_user_pages() does. Walk callbacks > become inspect-only: when they detect a range that needs to be faulted in, > they record it in struct hmm_vma_walk and return a private sentinel > (HMM_FAULT_PENDING). The outer loop in hmm_range_fault() then drops out of > walk_page_range(), invokes a new helper hmm_do_fault() that calls > handle_mm_fault() with only mmap_lock held, and restarts the walk so the > now-present entries are collected into hmm_pfns. > > No functional change for existing callers. As a side effect the hugetlb > callback no longer needs the hugetlb_vma_{un}lock_read dance, and every > fault-path exit from the callbacks now releases the pte spinlock on a > single, common path. This refactor is also a precursor for adding an > unlockable variant of hmm_range_fault() in a follow-up patch. > > Reviewed-by: Jason Gunthorpe > Signed-off-by: Stanislav Kinsburskii > --- [...] > > pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT); > @@ -564,21 +561,8 @@ static int hmm_vma_walk_hugetlb_entry(pte_t *pte, unsigned long hmask, > required_fault = > hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, cpu_flags); > if (required_fault) { > - int ret; > - > spin_unlock(ptl); > - hugetlb_vma_unlock_read(vma); > - /* > - * Avoid deadlock: drop the vma lock before calling > - * hmm_vma_fault(), which will itself potentially take and > - * drop the vma lock. This is also correct from a > - * protection point of view, because there is no further > - * use here of either pte or ptl after dropping the vma > - * lock. > - */ > - ret = hmm_vma_fault(addr, end, required_fault, walk); > - hugetlb_vma_lock_read(vma); > - return ret; > + return hmm_record_fault(addr, end, required_fault, walk); Yes, that looks much better, as discussed. The downside is another vma_lookup() in hmm_do_fault(). > } > > pfn = pte_pfn(entry) + ((start & ~hmask) >> PAGE_SHIFT); > @@ -637,6 +621,44 @@ static const struct mm_walk_ops hmm_walk_ops = { > .walk_lock = PGWALK_RDLOCK, > }; > > +/* > + * hmm_do_fault - fault in a range recorded by a walk callback > + * > + * Called from the outer loop in hmm_range_fault() after a callback > + * returned HMM_FAULT_PENDING. At this point we hold only mmap_lock; > + * the page-table spinlock and any hugetlb_vma_lock acquired by the walk > + * framework have already been released by the unwind. > + * > + * Returns -EBUSY on success (all pages faulted, caller should re-walk). > + * Returns a negative errno on failure. > + */ > +static int hmm_do_fault(struct mm_struct *mm, > + struct hmm_vma_walk *hmm_vma_walk) > +{ > + unsigned long addr = hmm_vma_walk->last; > + unsigned long end = hmm_vma_walk->end; > + unsigned int required_fault = hmm_vma_walk->required_fault; end and required_fault could be const. Reviewed-by: David Hildenbrand (Arm) -- Cheers, David