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AJvYcCVC6BG89ggY/bHFufhmxG2VEck7rxdOSx+zgTNeCXqUlt4QNmJY2tUlJDhoqg4cTzMWmzW61uVzeQ==@kvack.org X-Gm-Message-State: AOJu0Yy6PS1s7YfT5SKzXDiOSf7eIQ6PD5gePo2br/IH6/hdjlSqYHrz 6HIhWdRrBDJKsZFBw/hrunatlKnQTu4XkpFRlwJL9ysbB8/mQ/IIpZ2eZKfup51dcr24CxZ73dj AOkzYnCwVPHjnrC2tZuUPNcVqI9zzkwZVD6JQpSFc X-Gm-Gg: ASbGnctbCeDheNF2XMOxOCVIOv45pbOvGBHV3TNjplJYmLqqCp4bq+GsmV9IRtgJ+mM nrlqIHNeCm4QM/zBPB8P2n13YqutlZIubFTukLgIuZrMBUnAB+89+UaFex/TAzygyoffs9PUfsb da5KnDjfWaMYcNSTkUaBg/lWY1hMHsKcuj7wqUQ+dWqEI3FJhgsrQn3/lqfZvBFqcFmkW2V9XZo 6jeCzccMx9bDZFYj6yn+Uzs+RoXeMGxDqxqxA== X-Google-Smtp-Source: AGHT+IFP6oWJuEB+t8FqlHu4nrpKTJN8ggoRVw1wyBCG9QSsTFoeUzkMrBL9SWqFRe5Hi7JIvL+AIijCHTWQWxdq+8E= X-Received: by 2002:a05:622a:93:b0:4a9:e34a:58a1 with SMTP id d75a77b69052e-4af0088e8c4mr4626841cf.21.1754062257422; Fri, 01 Aug 2025 08:30:57 -0700 (PDT) MIME-Version: 1.0 References: <20250731151919.212829-1-surenb@google.com> <20250731151919.212829-2-surenb@google.com> In-Reply-To: From: Suren Baghdasaryan Date: Fri, 1 Aug 2025 08:30:45 -0700 X-Gm-Features: Ac12FXydoPHt9DYmIUpoqeSH4RwHYO1K7tIS4LC4But0n7b4ZrC_qkGPggvaHZo Message-ID: Subject: Re: [PATCH v2 2/2] mm: change vma_start_read() to drop RCU lock on failure To: Vlastimil Babka Cc: akpm@linux-foundation.org, jannh@google.com, Liam.Howlett@oracle.com, lorenzo.stoakes@oracle.com, pfalcato@suse.de, linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: E4C4940017 X-Stat-Signature: kwnkfp1a7k9cgzkwmku8jo4fs5one4be X-HE-Tag: 1754062258-763550 X-HE-Meta: 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 N55u+5wF 3ljL8i8k8w+Ll9DgoP0rn3u9veId8absLLoFKYrAoytme81SDBa7KV5sjIDGwlRQu6UHxCl3yM7I5YAd7J+N5cPVHqE0dj2T9fTL1hMJDGGiHmLNhjJAvbu4RWE+wlVnZw0xf0uMsM7gdgHe3fC0UxZCIQnYrAzssf8G62zxTqOW3NKIywUFf+8uJPhnzLKr65YWklAiK/wmHthpfjcmxXs9u/w3U4yhe1pLPQ+e+8I6EHOm0II+DT5yXP8sACqa8l3DWsjPFhhdzvYTkMOZNWP1+WFtCwvzPg9U44Xe1yW0mt3AxEAcbX3+J9qu4cft+HWDq5TQK/vPvy4Q= 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 Fri, Aug 1, 2025 at 2:09=E2=80=AFAM Vlastimil Babka wro= te: > > On 7/31/25 17:19, Suren Baghdasaryan wrote: > > vma_start_read() can drop and reacquire RCU lock in certain failure > > cases. It's not apparent that the RCU session started by the caller of > > this function might be interrupted when vma_start_read() fails to lock > > the vma. This might become a source of subtle bugs and to prevent that > > we change the locking rules for vma_start_read() to drop RCU read lock > > upon failure. This way it's more obvious that RCU-protected objects are > > unsafe after vma locking fails. > > > > Suggested-by: Vlastimil Babka > > IIRC you considered it yourself, I just convinced you to try :) > > > Signed-off-by: Suren Baghdasaryan > > I thought we didn't need the drop rcu lock for -EAGAIN, but that would ju= st > made it more complex for little gain, so this looks good to me. Yes, we technically don't but this way it's simpler to explain (drop RCU on any failure). > > Reviewed-by: Vlastimil Babka > > Nit: > > > @@ -223,11 +227,13 @@ struct vm_area_struct *lock_vma_under_rcu(struct = mm_struct *mm, > > MA_STATE(mas, &mm->mm_mt, address, address); > > struct vm_area_struct *vma; > > > > - rcu_read_lock(); > > retry: > > + rcu_read_lock(); > > vma =3D mas_walk(&mas); > > - if (!vma) > > + if (!vma) { > > + rcu_read_unlock(); > > goto inval; > > + } > > > > vma =3D vma_start_read(mm, vma); > > if (IS_ERR_OR_NULL(vma)) { > > @@ -241,6 +247,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm= _struct *mm, > > /* Failed to lock the VMA */ > > goto inval; > > } > > + > > + rcu_read_unlock(); > > Would it make sense to put this under the comment below? > > > + > > /* > > * At this point, we have a stable reference to a VMA: The VMA is > > * locked and we know it hasn't already been isolated. > > Give it continues like this: > > * From here on, we can access the VMA without worrying about whi= ch > > * fields are accessible for RCU readers. Yep, will change. Thanks! > > > @@ -249,16 +258,14 @@ struct vm_area_struct *lock_vma_under_rcu(struct = mm_struct *mm, > > */ > > > > /* Check if the vma we locked is the right one. */ > > - if (unlikely(address < vma->vm_start || address >=3D vma->vm_end)= ) > > - goto inval_end_read; > > + if (unlikely(address < vma->vm_start || address >=3D vma->vm_end)= ) { > > + vma_end_read(vma); > > + goto inval; > > + } > > > > - rcu_read_unlock(); > > return vma; > > > > -inval_end_read: > > - vma_end_read(vma); > > inval: > > - rcu_read_unlock(); > > count_vm_vma_lock_event(VMA_LOCK_ABORT); > > return NULL; > > }