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Howlett" , akpm@linux-foundation.org, peterx@redhat.com, jannh@google.com, hannes@cmpxchg.org, mhocko@kernel.org, paulmck@kernel.org, shuah@kernel.org, adobriyan@gmail.com, brauner@kernel.org, josef@toxicpanda.com, yebin10@huawei.com, linux@weissschuh.net, willy@infradead.org, osalvador@suse.de, andrii@kernel.org, ryan.roberts@arm.com, christophe.leroy@csgroup.eu, tjmercier@google.com, kaleshsingh@google.com, aha310510@gmail.com, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org Subject: Re: [PATCH v6 7/8] fs/proc/task_mmu: read proc/pid/maps under per-vma lock Message-ID: <7d878566-f445-4fc2-9d04-eb8b38024c9b@lucifer.local> References: <5ec10376-6a5f-4a94-9880-e59f1b6d425f@suse.cz> <19d46c33-bd5e-41d1-88ad-3db071fa1bed@lucifer.local> <0b8617c1-a150-426f-8fa6-9ab3b5bcfa1e@redhat.com> <8026c455-6237-47e3-98af-e3acb90dba25@suse.cz> <5f8d3100-a0dd-4da3-8797-f097e063ca97@lucifer.local> 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List-Unsubscribe: On Tue, Jul 15, 2025 at 06:10:16PM +0100, Lorenzo Stoakes wrote: > > For PROCMAP_QUERY, we need priv->mm, but the newly added locked_vma > > and locked_vma don't need to be persisted between ioctl calls. So we > > can just add those two fields into a small struct, and for seq_file > > case have it in priv, but for PROCMAP_QUERY just have it on the stack. > > The code can be written to accept this struct to maintain the state, > > which for PROCMAP_QUERY ioctl will be very short-lived on the stack > > one. > > > > Would that work? > > Yeah that's a great idea actually, the stack would obviously give us the > per-query invocation thing. Nice! > > I am kicking myself because I jokingly suggested (off-list) that a helper > struct would be the answer to everything (I do love them) and of > course... here we are :P Hm but actually we'd have to invert things I think, what I mean is - since these fields can be updated at any time by racing threads, we can't have _anything_ in the priv struct that is mutable. So instead we should do something like: struct proc_maps_state { const struct proc_maps_private *priv; bool mmap_locked; struct vm_area_struct *locked_vma; struct vma_iterator iter; loff_t last_pos; }; static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned long arg) { struct seq_file *seq = file->private_data; struct proc_maps_private *priv = seq->private; struct proc_maps_state state = { .priv = priv, }; switch (cmd) { case PROCMAP_QUERY: return do_procmap_query(state, (void __user *)arg); default: return -ENOIOCTLCMD; } } And then we have a stack-based thing with the bits that change and a read-only pointer to the bits that must remain static. And we can enforce that with const... We'd have to move the VMI and last_pos out too to make it const. Anyway the general idea should work!