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charset="UTF-8" X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: A2C106001982 X-Stat-Signature: t6kf6oi9acbti98pynpr1o3jcbn6ajuy Authentication-Results: imf10.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=MHF8oRkD; spf=pass (imf10.hostedemail.com: domain of surenb@google.com designates 209.85.219.171 as permitted sender) smtp.mailfrom=surenb@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1638998689-161148 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: On Wed, Dec 8, 2021 at 11:22 AM Matthew Wilcox wrote: > > On Wed, Dec 08, 2021 at 11:13:42AM -0800, Suren Baghdasaryan wrote: > > On Wed, Dec 8, 2021 at 8:50 AM Suren Baghdasaryan wrote: > > > > > > On Wed, Dec 8, 2021 at 8:05 AM Matthew Wilcox wrote: > > > > > > > > On Wed, Dec 08, 2021 at 04:51:58PM +0100, Michal Hocko wrote: > > > > > On Wed 08-12-21 15:01:24, Matthew Wilcox wrote: > > > > > > On Tue, Dec 07, 2021 at 03:08:19PM -0800, Suren Baghdasaryan wrote: > > > > > > > > > /** > > > > > > > > > * @close: Called when the VMA is being removed from the MM. > > > > > > > > > * Context: Caller holds mmap_lock. > > > > > > > > > > > > > > BTW, is the caller always required to hold mmap_lock for write or it > > > > > > > *might* hold it? > > > > > > > > > > > > __do_munmap() might hold it for read, thanks to: > > > > > > > > > > > > if (downgrade) > > > > > > mmap_write_downgrade(mm); > > > > > > > > > > > > Should probably say: > > > > > > > > > > > > * Context: User context. May sleep. Caller holds mmap_lock. > > > > > > > > > > > > I don't think we should burden the implementor of the vm_ops with the > > > > > > knowledge that the VM chooses to not hold the mmap_lock under certain > > > > > > circumstances when it doesn't matter whether it's holding the mmap_lock > > > > > > or not. > > > > > > > > > > If we document it like that some code might depend on that lock to be > > > > > held. I think we only want to document that the holder itself is not > > > > > allowed to take mmap sem or a depending lock. > > > > > > > > The only place where we're not currently holding the mmap_lock is at > > > > task exit, where the mmap_lock is effectively held because nobody else > > > > can modify the task's mm. Besides, Suren is changing that in this patch > > > > series anyway, so it will be always true. > > > > > > Ok, I'll make it a separate patch after the patch that changes > > > exit_mmap and this statement will become always true. Sounds > > > reasonable? > > > > Actually, while today vma_ops->close is called with mmap_lock held, I > > believe we want this comment to reflect the restrictions on the > > callback itself, not on the user. IOW, we want to say that the > > callback should not take mmap_lock while the caller might or might not > > hold it. If so, I think *might* would make more sense here, like this: > > > > * Context: User context. May sleep. Caller might hold mmap_lock. > > > > WDYT? > > We're documenting the contract between the caller and the callee. > That implies responsibilities on both sides. For example, we're > placing requirements on the caller that they're not going to tear > down the VMA in interrupt context. So I preferred what previous-Suren > said to current-Suren, "this statement will become always true". > previous-Suren posted v4 at https://lore.kernel.org/all/20211208212211.2860249-1-surenb@google.com Thanks!