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Mon, 12 Oct 2020 15:08:23 -0700 (PDT) Subject: Re: Loophole in async page I/O To: Matthew Wilcox , io-uring@vger.kernel.org Cc: Johannes Weiner References: <20201012211355.GC20115@casper.infradead.org> From: Jens Axboe Message-ID: <14d97ab3-edf7-c72a-51eb-d335e2768b65@kernel.dk> Date: Mon, 12 Oct 2020 16:08:22 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 In-Reply-To: <20201012211355.GC20115@casper.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 10/12/20 3:13 PM, Matthew Wilcox wrote: > This one's pretty unlikely, but there's a case in buffered reads where > an IOCB_WAITQ read can end up sleeping. > > generic_file_buffered_read(): > page = find_get_page(mapping, index); > ... > if (!PageUptodate(page)) { > ... > if (iocb->ki_flags & IOCB_WAITQ) { > ... > error = wait_on_page_locked_async(page, > iocb->ki_waitq); > wait_on_page_locked_async(): > if (!PageLocked(page)) > return 0; > (back to generic_file_buffered_read): > if (!mapping->a_ops->is_partially_uptodate(page, > offset, iter->count)) > goto page_not_up_to_date_locked; > > page_not_up_to_date_locked: > if (iocb->ki_flags & (IOCB_NOIO | IOCB_NOWAIT)) { > unlock_page(page); > put_page(page); > goto would_block; > } > ... > error = mapping->a_ops->readpage(filp, page); > (will unlock page on I/O completion) > if (!PageUptodate(page)) { > error = lock_page_killable(page); > > So if we have IOCB_WAITQ set but IOCB_NOWAIT clear, we'll call ->readpage() > and wait for the I/O to complete. I can't quite figure out if this is > intentional -- I think not; if I understand the semantics right, we > should be returning -EIOCBQUEUED and punting to an I/O thread to > kick off the I/O and wait. > > I think the right fix is to return -EIOCBQUEUED from > wait_on_page_locked_async() if the page isn't locked. ie this: > > @@ -1258,7 +1258,7 @@ static int wait_on_page_locked_async(struct page *page, > struct wait_page_queue *wait) > { > if (!PageLocked(page)) > - return 0; > + return -EIOCBQUEUED; > return __wait_on_page_locked_async(compound_head(page), wait, false); > } > > But as I said, I'm not sure what the semantics are supposed to be. If NOWAIT isn't set, then the issue attempt is from the helper thread already, and IOCB_WAITQ shouldn't be set either (the latter doesn't matter for this discussion). So it's totally fine and expected to block at that point. Hmm actually, I believe that: commit c8d317aa1887b40b188ec3aaa6e9e524333caed1 Author: Hao Xu Date: Tue Sep 29 20:00:45 2020 +0800 io_uring: fix async buffered reads when readahead is disabled maybe messed up that case, so we could block off the retry-path. I'll take a closer look, looks like that can be the case if read-ahead is disabled. In general, we can only return -EIOCBQUEUED if the IO has been started or is in progress already. That means we can safely rely on being told when it's unlocked/done. If we need to block, we should be returning -EAGAIN, which would punt to a worker thread. -- Jens Axboe