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([2605:e000:100e:8c61:8568:4ec4:ebd3:32d1]) by smtp.gmail.com with ESMTPSA id go1sm10608240pjb.26.2020.05.24.09.30.43 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 24 May 2020 09:30:43 -0700 (PDT) Subject: Re: [PATCH 05/12] mm: support async buffered reads in generic_file_buffered_read() To: Trond Myklebust , "io-uring@vger.kernel.org" Cc: "linux-mm@kvack.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" References: <20200523185755.8494-1-axboe@kernel.dk> <20200523185755.8494-6-axboe@kernel.dk> <264614fc4fa08df2b0899da1cd38bb07150cd7f3.camel@hammerspace.com> From: Jens Axboe Message-ID: <2fa7104a-ea85-55f2-692c-514eb3b88a88@kernel.dk> Date: Sun, 24 May 2020 10:30:42 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <264614fc4fa08df2b0899da1cd38bb07150cd7f3.camel@hammerspace.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: io-uring-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 5/24/20 8:05 AM, Trond Myklebust wrote: > On Sat, 2020-05-23 at 12:57 -0600, Jens Axboe wrote: >> Use the async page locking infrastructure, if IOCB_WAITQ is set in >> the >> passed in iocb. The caller must expect an -EIOCBQUEUED return value, >> which means that IO is started but not done yet. This is similar to >> how >> O_DIRECT signals the same operation. Once the callback is received by >> the caller for IO completion, the caller must retry the operation. >> >> Signed-off-by: Jens Axboe >> --- >> mm/filemap.c | 33 ++++++++++++++++++++++++++------- >> 1 file changed, 26 insertions(+), 7 deletions(-) >> >> diff --git a/mm/filemap.c b/mm/filemap.c >> index c746541b1d49..a3b86c9acdc8 100644 >> --- a/mm/filemap.c >> +++ b/mm/filemap.c >> @@ -1219,6 +1219,14 @@ static int __wait_on_page_locked_async(struct >> page *page, >> return ret; >> } >> >> +static int wait_on_page_locked_async(struct page *page, >> + struct wait_page_queue *wait) >> +{ >> + if (!PageLocked(page)) >> + return 0; >> + return __wait_on_page_locked_async(compound_head(page), wait, >> false); >> +} >> + >> /** >> * put_and_wait_on_page_locked - Drop a reference and wait for it to >> be unlocked >> * @page: The page to wait for. >> @@ -2058,17 +2066,25 @@ static ssize_t >> generic_file_buffered_read(struct kiocb *iocb, >> index, last_index - index); >> } >> if (!PageUptodate(page)) { >> - if (iocb->ki_flags & IOCB_NOWAIT) { >> - put_page(page); >> - goto would_block; >> - } >> - >> /* >> * See comment in do_read_cache_page on why >> * wait_on_page_locked is used to avoid >> unnecessarily >> * serialisations and why it's safe. >> */ >> - error = wait_on_page_locked_killable(page); >> + if (iocb->ki_flags & IOCB_WAITQ) { >> + if (written) { >> + put_page(page); >> + goto out; >> + } >> + error = wait_on_page_locked_async(page, >> + iocb- >>> private); > > If it is being used in 'generic_file_buffered_read()' as storage for a > wait queue, then it is hard to consider this a 'private' field. private isn't the prettiest, and in fact this one in particular is a bit of a mess. It's not clear if it's caller or callee owned. It's generally not used, outside of the old usb gadget code, iomap O_DIRECT, and ocfs2. With FMODE_BUF_RASYNC, the fs obviously can't set it if it uses ->private for buffered IO. > Perhaps either rename and add type checking, or else add a separate > field altogether to struct kiocb? I'd hate to add a new field and increase the size of the kiocb... One alternative is to do: union { void *private; struct wait_page_queue *ki_waitq; }; and still use IOCB_WAITQ to say that ->ki_waitq is valid. There's also 4 bytes of padding in the kiocb struct. And some fields are only used for O_DIRECT as well, eg ->ki_cookie which is just used for polled O_DIRECT. So we could also do: union { unsigned int ki_cookie; struct wait_page_queue *ki_waitq; }; and still not grow the kiocb. How about we go with this approach, and also add: if (kiocb->ki_flags & IOCB_HIPRI) return -EOPNOTSUPP; to kiocb_wait_page_queue_init() to make sure that this combination isn't valid? -- Jens Axboe