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Sat, 17 Oct 2020 08:31:01 -0700 (PDT) To: linux-mm , Andrew Morton , Johannes Weiner , Kent Overstreet From: Jens Axboe Subject: [PATCH] mm: mark async iocb read as NOWAIT once some data has been, copied Message-ID: <59eae433-9b30-f094-534d-eb5cf0a40a80@kernel.dk> Date: Sat, 17 Oct 2020 09:30:59 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0 MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit 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: Once we've copied some data for an iocb that is marked with IOCB_WAITQ, we should no longer attempt to async lock a new page. Instead make sure we return the copied amount, and let the caller retry, instead of returning -EIOCBQUEUED for a new page. This should only be possible with read-ahead disabled on the below device, and multiple threads racing on the same file. Haven't been able to reproduce on anything else. Cc: stable@vger.kernel.org # v5.9 Fixes: 1a0a7853b901 ("mm: support async buffered reads in generic_file_buffered_read()") Reported-by: Kent Overstreet Signed-off-by: Jens Axboe --- mm/filemap.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index 1a6beaf69f49..029787fda56b 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2185,6 +2185,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, pgoff_t index; pgoff_t last_index; pgoff_t prev_index; + ssize_t this_written = 0; unsigned long offset; /* offset into pagecache page */ unsigned int prev_offset; int error = 0; @@ -2207,6 +2208,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, cond_resched(); find_page: + /* + * If we've already successfully copied some data, then we + * can no longer safely return -EIOCBQUEUED. Hence mark + * an async read NOWAIT at that point. + */ + if (this_written && (iocb->ki_flags & IOCB_WAITQ)) + iocb->ki_flags |= IOCB_NOWAIT; + if (fatal_signal_pending(current)) { error = -EINTR; goto out; @@ -2239,7 +2248,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, * serialisations and why it's safe. */ if (iocb->ki_flags & IOCB_WAITQ) { - if (written) { + if (this_written) { put_page(page); goto out; } @@ -2328,7 +2337,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, prev_offset = offset; put_page(page); - written += ret; + this_written += ret; if (!iov_iter_count(iter)) goto out; if (ret < nr) { @@ -2448,6 +2457,7 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, *ppos = ((loff_t)index << PAGE_SHIFT) + offset; file_accessed(filp); + written += this_written; return written ? written : error; } EXPORT_SYMBOL_GPL(generic_file_buffered_read); -- 2.28.0 -- Jens Axboe