| Message ID | 59eae433-9b30-f094-534d-eb5cf0a40a80@kernel.dk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | mm: mark async iocb read as NOWAIT once some data has been, copied | expand |
On Sat, Oct 17, 2020 at 09:30:59AM -0600, Jens Axboe wrote: > 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. Wouldn't this do the job just as well? @@ -2211,6 +2211,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, put_page(page); written += ret; + if (iocb->ki_flags & IOCB_WAITQ) + iocb->ki_flags |= IOCB_NOWAIT; if (!iov_iter_count(iter)) goto out; if (ret < nr) {
On 10/17/20 10:28 AM, Matthew Wilcox wrote: > On Sat, Oct 17, 2020 at 09:30:59AM -0600, Jens Axboe wrote: >> 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. > > Wouldn't this do the job just as well? > > @@ -2211,6 +2211,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, > > put_page(page); > written += ret; > + if (iocb->ki_flags & IOCB_WAITQ) > + iocb->ki_flags |= IOCB_NOWAIT; > if (!iov_iter_count(iter)) > goto out; > if (ret < nr) { Indeed, that's cleaner. Let me re-test with that just to be on the safe side, and I'll post a new one.
On Sat, Oct 17, 2020 at 11:30:47AM -0600, Jens Axboe wrote: > On 10/17/20 10:28 AM, Matthew Wilcox wrote: > > On Sat, Oct 17, 2020 at 09:30:59AM -0600, Jens Axboe wrote: > >> 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. > > > > Wouldn't this do the job just as well? > > > > @@ -2211,6 +2211,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, > > > > put_page(page); > > written += ret; > > + if (iocb->ki_flags & IOCB_WAITQ) > > + iocb->ki_flags |= IOCB_NOWAIT; > > if (!iov_iter_count(iter)) > > goto out; > > if (ret < nr) { > > Indeed, that's cleaner. Let me re-test with that just to be on the safe > side, and I'll post a new one. generic_file_buffered_read() has written passed in, which can be nonzero (DIO fallback) - we probably want the check to be at the top of the loop.
On 10/17/20 1:13 PM, Kent Overstreet wrote: > On Sat, Oct 17, 2020 at 11:30:47AM -0600, Jens Axboe wrote: >> On 10/17/20 10:28 AM, Matthew Wilcox wrote: >>> On Sat, Oct 17, 2020 at 09:30:59AM -0600, Jens Axboe wrote: >>>> 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. >>> >>> Wouldn't this do the job just as well? >>> >>> @@ -2211,6 +2211,8 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb, >>> >>> put_page(page); >>> written += ret; >>> + if (iocb->ki_flags & IOCB_WAITQ) >>> + iocb->ki_flags |= IOCB_NOWAIT; >>> if (!iov_iter_count(iter)) >>> goto out; >>> if (ret < nr) { >> >> Indeed, that's cleaner. Let me re-test with that just to be on the safe >> side, and I'll post a new one. > > generic_file_buffered_read() has written passed in, which can be nonzero (DIO > fallback) - we probably want the check to be at the top of the loop. Yeah good point. That calling convention is... miserable. I'll move it up top.
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);
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 <kent.overstreet@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk> --- mm/filemap.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-)