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 <haoxu@xxxxxxxxxxxxxxxxx>
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
