在 2020/10/13 上午6:08, Jens Axboe 写道:
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.
Hi Jens,
My undertanding of io_uring buffered reads process after the commit
c8d317aa1887b40b188ec3aaa6e9e524333caed1 has been merged is:
the first io_uring IO try is with IOCB_NOWAIT, the second retry in the
same context is with IOCB_WAITQ but without IOCB_NOWAIT.
so in Matthew's case, lock_page_async() will be called after calling
mapping->a_ops->readpage(), So it won't end up sleeping.
Actually this case is what happens when readahead is disabled or somehow
skipped for reasons like blk_cgroup_congested() returns true. And this
case is my commit c8d317aa1887b40b188e for.
Regards,
Hao
