在 2020/10/13 上午5:13, Matthew Wilcox 写道:
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.
Hi Matthew,
which kernel version are you use, I believe I've fixed this case in the
commit c8d317aa1887b40b188ec3aaa6e9e524333caed1
in this commit, I did the modification:
diff --git a/mm/filemap.c b/mm/filemap.c
index 1aaea26556cc..ea383478fc22 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2267,7 +2267,11 @@ ssize_t generic_file_buffered_read(struct kiocb
*iocb,
}
if (!PageUptodate(page)) {
- error = lock_page_killable(page);
+ if (iocb->ki_flags & IOCB_WAITQ)
+ error = lock_page_async(page,
iocb->ki_waitq);
+ else
+ error = lock_page_killable(page);
+
if (unlikely(error))
goto readpage_error;
if (!PageUptodate(page)) {
lock_page_killable() won't be called in this case.
