On Tue, Oct 13, 2020 at 01:13:48PM +0800, Hao_Xu wrote:
> 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
Ah, I don't have that commit in my tree.
Nevertheless, there is still a problem. The ->readpage implementation
is not required to execute asynchronously. For example, it may enter
page reclaim by using GFP_KERNEL. Indeed, I feel it is better if it
works synchronously as it can then report the actual error from an I/O
instead of the almost-meaningless -EIO.
This patch series documents 12 filesystems which implement ->readpage
in a synchronous way today (for at least some cases) and converts iomap
to be synchronous (making two more filesystems synchronous).
https://lore.kernel.org/linux-fsdevel/20201009143104.22673-1-willy@xxxxxxxxxxxxx/
