Re: [PATCH] Btrfs: add another missing end_page_writeback on submit_extent_page failure

[Liu Bo <bo.li.liu@oracle.com>]

On Sat, Feb 04, 2017 at 09:42:17PM +0900, takafumi-sslab wrote:
> 
> > (But it could be changed after subpagesize block patchset, and there is
> > more work rather than just adding a end_page_writeback, e.g. writepage
> > endio also needs to be updated).
> 
> Ok... the discussion become complicated.
> So, let me make this clear.
> 
> you think
> a) this is a bug;
> we need to clear the writeback bit in the error handling if the bit remains.
> 
> b) however, the way of fixing this bug has some concerns.　( and now we
> discuss the best solution )
> 
> Is my understanding correct?

Sorry for making you confused even more, to clarify it, I don't think
the bug could exist in the current btrfs because blocksize is equal to
PAGE_SIZE so that @nr in __extent_writepage could only be 0 or 1.

a) __extent_writepage has handled the case when nr == 0.
b) when nr == 1, the page is marked with writeback bit and added into a
   bio, thus we have bio_end to deal with page bits.

So I don't think the patch is necessary for now.

But as I said, the fact (nr == 0 or 1) would be changed if the
subpagesize blocksize is supported.

Thanks,

-liubo

> 
> Sincerely,
> 
> -takafumi
> > 
> > Thanks,
> > 
> > -liubo
> > > Sincerely,
> > > 
> > > On 2017/01/31 5:09, Liu Bo wrote:
> > > > On Fri, Jan 13, 2017 at 03:12:31PM +0900, takafumi-sslab wrote:
> > > > > Thanks for your replying.
> > > > > 
> > > > > I understand this bug is more complicated than I expected.
> > > > > I classify error cases under submit_extent_page() below
> > > > > 
> > > > > A: ENOMEM error at btrfs_bio_alloc() in submit_extent_page()
> > > > > I first assumed this case and sent the mail.
> > > > > When bio_ret is NULL, submit_extent_page() calls btrfs_bio_alloc().
> > > > > Then, btrfs_bio_alloc() may fail and submit_extent_page() returns -ENOMEM.
> > > > > In this case, bio_endio() is not called and the page's writeback bit
> > > > > remains.
> > > > > So, there is a need to call end_page_writeback() in the error handling.
> > > > > 
> > > > > B: errors under submit_one_bio() of submit_extent_page()
> > > > > Errors that occur under submit_one_bio() handles at bio_endio(), and
> > > > > bio_endio() would call end_page_writeback().
> > > > > 
> > > > > Therefore, as you mentioned in the last mail, simply adding
> > > > > end_page_writeback() like my last email and commit 55e3bd2e0c2e1 can
> > > > > conflict in the case of B.
> > > > > To avoid such conflict, one easy solution is adding PageWriteback() check
> > > > > too.
> > > > > 
> > > > > How do you think of this solution?
> > > > (sorry for the late reply.)
> > > > 
> > > > I think its caller, "__extent_writepage", has covered the above case
> > > > by setting page writeback again.
> > > > 
> > > > Thanks,
> > > > 
> > > > -liubo
> > > > > Sincerely,
> > > > > 
> > > > > On 2016/12/22 15:20, Liu Bo wrote:
> > > > > > On Fri, Dec 16, 2016 at 03:41:50PM +0900, Takafumi Kubota wrote:
> > > > > > > This is actually inspired by Filipe's patch(55e3bd2e0c2e1).
> > > > > > > 
> > > > > > > When submit_extent_page() in __extent_writepage_io() fails,
> > > > > > > Btrfs misses clearing a writeback bit of the failed page.
> > > > > > > This causes the false under-writeback page.
> > > > > > > Then, another sync task hangs in filemap_fdatawait_range(),
> > > > > > > because it waits the false under-writeback page.
> > > > > > > 
> > > > > > > CPU0                            CPU1
> > > > > > > 
> > > > > > > __extent_writepage_io()
> > > > > > >      ret = submit_extent_page() // fail
> > > > > > > 
> > > > > > >      if (ret)
> > > > > > >        SetPageError(page)
> > > > > > >        // miss clearing the writeback bit
> > > > > > > 
> > > > > > >                                    sync()
> > > > > > >                                      ...
> > > > > > >                                      filemap_fdatawait_range()
> > > > > > >                                        wait_on_page_writeback(page);
> > > > > > >                                        // wait the false under-writeback page
> > > > > > > 
> > > > > > > Signed-off-by: Takafumi Kubota <takafumi.kubota1012@sslab.ics.keio.ac.jp>
> > > > > > > ---
> > > > > > >     fs/btrfs/extent_io.c | 4 +++-
> > > > > > >     1 file changed, 3 insertions(+), 1 deletion(-)
> > > > > > > 
> > > > > > > diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> > > > > > > index 1e67723..ef9793b 100644
> > > > > > > --- a/fs/btrfs/extent_io.c
> > > > > > > +++ b/fs/btrfs/extent_io.c
> > > > > > > @@ -3443,8 +3443,10 @@ static noinline_for_stack int __extent_writepage_io(struct inode *inode,
> > > > > > >     					 bdev->bio, max_nr,
> > > > > > >     					 end_bio_extent_writepage,
> > > > > > >     					 0, 0, 0, false);
> > > > > > > -		if (ret)
> > > > > > > +		if (ret) {
> > > > > > >     			SetPageError(page);
> > > > > > > +			end_page_writeback(page);
> > > > > > > +		}
> > > > > > OK...this could be complex as we don't know which part in
> > > > > > submit_extent_page gets the error, if the page has been added into bio
> > > > > > and bio_end would call end_page_writepage(page) as well, so whichever
> > > > > > comes later, the BUG() in end_page_writeback() would complain.
> > > > > > 
> > > > > > Looks like commit 55e3bd2e0c2e1 also has the same problem although I
> > > > > > gave it my reviewed-by.
> > > > > > 
> > > > > > Thanks,
> > > > > > 
> > > > > > -liubo
> > > > > > 
> > > > > > >     		cur = cur + iosize;
> > > > > > >     		pg_offset += iosize;
> > > > > > > -- 
> > > > > > > 1.9.3
> > > > > > > 
> > > > > > > --
> > > > > > > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in
> > > > > > > the body of a message to majordomo@vger.kernel.org
> > > > > > > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> > > > > -- 
> > > > > Keio University
> > > > > System Software Laboratory
> > > > > Takafumi Kubota
> > > > > takafumi.kubota1012@sslab.ics.keio.jp
> > > > > 
> > > -- 
> > > Keio University
> > > System Software Laboratory
> > > Takafumi Kubota
> > > takafumi.kubota1012@sslab.ics.keio.jp
> > > 
> 
> -- 
> Keio University
> System Software Laboratory
> Takafumi Kubota
> takafumi.kubota1012@sslab.ics.keio.jp
>