Messages in this thread Patch in this message | | | Date | Wed, 1 Feb 2017 08:19:32 -0800 | From | Liu Bo <> | Subject | Re: [PATCH] Btrfs: add another missing end_page_writeback on submit_extent_page failure |
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On Wed, Feb 01, 2017 at 12:27:24PM +0900, takafumi-sslab wrote: > Thanks for your reply. > > I think you mentioned about the below if-block in __extent_writepage(). > > if (nr == 0) { > /* make sure the mapping tag for page dirty gets cleared */ > set_page_writeback(page); > end_page_writeback(page); > }
Right.
> > However, this if-block only works when nr is 0, and does not work in the > case we indicated. > According to the below codes that we excerpt from __extent_writepage_io(), > nr is incremented even if submit_extent_page() fails. > Therefore, we should safely clear the writeback bit in the error handling > after the fail of submit_extent_page() call > > while (cur <= end) { > ... > ret = submit_extent_page(...); > if (ret) > SetPageError(page); > > cur = cur + iosize; > pg_offset += iosize; > nr++; > } > done: > *nr_ret = nr; // *nr_ret is nr of __extent_writepage()
It'd be either nr == 0 or nr == 1,
right now (nr > 1) couldn't happen on btrfs because it assumes blocksize == PAGE_SIZE, so we could only submit_extent_page once for each page, and bio_ret is used to batch bio submit.
(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).
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 >
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