Re: [PATCH v2 2/3] iomap: support partial page discard on writeback block mapping failure

Darrick J. Wong
2020-10-29T16:13:13Z

On Thu, Oct 29, 2020 at 12:07:32PM -0400, Brian Foster wrote:
> On Thu, Oct 29, 2020 at 08:27:18AM -0700, Darrick J. Wong wrote:
> > On Thu, Oct 29, 2020 at 09:23:24AM -0400, Brian Foster wrote:
> > > iomap writeback mapping failure only calls into ->discard_page() if
> > > the current page has not been added to the ioend. Accordingly, the
> > > XFS callback assumes a full page discard and invalidation. This is
> > > problematic for sub-page block size filesystems where some portion
> > > of a page might have been mapped successfully before a failure to
> > > map a delalloc block occurs. ->discard_page() is not called in that
> > > error scenario and the bio is explicitly failed by iomap via the
> > > error return from ->prepare_ioend(). As a result, the filesystem
> > > leaks delalloc blocks and corrupts the filesystem block counters.
> > >
> > > Since XFS is the only user of ->discard_page(), tweak the semantics
> > > to invoke the callback unconditionally on mapping errors and provide
> > > the file offset that failed to map. Update xfs_discard_page() to
> > > discard the corresponding portion of the file and pass the range
> > > along to iomap_invalidatepage(). The latter already properly handles
> > > both full and sub-page scenarios by not changing any iomap or page
> > > state on sub-page invalidations.
> > >
> > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > > ---
> > > fs/iomap/buffered-io.c | 15 ++++++++-------
> > > fs/xfs/xfs_aops.c | 13 +++++++------
> > > include/linux/iomap.h | 2 +-
> > > 3 files changed, 16 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > index bcfc288dba3f..d1f04eabc7e4 100644
> > > --- a/fs/iomap/buffered-io.c
> > > +++ b/fs/iomap/buffered-io.c
> > > @@ -1412,14 +1412,15 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
> > > * appropriately.
> > > */
> > > if (unlikely(error)) {
> > > + /*
> > > + * Let the filesystem know what portion of the current page
> > > + * failed to map. If the page wasn't been added to ioend, it
> > > + * won't be affected by I/O completion and we must unlock it
> > > + * now.
> > > + */
> > > + if (wpc->ops->discard_page)
> > > + wpc->ops->discard_page(page, file_offset);
> > > if (!count) {
> > > - /*
> > > - * If the current page hasn't been added to ioend, it
> > > - * won't be affected by I/O completions and we must
> > > - * discard and unlock it right here.
> > > - */
> > > - if (wpc->ops->discard_page)
> > > - wpc->ops->discard_page(page);
> > > ClearPageUptodate(page);
> > > unlock_page(page);
> > > goto done;
> > > diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
> > > index b35611882ff9..46920c530b20 100644
> > > --- a/fs/xfs/xfs_aops.c
> > > +++ b/fs/xfs/xfs_aops.c
> > > @@ -527,13 +527,14 @@ xfs_prepare_ioend(
> > > */
> > > static void
> > > xfs_discard_page(
> > > - struct page *page)
> > > + struct page *page,
> > > + loff_t fileoff)
> > > {
> > > struct inode *inode = page->mapping->host;
> > > struct xfs_inode *ip = XFS_I(inode);
> > > struct xfs_mount *mp = ip->i_mount;
> > > - loff_t offset = page_offset(page);
> > > - xfs_fileoff_t start_fsb = XFS_B_TO_FSBT(mp, offset);
> > > + unsigned int pageoff = offset_in_page(fileoff);
> > > + xfs_fileoff_t start_fsb = XFS_B_TO_FSBT(mp, fileoff);
> > > int error;
> > >
> > > if (XFS_FORCED_SHUTDOWN(mp))
> > > @@ -541,14 +542,14 @@ xfs_discard_page(
> > >
> > > xfs_alert_ratelimited(mp,
> > > "page discard on page "PTR_FMT", inode 0x%llx, offset %llu.",
> > > - page, ip->i_ino, offset);
> > > + page, ip->i_ino, fileoff);
> > >
> > > error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
> > > - PAGE_SIZE / i_blocksize(inode));
> > > + (PAGE_SIZE - pageoff) / i_blocksize(inode));
> >
> > Er... could you rebase this against 5.10-rc1, please? willy changed
> > that line to not use PAGE_SIZE directly.
> >
>
> Sure.. (note that there's still a PAGE_SIZE usage in the
> iomap_invalidatepage() call).

<nod> I think he has more for 5.11, but in the meantime it's just fixing
the merge conflicts. :)

> > I /think/ the way to resolve the merge conflict here is to change this
> > last argument to:
> >
> > (i_blocks_per_page(page) - pageoff) / i_blocksize(inode)
> >
>
> Hmm... pageoff is bytes so that doesn't look quite right. How about
> something like this?
>
> ...
> unsigned int pageoff = offset_in_page(fileoff);
> xfs_fileoff_t pageoff_fsb = XFS_B_TO_FSBT(mp, pageoff);
>
> ...
> error = xfs_bmap_punch_delalloc_range(ip, start_fsb,
> i_blocks_per_page(inode, page) - pageoff_fsb);
> ...

You could probably combine the two variables, but otherwise that looks
fine to me.

--D

>
> Brian
>
> > --D
> >
> > > if (error && !XFS_FORCED_SHUTDOWN(mp))
> > > xfs_alert(mp, "page discard unable to remove delalloc mapping.");
> > > out_invalidate:
> > > - iomap_invalidatepage(page, 0, PAGE_SIZE);
> > > + iomap_invalidatepage(page, pageoff, PAGE_SIZE - pageoff);
> > > }
> > >
> > > static const struct iomap_writeback_ops xfs_writeback_ops = {
> > > diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> > > index 4d1d3c3469e9..36e0ab19210a 100644
> > > --- a/include/linux/iomap.h
> > > +++ b/include/linux/iomap.h
> > > @@ -220,7 +220,7 @@ struct iomap_writeback_ops {
> > > * Optional, allows the file system to discard state on a page where
> > > * we failed to submit any I/O.
> > > */
> > > - void (*discard_page)(struct page *page);
> > > + void (*discard_page)(struct page *page, loff_t fileoff);
> > > };
> > >
> > > struct iomap_writepage_ctx {
> > > --
> > > 2.25.4
> > >
> >
>