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

Brian Foster
2020-10-29T16:07:43Z

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).

> 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);
...

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
> >
>