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Date: Mon, 9 Jan 2023 13:46:42 +0100
From: Andreas Gruenbacher <agruenba@...hat.com>
To: Dave Chinner <dchinner@...hat.com>
Cc: Christoph Hellwig <hch@...radead.org>,
"Darrick J . Wong" <djwong@...nel.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
Matthew Wilcox <willy@...radead.org>,
linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-ext4@...r.kernel.org, cluster-devel@...hat.com,
Christoph Hellwig <hch@....de>
Subject: Re: [RFC v6 04/10] iomap: Add iomap_get_folio helper
On Sun, Jan 8, 2023 at 10:33 PM Dave Chinner <david@...morbit.com> wrote:
> On Sun, Jan 08, 2023 at 08:40:28PM +0100, Andreas Gruenbacher wrote:
> > Add an iomap_get_folio() helper that gets a folio reference based on
> > an iomap iterator and an offset into the address space. Use it in
> > iomap_write_begin().
> >
> > Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
> > Reviewed-by: Darrick J. Wong <djwong@...nel.org>
> > Reviewed-by: Christoph Hellwig <hch@....de>
> > ---
> > fs/iomap/buffered-io.c | 39 ++++++++++++++++++++++++++++++---------
> > include/linux/iomap.h | 1 +
> > 2 files changed, 31 insertions(+), 9 deletions(-)
> >
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index d4b444e44861..de4a8e5f721a 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -457,6 +457,33 @@ bool iomap_is_partially_uptodate(struct folio *folio, size_t from, size_t count)
> > }
> > EXPORT_SYMBOL_GPL(iomap_is_partially_uptodate);
> >
> > +/**
> > + * iomap_get_folio - get a folio reference for writing
> > + * @iter: iteration structure
> > + * @pos: start offset of write
> > + *
> > + * Returns a locked reference to the folio at @pos, or an error pointer if the
> > + * folio could not be obtained.
> > + */
> > +struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos)
> > +{
> > + unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
> > + struct folio *folio;
> > +
> > + if (iter->flags & IOMAP_NOWAIT)
> > + fgp |= FGP_NOWAIT;
> > +
> > + folio = __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
> > + fgp, mapping_gfp_mask(iter->inode->i_mapping));
> > + if (folio)
> > + return folio;
> > +
> > + if (iter->flags & IOMAP_NOWAIT)
> > + return ERR_PTR(-EAGAIN);
> > + return ERR_PTR(-ENOMEM);
> > +}
> > +EXPORT_SYMBOL_GPL(iomap_get_folio);
>
> Hmmmm.
>
> This is where things start to get complex. I have sent a patch to
> fix a problem with iomap_zero_range() failing to zero cached dirty
> pages over UNWRITTEN extents, and that requires making FGP_CREAT
> optional. This is an iomap bug, and needs to be fixed in the core
> iomap code:
>
> https://lore.kernel.org/linux-xfs/20221201005214.3836105-1-david@fromorbit.com/
>
> Essentially, we need to pass fgp flags to iomap_write_begin() need
> so the callers can supply a 0 or FGP_CREAT appropriately. This
> allows iomap_write_begin() to act only on pre-cached pages rather
> than always instantiating a new page if one does not exist in cache.
>
> This allows that iomap_write_begin() to return a NULL folio
> successfully, and this is perfectly OK for callers that pass in fgp
> = 0 as they are expected to handle a NULL folio return indicating
> there was no cached data over the range...
>
> Exposing the folio allocation as an external interface makes bug
> fixes like this rather messy - it's taking a core abstraction (iomap
> hides all the folio and page cache manipulations from the
> filesystem) and punching a big hole in it by requiring filesystems
> to actually allocation page cache folios on behalf of the iomap
> core.
>
> Given that I recently got major push-back for fixing an XFS-only bug
> by walking the page cache directly instead of abstracting it via the
> iomap core, punching an even bigger hole in the abstraction layer to
> fix a GFS2-only problem is just as bad....
We can handle that by adding a new IOMAP_NOCREATE iterator flag and
checking for that in iomap_get_folio(). Your patch then turns into
the below.
Thanks,
Andreas
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index dacc7c80b20d..34b335a89527 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -470,6 +470,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos)
unsigned fgp = FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE | FGP_NOFS;
struct folio *folio;
+ if (!(iter->flags & IOMAP_NOCREATE))
+ fgp |= FGP_CREAT;
if (iter->flags & IOMAP_NOWAIT)
fgp |= FGP_NOWAIT;
@@ -478,6 +480,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos)
if (folio)
return folio;
+ if (iter->flags & IOMAP_NOCREATE)
+ return ERR_PTR(-ENODATA);
if (iter->flags & IOMAP_NOWAIT)
return ERR_PTR(-EAGAIN);
return ERR_PTR(-ENOMEM);
@@ -1162,8 +1166,12 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
loff_t written = 0;
/* already zeroed? we're done. */
- if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
+ if (srcmap->type == IOMAP_HOLE)
return length;
+ /* only do page cache lookups over unwritten extents */
+ iter->flags &= ~IOMAP_NOCREATE;
+ if (srcmap->type == IOMAP_UNWRITTEN)
+ iter->flags |= IOMAP_NOCREATE;
do {
struct folio *folio;
@@ -1172,8 +1180,19 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
size_t bytes = min_t(u64, SIZE_MAX, length);
status = iomap_write_begin(iter, pos, bytes, &folio);
- if (status)
+ if (status) {
+ if (status == -ENODATA) {
+ /*
+ * No folio was found, so skip to the start of
+ * the next potential entry in the page cache
+ * and continue from there.
+ */
+ if (bytes > PAGE_SIZE - offset_in_page(pos))
+ bytes = PAGE_SIZE - offset_in_page(pos);
+ goto loop_continue;
+ }
return status;
+ }
if (iter->iomap.flags & IOMAP_F_STALE)
break;
@@ -1181,6 +1200,19 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
if (bytes > folio_size(folio) - offset)
bytes = folio_size(folio) - offset;
+ /*
+ * If the folio over an unwritten extent is clean, then we
+ * aren't going to touch the data in it at all. We don't want to
+ * mark it dirty or change the uptodate state of data in the
+ * page, so we just unlock it and skip to the next range over
+ * the unwritten extent we need to check.
+ */
+ if (srcmap->type == IOMAP_UNWRITTEN &&
+ !folio_test_dirty(folio)) {
+ folio_unlock(folio);
+ goto loop_continue;
+ }
+
folio_zero_range(folio, offset, bytes);
folio_mark_accessed(folio);
@@ -1188,6 +1220,7 @@ static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
if (WARN_ON_ONCE(bytes == 0))
return -EIO;
+loop_continue:
pos += bytes;
length -= bytes;
written += bytes;
diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index 515318dfbc38..87b9d9aba4bb 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c
@@ -841,17 +841,7 @@ xfs_setattr_size(
trace_xfs_zero_eof(ip, oldsize, newsize - oldsize);
error = xfs_zero_range(ip, oldsize, newsize - oldsize,
&did_zeroing);
- } else {
- /*
- * iomap won't detect a dirty page over an unwritten block (or a
- * cow block over a hole) and subsequently skips zeroing the
- * newly post-EOF portion of the page. Flush the new EOF to
- * convert the block before the pagecache truncate.
- */
- error = filemap_write_and_wait_range(inode->i_mapping, newsize,
- newsize);
- if (error)
- return error;
+ } else if (newsize != oldsize) {
error = xfs_truncate_page(ip, newsize, &did_zeroing);
}
diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 3e6c34b03c89..55f195866f00 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -164,6 +164,7 @@ struct iomap_folio_ops {
#else
#define IOMAP_DAX 0
#endif /* CONFIG_FS_DAX */
+#define IOMAP_NOCREATE (1 << 9) /* look up folios without FGP_CREAT */
struct iomap_ops {
/*
--
2.38.1
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