Merge tag 'xfs-iomap-stale-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs into xfs-6.2-mergeB

xfs, iomap: fix data corruption due to stale cached iomaps

This patch series fixes a data corruption that occurs in a specific
multi-threaded write workload. The workload combined
racing unaligned adjacent buffered writes with low memory conditions
that caused both writeback and memory reclaim to race with the
writes.

The result of this was random partial blocks containing zeroes
instead of the correct data.  The underlying problem is that iomap
caches the write iomap for the duration of the write() operation,
but it fails to take into account that the extent underlying the
iomap can change whilst the write is in progress.

The short story is that an iomap can span mutliple folios, and so
under low memory writeback can be cleaning folios the write()
overlaps. Whilst the overlapping data is cached in memory, this
isn't a problem, but because the folios are now clean they can be
reclaimed. Once reclaimed, the write() does the wrong thing when
re-instantiating partial folios because the iomap no longer reflects
the underlying state of the extent. e.g. it thinks the extent is
unwritten, so it zeroes the partial range, when in fact the
underlying extent is now written and so it should have read the data
from disk.  This is how we get random zero ranges in the file
instead of the correct data.

The gory details of the race condition can be found here:

https://lore.kernel.org/linux-xfs/20220817093627.GZ3600936@dread.disaster.area/

Fixing the problem has two aspects. The first aspect of the problem
is ensuring that iomap can detect a stale cached iomap during a
write in a race-free manner. We already do this stale iomap
detection in the writeback path, so we have a mechanism for
detecting that the iomap backing the data range may have changed
and needs to be remapped.

In the case of the write() path, we have to ensure that the iomap is
validated at a point in time when the page cache is stable and
cannot be reclaimed from under us. We also need to validate the
extent before we start performing any modifications to the folio
state or contents. Combine these two requirements together, and the
only "safe" place to validate the iomap is after we have looked up
and locked the folio we are going to copy the data into, but before
we've performed any initialisation operations on that folio.

If the iomap fails validation, we then mark it stale, unlock the
folio and end the write. This effectively means a stale iomap
results in a short write. Filesystems should already be able to
handle this, as write operations can end short for many reasons and
need to iterate through another mapping cycle to be completed. Hence
the iomap changes needed to detect and handle stale iomaps during
write() operations is relatively simple...

However, the assumption is that filesystems should already be able
to handle write failures safely, and that's where the second
(first?) part of the problem exists. That is, handling a partial
write is harder than just "punching out the unused delayed
allocation extent". This is because mmap() based faults can race
with writes, and if they land in the delalloc region that the write
allocated, then punching out the delalloc region can cause data
corruption.

This data corruption problem is exposed by generic/346 when iomap is
converted to detect stale iomaps during write() operations. Hence
write failure in the filesytems needs to handle the fact that the
write() in progress doesn't necessarily own the data in the page
cache over the range of the delalloc extent it just allocated.

As a result, we can't just truncate the page cache over the range
the write() didn't reach and punch all the delalloc extent. We have
to walk the page cache over the untouched range and skip over any
dirty data region in the cache in that range. Which is ....
non-trivial.

That is, iterating the page cache has to handle partially populated
folios (i.e. block size < page size) that contain data. The data
might be discontiguous within a folio. Indeed, there might be
*multiple* discontiguous data regions within a single folio. And to
make matters more complex, multi-page folios mean we just don't know
how many sub-folio regions we might have to iterate to find all
these regions. All the corner cases between the conversions and
rounding between filesystem block size, folio size and multi-page
folio size combined with unaligned write offsets kept breaking my
brain.

However, if we convert the code to track the processed
write regions by byte ranges instead of fileystem block or page
cache index, we could simply use mapping_seek_hole_data() to find
the start and end of each discrete data region within the range we
needed to scan. SEEK_DATA finds the start of the cached data region,
SEEK_HOLE finds the end of the region. These are byte based
interfaces that understand partially uptodate folio regions, and so
can iterate discrete sub-folio data regions directly. This largely
solved the problem of discovering the dirty regions we need to keep
the delalloc extent over.

However, to use mapping_seek_hole_data() without needing to export
it, we have to move all the delalloc extent cleanup to the iomap
core and so now the iomap core can clean up delayed allocation
extents in a safe, sane and filesystem neutral manner.

With all this done, the original data corruption never occurs
anymore, and we now have a generic mechanism for ensuring that page
cache writes do not do the wrong thing when writeback and reclaim
change the state of the physical extent and/or page cache contents
whilst the write is in progress.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>

* tag 'xfs-iomap-stale-fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/dgc/linux-xfs:
  xfs: drop write error injection is unfixable, remove it
  xfs: use iomap_valid method to detect stale cached iomaps
  iomap: write iomap validity checks
  xfs: xfs_bmap_punch_delalloc_range() should take a byte range
  iomap: buffered write failure should not truncate the page cache
  xfs,iomap: move delalloc punching to iomap
  xfs: use byte ranges for write cleanup ranges
  xfs: punching delalloc extents on write failure is racy
  xfs: write page faults in iomap are not buffered writes

1 files changed, 39 insertions, 8 deletions

diff --git a/include/linux/iomap.h b/include/linux/iomap.h
index 238a03087e17..0983dfc9a203 100644
--- a/include/linux/iomap.h
+++ b/include/linux/iomap.h
@@ -49,26 +49,35 @@ struct vm_fault;
  *
  * IOMAP_F_BUFFER_HEAD indicates that the file system requires the use of
  * buffer heads for this mapping.
+ *
+ * IOMAP_F_XATTR indicates that the iomap is for an extended attribute extent
+ * rather than a file data extent.
  */
-#define IOMAP_F_NEW		0x01
-#define IOMAP_F_DIRTY		0x02
-#define IOMAP_F_SHARED		0x04
-#define IOMAP_F_MERGED		0x08
-#define IOMAP_F_BUFFER_HEAD	0x10
-#define IOMAP_F_ZONE_APPEND	0x20
+#define IOMAP_F_NEW		(1U << 0)
+#define IOMAP_F_DIRTY		(1U << 1)
+#define IOMAP_F_SHARED		(1U << 2)
+#define IOMAP_F_MERGED		(1U << 3)
+#define IOMAP_F_BUFFER_HEAD	(1U << 4)
+#define IOMAP_F_ZONE_APPEND	(1U << 5)
+#define IOMAP_F_XATTR		(1U << 6)
 
 /*
  * Flags set by the core iomap code during operations:
  *
  * IOMAP_F_SIZE_CHANGED indicates to the iomap_end method that the file size
  * has changed as the result of this write operation.
+ *
+ * IOMAP_F_STALE indicates that the iomap is not valid any longer and the file
+ * range it covers needs to be remapped by the high level before the operation
+ * can proceed.
  */
-#define IOMAP_F_SIZE_CHANGED	0x100
+#define IOMAP_F_SIZE_CHANGED	(1U << 8)
+#define IOMAP_F_STALE		(1U << 9)
 
 /*
  * Flags from 0x1000 up are for file system specific usage:
  */
-#define IOMAP_F_PRIVATE		0x1000
+#define IOMAP_F_PRIVATE		(1U << 12)
 
 
 /*
@@ -89,6 +98,7 @@ struct iomap {
 	void			*inline_data;
 	void			*private; /* filesystem private */
 	const struct iomap_page_ops *page_ops;
+	u64			validity_cookie; /* used with .iomap_valid() */
 };
 
 static inline sector_t iomap_sector(const struct iomap *iomap, loff_t pos)
@@ -128,6 +138,23 @@ struct iomap_page_ops {
 	int (*page_prepare)(struct inode *inode, loff_t pos, unsigned len);
 	void (*page_done)(struct inode *inode, loff_t pos, unsigned copied,
 			struct page *page);
+
+	/*
+	 * Check that the cached iomap still maps correctly to the filesystem's
+	 * internal extent map. FS internal extent maps can change while iomap
+	 * is iterating a cached iomap, so this hook allows iomap to detect that
+	 * the iomap needs to be refreshed during a long running write
+	 * operation.
+	 *
+	 * The filesystem can store internal state (e.g. a sequence number) in
+	 * iomap->validity_cookie when the iomap is first mapped to be able to
+	 * detect changes between mapping time and whenever .iomap_valid() is
+	 * called.
+	 *
+	 * This is called with the folio over the specified file position held
+	 * locked by the iomap code.
+	 */
+	bool (*iomap_valid)(struct inode *inode, const struct iomap *iomap);
 };
 
 /*
@@ -226,6 +253,10 @@ static inline const struct iomap *iomap_iter_srcmap(const struct iomap_iter *i)
 
 ssize_t iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *from,
 		const struct iomap_ops *ops);
+int iomap_file_buffered_write_punch_delalloc(struct inode *inode,
+		struct iomap *iomap, loff_t pos, loff_t length, ssize_t written,
+		int (*punch)(struct inode *inode, loff_t pos, loff_t length));
+
 int iomap_read_folio(struct folio *folio, const struct iomap_ops *ops);
 void iomap_readahead(struct readahead_control *, const struct iomap_ops *ops);
 bool iomap_is_partially_uptodate(struct folio *, size_t from, size_t count);