[PATCH v2 2/2] iomap: make zero range flush conditional on unwritten mappings

Brian Foster
2024-08-28T18:18:23Z

iomap_zero_range() flushes pagecache to mitigate consistency
problems with dirty pagecache and unwritten mappings. The flush is
unconditional over the entire range because checking pagecache state
after mapping lookup is racy with writeback and reclaim. There are
ways around this using iomap's mapping revalidation mechanism, but
this is not supported by all iomap based filesystems and so is not a
generic solution.

There is another way around this limitation that is good enough to
filter the flush for most cases in practice. If we check for dirty
pagecache over the target range (instead of unconditionally flush),
we can keep track of whether the range was dirty before lookup and
defer the flush until/unless we see a combination of dirty cache
backed by an unwritten mapping. We don't necessarily know whether
the dirty cache was backed by the unwritten maping or some other
(written) part of the range, but the impliciation of a false
positive here is a spurious flush and thus relatively harmless.

Note that we also flush for hole mappings because iomap_zero_range()
is used for partial folio zeroing in some cases. For example, if a
folio straddles EOF on a sub-page FSB size fs, the post-eof portion
is hole-backed and dirtied/written via mapped write, and then i_size
increases before writeback can occur (which otherwise zeroes the
post-eof portion of the EOF folio), then the folio becomes
inconsistent with disk until reclaimed. A flush in this case
executes partial zeroing from writeback, and iomap knows that there
is otherwise no I/O to submit for hole backed mappings.

Signed-off-by: Brian Foster <bfoster@redhat.com>
---
fs/iomap/buffered-io.c | 57 +++++++++++++++++++++++++++++++++++-------
1 file changed, 48 insertions(+), 9 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index 3e846f43ff48..a6e897e6e303 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -1393,16 +1393,47 @@ iomap_file_unshare(struct inode *inode, loff_t pos, loff_t len,
}
EXPORT_SYMBOL_GPL(iomap_file_unshare);

-static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero)
+/*
+ * Flush the remaining range of the iter and mark the current mapping stale.
+ * This is used when zero range sees an unwritten mapping that may have had
+ * dirty pagecache over it.
+ */
+static inline int iomap_zero_iter_flush_and_stale(struct iomap_iter *i)
+{
+ struct address_space *mapping = i->inode->i_mapping;
+ loff_t end = i->pos + i->len - 1;
+
+ i->iomap.flags |= IOMAP_F_STALE;
+ return filemap_write_and_wait_range(mapping, i->pos, end);
+}
+
+static loff_t iomap_zero_iter(struct iomap_iter *iter, bool *did_zero,
+ bool *range_dirty)
{
const struct iomap *srcmap = iomap_iter_srcmap(iter);
loff_t pos = iter->pos;
loff_t length = iomap_length(iter);
loff_t written = 0;

- /* already zeroed? we're done. */
- if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN)
+ /*
+ * We can skip pre-zeroed mappings so long as either the mapping was
+ * clean before we started or we've flushed at least once since.
+ * Otherwise we don't know whether the current mapping had dirty
+ * pagecache, so flush it now, stale the current mapping, and proceed
+ * from there.
+ *
+ * The hole case is intentionally included because this is (ab)used to
+ * handle partial folio zeroing in some cases. Hole backed post-eof
+ * ranges can be dirtied via mapped write and the flush triggers
+ * writeback time post-eof zeroing.
+ */
+ if (srcmap->type == IOMAP_HOLE || srcmap->type == IOMAP_UNWRITTEN) {
+ if (*range_dirty) {
+ *range_dirty = false;
+ return iomap_zero_iter_flush_and_stale(iter);
+ }
return length;
+ }

do {
struct folio *folio;
@@ -1450,19 +1481,27 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
.flags = IOMAP_ZERO,
};
int ret;
+ bool range_dirty;

/*
* Zero range wants to skip pre-zeroed (i.e. unwritten) mappings, but
* pagecache must be flushed to ensure stale data from previous
- * buffered writes is not exposed.
+ * buffered writes is not exposed. A flush is only required for certain
+ * types of mappings, but checking pagecache after mapping lookup is
+ * racy with writeback and reclaim.
+ *
+ * Therefore, check the entire range first and pass along whether any
+ * part of it is dirty. If so and an underlying mapping warrants it,
+ * flush the cache at that point. This trades off the occasional false
+ * positive (and spurious flush, if the dirty data and mapping don't
+ * happen to overlap) for simplicity in handling a relatively uncommon
+ * situation.
*/
- ret = filemap_write_and_wait_range(inode->i_mapping,
- pos, pos + len - 1);
- if (ret)
- return ret;
+ range_dirty = filemap_range_needs_writeback(inode->i_mapping,
+ pos, pos + len - 1);

while ((ret = iomap_iter(&iter, ops)) > 0)
- iter.processed = iomap_zero_iter(&iter, did_zero);
+ iter.processed = iomap_zero_iter(&iter, did_zero, &range_dirty);
return ret;
}
EXPORT_SYMBOL_GPL(iomap_zero_range);
--
2.45.0