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-rw-r--r--fs/btrfs/inode.c28
1 files changed, 28 insertions, 0 deletions
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 76e530f76e3c..5bbea5ec31fc 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7600,6 +7600,34 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
}
len = min(len, em->len - (start - em->start));
+
+ /*
+ * If we have a NOWAIT request and the range contains multiple extents
+ * (or a mix of extents and holes), then we return -EAGAIN to make the
+ * caller fallback to a context where it can do a blocking (without
+ * NOWAIT) request. This way we avoid doing partial IO and returning
+ * success to the caller, which is not optimal for writes and for reads
+ * it can result in unexpected behaviour for an application.
+ *
+ * When doing a read, because we use IOMAP_DIO_PARTIAL when calling
+ * iomap_dio_rw(), we can end up returning less data then what the caller
+ * asked for, resulting in an unexpected, and incorrect, short read.
+ * That is, the caller asked to read N bytes and we return less than that,
+ * which is wrong unless we are crossing EOF. This happens if we get a
+ * page fault error when trying to fault in pages for the buffer that is
+ * associated to the struct iov_iter passed to iomap_dio_rw(), and we
+ * have previously submitted bios for other extents in the range, in
+ * which case iomap_dio_rw() may return us EIOCBQUEUED if not all of
+ * those bios have completed by the time we get the page fault error,
+ * which we return back to our caller - we should only return EIOCBQUEUED
+ * after we have submitted bios for all the extents in the range.
+ */
+ if ((flags & IOMAP_NOWAIT) && len < length) {
+ free_extent_map(em);
+ ret = -EAGAIN;
+ goto unlock_err;
+ }
+
if (write) {
ret = btrfs_get_blocks_direct_write(&em, inode, dio_data,
start, len);