Re: [PATCH 3/3] iomap: fall back to buffered writes for invalidation failures

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On 7/22/20 12:01 AM, Christoph Hellwig wrote:
Failing to invalid the page cache means data in incoherent, which is
a very bad state for the system.  Always fall back to buffered I/O
through the page cache if we can't invalidate mappings.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Acked-by: Dave Chinner <dchinner@xxxxxxxxxx>
Reviewed-by: Goldwyn Rodrigues <rgoldwyn@xxxxxxxx>


Reviewed-by: Ritesh Harjani <riteshh@xxxxxxxxxxxxx>
 mainly for ext4 part.


---
  fs/ext4/file.c       |  2 ++
  fs/gfs2/file.c       |  3 ++-
  fs/iomap/direct-io.c | 16 +++++++++++-----
  fs/iomap/trace.h     |  1 +
  fs/xfs/xfs_file.c    |  4 ++--
  fs/zonefs/super.c    |  7 +++++--
  6 files changed, 23 insertions(+), 10 deletions(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 2a01e31a032c4c..129cc1dd6b7952 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -544,6 +544,8 @@ static ssize_t ext4_dio_write_iter(struct kiocb *iocb, struct iov_iter *from)
  		iomap_ops = &ext4_iomap_overwrite_ops;
  	ret = iomap_dio_rw(iocb, from, iomap_ops, &ext4_dio_write_ops,
  			   is_sync_kiocb(iocb) || unaligned_io || extend);
+	if (ret == -ENOTBLK)
+		ret = 0;

  	if (extend)
  		ret = ext4_handle_inode_extension(inode, offset, ret, count);
diff --git a/fs/gfs2/file.c b/fs/gfs2/file.c
index bebde537ac8cf2..b085a3bea4f0fd 100644
--- a/fs/gfs2/file.c
+++ b/fs/gfs2/file.c
@@ -835,7 +835,8 @@ static ssize_t gfs2_file_direct_write(struct kiocb *iocb, struct iov_iter *from)

  	ret = iomap_dio_rw(iocb, from, &gfs2_iomap_ops, NULL,
  			   is_sync_kiocb(iocb));
-
+	if (ret == -ENOTBLK)
+		ret = 0;
  out:
  	gfs2_glock_dq(&gh);
  out_uninit:
diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
index 190967e87b69e4..c1aafb2ab99072 100644
--- a/fs/iomap/direct-io.c
+++ b/fs/iomap/direct-io.c
@@ -10,6 +10,7 @@
  #include <linux/backing-dev.h>
  #include <linux/uio.h>
  #include <linux/task_io_accounting_ops.h>
+#include "trace.h"

  #include "../internal.h"

@@ -401,6 +402,9 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
   * can be mapped into multiple disjoint IOs and only a subset of the IOs issued
   * may be pure data writes. In that case, we still need to do a full data sync
   * completion.
+ *
+ * Returns -ENOTBLK In case of a page invalidation invalidation failure for
+ * writes.  The callers needs to fall back to buffered I/O in this case.
   */
  ssize_t
  iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
@@ -478,13 +482,15 @@ iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
  	if (iov_iter_rw(iter) == WRITE) {
  		/*
  		 * Try to invalidate cache pages for the range we are writing.
-		 * If this invalidation fails, tough, the write will still work,
-		 * but racing two incompatible write paths is a pretty crazy
-		 * thing to do, so we don't support it 100%.
+		 * If this invalidation fails, let the caller fall back to
+		 * buffered I/O.
  		 */
  		if (invalidate_inode_pages2_range(mapping, pos >> PAGE_SHIFT,
-				end >> PAGE_SHIFT))
-			dio_warn_stale_pagecache(iocb->ki_filp);
+				end >> PAGE_SHIFT)) {
+			trace_iomap_dio_invalidate_fail(inode, pos, count);
+			ret = -ENOTBLK;
+			goto out_free_dio;
+		}

  		if (!wait_for_completion && !inode->i_sb->s_dio_done_wq) {
  			ret = sb_init_dio_done_wq(inode->i_sb);

Just as a note. So if the driver returns -ENOTBLK (from ->iomap_end)then
iomap considers it as a magic value to fall back to buffered-io and it
changes ret=0 And now with this patch, iomap could also return
-ENOTBLK if it gets an error while doing above operation and so the
driver is free to consider this as a fallback mechanism to buffered-io.



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