From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Prefaulting the write source buffer incurs an extra userspace access
in the common fast path. Make ntfs_compress_write() consistent with
generic_perform_write(): only touch userspace an extra time when
copy_page_from_iter_atomic() has failed to make progress.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Konstantin Komarov <almaz.alexandrovich@xxxxxxxxxxxxxxxxxxxx>
Cc: ntfs3@xxxxxxxxxxxxxxx
---
b/fs/ntfs3/file.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
diff -puN fs/ntfs3/file.c~ntfs-postfault fs/ntfs3/file.c
--- a/fs/ntfs3/file.c~ntfs-postfault 2025-01-29 09:03:33.371460504 -0800
+++ b/fs/ntfs3/file.c 2025-01-29 09:03:33.375460837 -0800
@@ -1092,11 +1092,6 @@ static ssize_t ntfs_compress_write(struc
frame_vbo = pos & ~(frame_size - 1);
index = frame_vbo >> PAGE_SHIFT;
- if (unlikely(fault_in_iov_iter_readable(from, bytes))) {
- err = -EFAULT;
- goto out;
- }
-
/* Load full frame. */
err = ntfs_get_frame_pages(mapping, index, pages,
pages_per_frame, &frame_uptodate);
@@ -1172,6 +1167,18 @@ static ssize_t ntfs_compress_write(struc
*/
cond_resched();
+ if (unlikely(!copied)) {
+ /*
+ * folios are now unlocked and faults on them can be
+ * handled. Ensure forward progress by trying to
+ * fault in 'from' in case it maps one of the folios.
+ */
+ if (fault_in_iov_iter_readable(from, bytes)) {
+ err = -EFAULT;
+ goto out;
+ }
+ }
+
pos += copied;
written += copied;
_
