[PATCH 3/7] ntfs3: Move prefaulting out of hot write path

[Dave Hansen <dave.hansen@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

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@linux.intel.com>
Cc: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Cc: ntfs3@lists.linux.dev
---

 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;
 
_