From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Prefaulting the write source buffer incurs an extra userspace access
in the common fast path. Make fuse_fill_write_pages() consistent with
generic_perform_write(): only touch userspace an extra time when
copy_folio_from_iter_atomic() has failed to make progress.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Miklos Szeredi <miklos@xxxxxxxxxx>
---
b/fs/fuse/file.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)
diff -puN fs/fuse/file.c~fuse-postfault fs/fuse/file.c
--- a/fs/fuse/file.c~fuse-postfault 2025-01-29 09:03:34.515555724 -0800
+++ b/fs/fuse/file.c 2025-01-29 09:03:34.523556390 -0800
@@ -1234,10 +1234,6 @@ static ssize_t fuse_fill_write_pages(str
bytes = min_t(size_t, bytes, fc->max_write - count);
again:
- err = -EFAULT;
- if (fault_in_iov_iter_readable(ii, bytes))
- break;
-
folio = __filemap_get_folio(mapping, index, FGP_WRITEBEGIN,
mapping_gfp_mask(mapping));
if (IS_ERR(folio)) {
@@ -1254,6 +1250,16 @@ static ssize_t fuse_fill_write_pages(str
if (!tmp) {
folio_unlock(folio);
folio_put(folio);
+
+ /*
+ * Ensure forward progress by faulting in
+ * while not holding the folio lock:
+ */
+ if (fault_in_iov_iter_readable(ii, bytes)) {
+ err = -EFAULT;
+ break;
+ }
+
goto again;
}
_
