[PATCH 7/7] netfs: Remove outdated comments about prefaulting
From: Dave Hansen
Date: Wed Jan 29 2025 - 13:26:33 EST
From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
I originally set out to make netfs_perform_write() behavior more
consistent with generic_perform_write(). However, netfs currently
treats a failure to make forward progress as a hard error and does not
retry where the generic code will loop around and retry.
Instead of a major code restructuring, just focus on improving the
comments.
The comment refers to a possible deadlock and to userspace address
checks. Neither of those things are a concern when using
copy_folio_from_iter_atomic() for atomic usercopies. It prevents
deadlocks by disabling page faults and it leverages user copy
functions that have their own access_ok() checks.
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: David Howells <dhowells@xxxxxxxxxx>
Cc: Jeff Layton <jlayton@xxxxxxxxxx>
Cc: netfs@xxxxxxxxxxxxxxx
---
b/fs/netfs/buffered_write.c | 13 +++----------
1 file changed, 3 insertions(+), 10 deletions(-)
diff -puN fs/netfs/buffered_write.c~netfs-postfault fs/netfs/buffered_write.c
--- a/fs/netfs/buffered_write.c~netfs-postfault 2025-01-29 09:03:38.167859749 -0800
+++ b/fs/netfs/buffered_write.c 2025-01-29 09:03:38.171860082 -0800
@@ -152,16 +152,9 @@ ssize_t netfs_perform_write(struct kiocb
offset = pos & (max_chunk - 1);
part = min(max_chunk - offset, iov_iter_count(iter));
- /* Bring in the user pages that we will copy from _first_ lest
- * we hit a nasty deadlock on copying from the same page as
- * we're writing to, without it being marked uptodate.
- *
- * Not only is this an optimisation, but it is also required to
- * check that the address is actually valid, when atomic
- * usercopies are used below.
- *
- * We rely on the page being held onto long enough by the LRU
- * that we can grab it below if this causes it to be read.
+ /* Bring in the user folios that are copied from before taking
+ * locks on the mapping folios. This helps ensure forward
+ * progress if they are the same folios.
*/
ret = -EFAULT;
if (unlikely(fault_in_iov_iter_readable(iter, part) == part))
_