From: Jinliang Zheng <alexjlzheng@tencent.com>

Similar to commit 665575cff098 ("filemap: move prefaulting out of hot
write path"), there's no need to do the faultin unconditionally. It is
more reasonable to perform faultin operation only when an exception
occurs.

And copy_folio_from_iter_atomic() short-circuits page fault handle logics
via pagefault_disable(), which prevents deadlock scenarios when both
source and destination buffers reside within the same folio. So it's
safe move prefaulting after copy failed.

Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
---
 fs/iomap/buffered-io.c | 25 ++++++++++---------------
 1 file changed, 10 insertions(+), 15 deletions(-)

diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
index fb4519158f3a..7ca3f3b9d57e 100644
--- a/fs/iomap/buffered-io.c
+++ b/fs/iomap/buffered-io.c
@@ -964,21 +964,6 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 		if (bytes > iomap_length(iter))
 			bytes = iomap_length(iter);
 
-		/*
-		 * Bring in the user page that we'll copy from _first_.
-		 * Otherwise there's a nasty deadlock on copying from the
-		 * same page as we're writing to, without it being marked
-		 * up-to-date.
-		 *
-		 * For async buffered writes the assumption is that the user
-		 * page has already been faulted in. This can be optimized by
-		 * faulting the user page.
-		 */
-		if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
-			status = -EFAULT;
-			break;
-		}
-
 		status = iomap_write_begin(iter, &folio, &offset, &bytes);
 		if (unlikely(status)) {
 			iomap_write_failed(iter->inode, iter->pos, bytes);
@@ -992,6 +977,12 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 		if (mapping_writably_mapped(mapping))
 			flush_dcache_folio(folio);
 
+		/*
+		 * copy_folio_from_iter_atomic() short-circuits page fault handle
+		 * logics via pagefault_disable(), to prevent deadlock scenarios
+		 * when both source and destination buffers reside within the same
+		 * folio (mmap, ...).
+		 */
 		copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);
 		written = iomap_write_end(iter, bytes, copied, folio) ?
 			  copied : 0;
@@ -1030,6 +1021,10 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
 				bytes = copied;
 				goto retry;
 			}
+			if (fault_in_iov_iter_readable(i, bytes) == bytes) {
+				status = -EFAULT;
+				break;
+			}
 		} else {
 			total_written += written;
 			iomap_iter_advance(iter, &written);
-- 
2.49.0

On Sat, Jul 26, 2025 at 05:09:56PM +0800, alexjlzheng@gmail.com wrote:
> @@ -992,6 +977,12 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>  		if (mapping_writably_mapped(mapping))
>  			flush_dcache_folio(folio);
>  
> +		/*
> +		 * copy_folio_from_iter_atomic() short-circuits page fault handle
> +		 * logics via pagefault_disable(), to prevent deadlock scenarios
> +		 * when both source and destination buffers reside within the same
> +		 * folio (mmap, ...).
> +		 */

Why did you change this comment from the one in 665575cff098?
The comment in that commit is correct.  This comment is so badly
mangled, it isn't even wrong.