+ filemap-prefault-writes-starting-beyond-eof.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: filemap: prefault writes starting beyond EOF
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     filemap-prefault-writes-starting-beyond-eof.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/filemap-prefault-writes-starting-beyond-eof.patch

This patch will later appear in the mm-hotfixes-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Hungyu Lin <dennylin0707@gmail.com>
Subject: filemap: prefault writes starting beyond EOF
Date: Wed, 04 Nov 2026 07:10:02 +0000

Since prefaulting moved to the slow write path, a buffered write can reach
->write_begin() and ->write_end() before discovering that its source
buffer is unreadable.  Several filesystems can extend i_size in these
callbacks even when no data was copied.  For example, a one-byte pwrite()
at offset 4096 into an empty file can fail with EFAULT but leave a
4096-byte file.

Prefault the source before ->write_begin() when the write position is
beyond EOF, and return EFAULT if no source bytes can be made readable. 
Overwrites and appends continue to skip this prefault.  Keep the existing
slow-path faulting for forward progress after a failed atomic copy.

This also leaves intentional zero-length ->write_end() calls used by
generic_cont_expand_simple() unchanged.  As with the previous prefaulting
behavior, a concurrent userspace unmap can still race with the later copy.

Link: https://lore.kernel.org/20261104071002.49693-1-dennylin0707@gmail.com
Fixes: 665575cff098 ("filemap: move prefaulting out of hot write path")
Signed-off-by: Hungyu Lin <dennylin0707@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Kiryl Shutsemau <kas@kernel.org>
Link: https://lore.kernel.org/all/at-XtWBQg3jv3H8Z@thinkstation/
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kiryl Shutsemau (Meta) <kas@kernel.org>.
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: <stable@vger.kernel.org>
---

 mm/filemap.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/mm/filemap.c~filemap-prefault-writes-starting-beyond-eof
+++ a/mm/filemap.c
@@ -4396,6 +4396,16 @@ retry:
 			break;
 		}
 
+		/*
+		 * Some filesystems can extend i_size even when nothing was copied.
+		 * Prefault writes beyond EOF to catch unreadable source buffers.
+		 */
+		if (unlikely(pos > i_size_read(mapping->host)) &&
+		    fault_in_iov_iter_readable(i, bytes) == bytes) {
+			status = -EFAULT;
+			break;
+		}
+
 		status = a_ops->write_begin(iocb, mapping, pos, bytes,
 						&folio, &fsdata);
 		if (unlikely(status < 0))
_

Patches currently in -mm which might be from dennylin0707@gmail.com are

filemap-prefault-writes-starting-beyond-eof.patch
mm-page-writeback-avoid-overflow-when-reporting-bdi-byte-limits.patch
mm-skip-empty-finite-ranges-in-truncate_pagecache_range.patch
delayacct-preserve-delay-extrema-when-aggregating-taskstats.patch
shmem-clamp-negative-used-block-samples-in-statfs.patch