Re: [PATCH] mm/shmem: fix uninitialized folio in shmem_symlink

[Matthew Wilcox <willy@infradead.org>]

On Wed, Dec 24, 2025 at 10:40:27PM +1300, Barry Song wrote:
> From: Barry Song <baohua@kernel.org>
> 
> Uninitialized folio allocated in shmem_symlink() may be accessed
> during swap-out, causing KMSAN BUG:

This would be an unfortunate way to fix it.  The vast majority of
symlinks are short, and we'll never access past the \0 in normal
operation, so we'll be dirtying a lot of cachelines essentially to (1)
shut up an automated tool and (2) optimise a corner case.

How about this instead which delays zeroing to swapout?

diff --git a/mm/shmem.c b/mm/shmem.c
index ec6c01378e9d..f3b3be1b50fe 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1636,6 +1636,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
 		folio_mark_uptodate(folio);
 	}
 
+	/* Zero out symlink tails to help with compression */
+	if (folio_test_owner_2(folio)) {
+		struct inode *inode = folio->mapping->host;
+		folio_zero_segment(folio, inode->i_size, folio_size(folio));
+		folio_clear_owner_2(folio);
+	}
+
 	if (!folio_alloc_swap(folio)) {
 		bool first_swapped = shmem_recalc_inode(inode, 0, nr_pages);
 		int error;
@@ -4133,6 +4140,7 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
 		memcpy(folio_address(folio), symname, len);
 		folio_mark_uptodate(folio);
 		folio_mark_dirty(folio);
+		folio_set_owner_2(folio);
 		folio_unlock(folio);
 		folio_put(folio);
 	}