[PATCH v4 02/18] f2fs: remove mappedtodisk flag usage in mmap write path

[Nanzhe Zhao <nzzhao.sigma@gmail.com>]

From: Nanzhe Zhao <zhaonanzhe@xiaomi.com>

PG_mappedtodisk was inherited from the nobh/buffer-head era.  However,
f2fs has no buffer heads and the flag is no longer meaningful.  The only
user of the flag is the "no holes" shortcut in f2fs_vm_page_mkwrite(),
which makes mmap write faults skip marking the folio dirty.  Moreover,
it causes the following race condition with filesystem freezing:

 CPU0 (write fault)     CPU1 (freeze_super)
 ----                   ----
 f2fs_vm_page_mkwrite()
 sb_start_pagefault()
 /* shortcut: no dirty */
 sb_end_pagefault()

                        sb_wait_write(SB_FREEZE_PAGEFAULT)
                        /* no waiter, passes */
                        sync_filesystem()
                        /* folio not dirty, skipped */

 finish_fault()
 /* PTE writable */
 fault_dirty_shared_page()
 folio_mark_dirty()
 /* dirty folio on a frozen fs */

It will also skip APP_MAPPED_IO iostat/REQ_TIME updates and post-EOF
folio zeroing.

Remove this shortcut in f2fs_vm_page_mkwrite() and stop setting it in
f2fs_read_single_page().

Reported-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/linux-f2fs-devel/496b913c-eb81-4fa8-be72-c6a06a10aba2@kernel.org/
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Barry Song <baohua@kernel.org>
Fixes: fbfa2cc58d53 ("f2fs: add file operations")
Cc: stable@vger.kernel.org
Signed-off-by: Nanzhe Zhao <zhaonanzhe@xiaomi.com>
---
 fs/f2fs/data.c | 1 -
 fs/f2fs/file.c | 6 ------
 2 files changed, 7 deletions(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 29c4a81eb..f2d2f0106 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -2481,7 +2481,6 @@ static int f2fs_read_single_page(struct inode *inode, struct fsverity_info *vi,
 got_it:
 	if ((map->m_flags & F2FS_MAP_MAPPED)) {
 		block_nr = map->m_pblk + block_in_file - map->m_lblk;
-		folio_set_mappedtodisk(folio);
 
 		if (!f2fs_is_valid_blkaddr(F2FS_I_SB(inode), block_nr,
 						DATA_GENERIC_ENHANCE_READ)) {
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index fc959cdb9..d680f6f8a 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -225,12 +225,6 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 	/* wait for GCed page writeback via generic cache */
 	f2fs_wait_on_block_writeback(inode, dn.data_blkaddr);
 
-	/*
-	 * check to see if the page is mapped already (no holes)
-	 */
-	if (folio_test_mappedtodisk(folio))
-		goto out_sem;
-
 	/* page is wholly or partially inside EOF */
 	if (((loff_t)(folio->index + 1) << PAGE_SHIFT) > isize) {
 		loff_t offset;
-- 
2.43.0