[PATCH 2/2] ntfs: map each LogFile page once in ntfs_check_logfile()

[Karl Mehltretter <kmehltretter@gmail.com>]

ntfs_check_logfile() calls kmap_local_folio() on every iteration of its
search loop but calls kunmap_local() only when it moves on to the next
folio. With 4 KiB pages the positions 0, 512, 1024 and 2048 are all in
the first page, so every mount leaves three entries on the kmap_local
stack of the mounting task.

64-bit kernels do not notice, kmap_local_folio() does not touch the
stack there. 32-bit kernels with HIGHMEM return from mount(2) with the
mappings in place. With the generic entry code that is a warning:

  DEBUG_LOCKS_WARN_ON(current->kmap_ctrl.idx)
  WARNING: include/linux/highmem-internal.h:17 at __do_fast_syscall_32+0x21e/0x550

The stack has 16 slots, so a process that mounts ntfs volumes five
times hits the BUG_ON() in kmap_local_idx_push() during the fifth
mount:

  kernel BUG at mm/highmem.c:480!

Map each page once, when it is read, and unmap that address.

Fixes: 5218cd102aec ("ntfs: update misc operations")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
Tested on top of patch 1 in QEMU with a fresh mkntfs volume and a small
program that calls mount(2) and umount(2) eight times in one process:
- 32-bit ARM (multi_v7_defconfig plus KCOV, virt, 1 GiB RAM, 256 MiB
  highmem) and x86-32 (i386_defconfig plus HIGHMEM4G, 2 GiB RAM): without
  this patch mounts one to four succeed and the fifth hits the BUG_ON()
  in kmap_local_idx_push() on both. With it all eight complete.
- x86-32 no longer warns when mount(2) returns. Three mounts in a row
  with mount(8) are clean on both.
The other kmap_local_folio() loops in fs/ntfs (lcnalloc.c, mft.c,
bitmap.c, compress.c) unmap on every path before they map again.

 fs/ntfs/logfile.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/fs/ntfs/logfile.c b/fs/ntfs/logfile.c
index 1404664dacc0..ce38ce87f8fc 100644
--- a/fs/ntfs/logfile.c
+++ b/fs/ntfs/logfile.c
@@ -457,7 +457,7 @@ bool ntfs_check_logfile(struct inode *log_vi, struct restart_page_header **rp)
 	struct ntfs_volume *vol = NTFS_SB(log_vi->i_sb);
 	struct address_space *mapping = log_vi->i_mapping;
 	struct folio *folio = NULL;
-	u8 *kaddr = NULL;
+	u8 *kaddr = NULL, *kbase = NULL;
 	struct restart_page_header *rstr1_ph = NULL;
 	struct restart_page_header *rstr2_ph = NULL;
 	int log_page_size, err;
@@ -511,7 +511,7 @@ bool ntfs_check_logfile(struct inode *log_vi, struct restart_page_header **rp)
 
 		if (!folio || folio->index != idx) {
 			if (folio) {
-				kunmap_local(kaddr);
+				kunmap_local(kbase);
 				folio_put(folio);
 			}
 			folio = read_mapping_folio(mapping, idx, NULL);
@@ -520,8 +520,9 @@ bool ntfs_check_logfile(struct inode *log_vi, struct restart_page_header **rp)
 						idx);
 				goto err_out;
 			}
+			kbase = kmap_local_folio(folio, 0);
 		}
-		kaddr = (u8 *)kmap_local_folio(folio, 0) + (pos & ~PAGE_MASK);
+		kaddr = kbase + (pos & ~PAGE_MASK);
 		/*
 		 * A non-empty block means the logfile is not empty while an
 		 * empty block after a non-empty block has been encountered
@@ -574,7 +575,7 @@ bool ntfs_check_logfile(struct inode *log_vi, struct restart_page_header **rp)
 		 * find a valid one further in the file.
 		 */
 		if (err != -EINVAL) {
-			kunmap_local(kaddr);
+			kunmap_local(kbase);
 			folio_put(folio);
 			goto err_out;
 		}
@@ -583,7 +584,7 @@ bool ntfs_check_logfile(struct inode *log_vi, struct restart_page_header **rp)
 			pos = NTFS_BLOCK_SIZE >> 1;
 	}
 	if (folio) {
-		kunmap_local(kaddr);
+		kunmap_local(kbase);
 		folio_put(folio);
 	}
 	if (logfile_is_empty) {
-- 
2.53.0