md: fix resync softlockup when bitmap size is less than array size

Is is reported that for dm-raid10, lvextend + lvchange --syncaction will
trigger following softlockup:

kernel:watchdog: BUG: soft lockup - CPU#3 stuck for 26s! [mdX_resync:6976]
CPU: 7 PID: 3588 Comm: mdX_resync Kdump: loaded Not tainted 6.9.0-rc4-next-20240419 #1
RIP: 0010:_raw_spin_unlock_irq+0x13/0x30
Call Trace:
 <TASK>
 md_bitmap_start_sync+0x6b/0xf0
 raid10_sync_request+0x25c/0x1b40 [raid10]
 md_do_sync+0x64b/0x1020
 md_thread+0xa7/0x170
 kthread+0xcf/0x100
 ret_from_fork+0x30/0x50
 ret_from_fork_asm+0x1a/0x30

And the detailed process is as follows:

md_do_sync
 j = mddev->resync_min
 while (j < max_sectors)
  sectors = raid10_sync_request(mddev, j, &skipped)
   if (!md_bitmap_start_sync(..., &sync_blocks))
    // md_bitmap_start_sync set sync_blocks to 0
    return sync_blocks + sectors_skippe;
  // sectors = 0;
  j += sectors;
  // j never change

Root cause is that commit 301867b1 ("md/raid10: check
slab-out-of-bounds in md_bitmap_get_counter") return early from
md_bitmap_get_counter(), without setting returned blocks.

Fix this problem by always set returned blocks from
md_bitmap_get_counter"(), as it used to be.

Noted that this patch just fix the softlockup problem in kernel, the
case that bitmap size doesn't match array size still need to be fixed.

Fixes: 301867b1 ("md/raid10: check slab-out-of-bounds in md_bitmap_get_counter")
Reported-and-tested-by: Nigel Croxon <ncroxon@redhat.com>
Closes: https://lore.kernel.org/all/71ba5272-ab07-43ba-8232-d2da642acb4e@redhat.com/Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20240422065824.2516-1-yukuai1@huaweicloud.comSigned-off-by: Song Liu <song@kernel.org>

drivers/md/md-bitmap.c

@@ -1424,7 +1424,7 @@ __acquires(bitmap->lock)
 	sector_t chunk = offset >> bitmap->chunkshift;
 	unsigned long page = chunk >> PAGE_COUNTER_SHIFT;
 	unsigned long pageoff = (chunk & PAGE_COUNTER_MASK) << COUNTER_BYTE_SHIFT;
-	sector_t csize;
+	sector_t csize = ((sector_t)1) << bitmap->chunkshift;
 	int err;
 
 	if (page >= bitmap->pages) {
@@ -1433,6 +1433,7 @@ __acquires(bitmap->lock)
 		 * End-of-device while looking for a whole page or
 		 * user set a huge number to sysfs bitmap_set_bits.
 		 */
+		*blocks = csize - (offset & (csize - 1));
 		return NULL;
 	}
 	err = md_bitmap_checkpage(bitmap, page, create, 0);
@@ -1441,8 +1442,7 @@ __acquires(bitmap->lock)
 	    bitmap->bp[page].map == NULL)
 		csize = ((sector_t)1) << (bitmap->chunkshift +
 					  PAGE_COUNTER_SHIFT);
-	else
-		csize = ((sector_t)1) << bitmap->chunkshift;
+
 	*blocks = csize - (offset & (csize - 1));
 
 	if (err < 0)