[PATCH] btrfs: disable large folios for systems with highmem

[BUG]
There is a bug report that on 32bit systems (i686), btrfs crashes when
trying to do zstd compression:

 BUG: unable to handle page fault for address: fffbc000
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 CPU: 0 UID: 0 PID: 61 Comm: kworker/u8:5 Tainted: G                 N  7.2.0-rc3-P3 #2 PREEMPTLAZY
 Hardware name: LENOVO 2007F2G/2007F2G, BIOS 79ETE7WW (2.27 ) 03/21/2011
 Workqueue: btrfs-delalloc btrfs_work_helper
 EIP: ZSTD_compressStream2+0x221/0x5fc
 Call Trace:
  ZSTD_compressStream+0xd/0x48
  zstd_compress_stream+0x8/0x10
  zstd_compress_bio+0x20a/0x564
  btrfs_compress_bio+0x94/0xc0
  compress_file_range+0x20a/0x380
  btrfs_work_helper+0xc1/0x1b4
  process_scheduled_works+0x15f/0x204
  worker_thread+0x10c/0x178
  kthread+0xe1/0xe8
  ret_from_fork+0x1d/0x14c
  ret_from_fork_asm+0x12/0x18
  entry_INT80_32+0xf0/0xf0
 CR2: 00000000fffbc000
 ---[ end trace 0000000000000000 ]---

[CAUSE]
Inside zstd_compress_bio(), we assume the whole page cache folio can be
mapped in one go.

However that assumption is not true on systems with CONFIG_HIGHMEM, the
pages of the large folio can be in HIGHMEM, which needs to be mapped
before access.

Meanwhile zstd_compress_bio() only map the page of a large folio where
the start filepos is, the remaining pages are not mapped, and accessing
the remaining pages will trigger the above crash.

[FIX]
Do not enable large folios when the kernel has CONFIG_HIGHMEM enabled.

This is the same handling for bs > ps support.

Link: https://github.com/kdave/btrfs-progs/issues/1146
Fixes: 9bce95edb1b4 ("btrfs: move large data folios out of experimental features")
Signed-off-by: Qu Wenruo <[email protected]>
---
 fs/btrfs/disk-io.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 37fc0d6b960d..5de8210070eb 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3477,7 +3477,15 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 	fs_info->sectorsize = sectorsize;
 	fs_info->sectorsize_bits = ilog2(sectorsize);
 	fs_info->block_min_order = ilog2(round_up(sectorsize, PAGE_SIZE) >> PAGE_SHIFT);
-	fs_info->block_max_order = calc_block_max_order(fs_info->sectorsize_bits);
+	/*
+	 * For HIGHMEM, a large folio cannot be mapped in one go, breaking a lot
+	 * of basic assumptions for btrfs IOs.
+	 * Disable large folios for such 32-bit systems.
+	 */
+	if (IS_ENABLED(CONFIG_HIGHMEM))
+		fs_info->block_max_order = fs_info->block_min_order;
+	else
+		fs_info->block_max_order = calc_block_max_order(fs_info->sectorsize_bits);
 	fs_info->csums_per_leaf = BTRFS_MAX_ITEM_SIZE(fs_info) / fs_info->csum_size;
 	fs_info->fs_devices->fs_info = fs_info;
 
-- 
2.54.0