From: Qu Wenruo Date: Mon, 20 Jul 2026 09:49:41 +0000 (+0930) Subject: btrfs: disable large folios for systems with highmem X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=7f03a417fc75fbe31a06199d8b609b8f313b5301;p=thirdparty%2Fkernel%2Flinux.git 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 Reported-by: Erhard Furtner Fixes: 9bce95edb1b4 ("btrfs: move large data folios out of experimental features") Reviewed-by: Jeff Layton Signed-off-by: Qu Wenruo Reviewed-by: David Sterba Signed-off-by: David Sterba --- diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c index 36332df9a0f1..87385b97c81b 100644 --- a/fs/btrfs/disk-io.c +++ b/fs/btrfs/disk-io.c @@ -3468,7 +3468,15 @@ 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->stripesize = stripesize; fs_info->fs_devices->fs_info = fs_info;