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

On Mon, Jul 20, 2026 at 07:19:41PM +0930, Qu Wenruo wrote:
> [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]>

Reviewed-by: David Sterba <[email protected]>