Re: [PATCH v2 3/7] block: take i_rwsem for the splice read path

[Hannes Reinecke]

On 8/28/26 3:49 PM, Tal Zussman wrote:
> def_blk_fops wires ->splice_read directly to filemap_splice_read(),
> which allocates folios based on mapping_min_folio_order() without any
> lock against set_blocksize(). A splice from a block device can race
> set_blocksize() raising the minimum folio order and insert a folio that
> is too small for the mapping. blkdev_read_iter() wraps filemap_read()
> in inode_lock_shared() for this reason, but the splice path was missed.
>
> Splicing from a block device while toggling the block size between 512
> bytes and 64K with BLKBSZSET hits this within seconds on a
> CONFIG_DEBUG_VM kernel:
>
>    page dumped because: VM_BUG_ON_FOLIO(folio_order(folio) < mapping_min_folio_order(mapping))
>    kernel BUG at mm/filemap.c:858!
>    Oops: invalid opcode: 0000 [#1] SMP NOPTI
>    RIP: 0010:__filemap_add_folio+0x51c/0x570
>    Call Trace:
>     filemap_add_folio+0x64/0x140
>     page_cache_ra_order+0x1dd/0x3d0
>     filemap_get_pages+0x153/0x760
>     filemap_splice_read+0x13f/0x300
>     splice_file_to_pipe+0xc0/0xd0
>     do_splice+0x6a8/0x890
>     __do_splice+0xb0/0x210
>     __x64_sys_splice+0x80/0x100
>     do_syscall_64+0x10e/0x520
>     entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> Take inode_lock_shared() around filemap_splice_read(), like the read
> path does.
>
> Fixes: 3c20917120ce ("block/bdev: enable large folio support for large logical block sizes")
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> ---
>   block/fops.c | 18 +++++++++++++++++-
>   1 file changed, 17 insertions(+), 1 deletion(-)
Reviewed-by: Hannes Reinecke <hare@xxxxxxxxxx>

Cheers,

Hannes
--
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@xxxxxxx                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich