Re: [PATCH for-6.10 1/2] dm-crypt: stop constraining max_segment_size to PAGE_SIZE

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 11 Apr 2024, Mike Snitzer wrote:

> This change effectively reverts commit 586b286b110e ("dm crypt:
> constrain crypt device's max_segment_size to PAGE_SIZE") and relies on
> block core's late bio-splitting to ensure that dm-crypt's encryption
> bios are split accordingly if they exceed the underlying device's
> limits (e.g. max_segment_size).
> 
> Commit 586b286b110e was applied as a 4.3 fix for the benefit of
> stable@ kernels 4.0+ just after block core's late bio-splitting was
> introduced in 4.3 with commit 54efd50bfd873 ("block: make
> generic_make_request handle arbitrarily sized bios"). Given block
> core's late bio-splitting it is past time that dm-crypt make use of
> it.
> 
> Also, given the recent need to revert meaningful progress that was
> attempted during the 6.9 merge window (see commit bff4b74625fe Revert
> "dm: use queue_limits_set") this change allows DM core to safely make
> use of queue_limits_set() without risk of breaking dm-crypt on NVMe.
> Though it should be noted this commit isn't a prereq for reinstating
> DM core's use of queue_limits_set() because blk_validate_limits() was
> made less strict with commit b561ea56a264 ("block: allow device to
> have both virt_boundary_mask and max segment size").
> 
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>

> ---
>  drivers/md/dm-crypt.c | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> index 5bfa35760167..f43a2c0b3d77 100644
> --- a/drivers/md/dm-crypt.c
> +++ b/drivers/md/dm-crypt.c
> @@ -1656,8 +1656,8 @@ static void crypt_free_buffer_pages(struct crypt_config *cc, struct bio *clone);
>  
>  /*
>   * Generate a new unfragmented bio with the given size
> - * This should never violate the device limitations (but only because
> - * max_segment_size is being constrained to PAGE_SIZE).
> + * This should never violate the device limitations (but if it did then block
> + * core should split the bio as needed).
>   *
>   * This function may be called concurrently. If we allocate from the mempool
>   * concurrently, there is a possibility of deadlock. For example, if we have
> @@ -3717,14 +3717,6 @@ static void crypt_io_hints(struct dm_target *ti, struct queue_limits *limits)
>  {
>  	struct crypt_config *cc = ti->private;
>  
> -	/*
> -	 * Unfortunate constraint that is required to avoid the potential
> -	 * for exceeding underlying device's max_segments limits -- due to
> -	 * crypt_alloc_buffer() possibly allocating pages for the encryption
> -	 * bio that are not as physically contiguous as the original bio.
> -	 */
> -	limits->max_segment_size = PAGE_SIZE;
> -
>  	limits->logical_block_size =
>  		max_t(unsigned int, limits->logical_block_size, cc->sector_size);
>  	limits->physical_block_size =
> -- 
> 2.40.0
>