Re: [PATCH v2 3/4] btrfs: add checksum implementation selection after mount

[Anand Jain <anand.jain@oracle.com>]

On 02/08/2022 20:28, David Sterba wrote:
> The sysfs file /sys/fs/btrfs/FSID/checksum shows the filesystem checksum
> and the crypto module implementing it.


> In the scenario when there's only
> the default generic implementation available during mount it's selected,
> even if there's an unloaded module with accelerated version.

> This happens with sha256 that's often built-in so the crypto API may not
> trigger loading the modules and select the fastest implementation.

Hmm ok.
What happens in the scenario if the accelerated module is loaded? Will 
the crypto API use the accelerated module?

- Anand

> Such
> filesystem could be left using in the generic for the whole time.
> Remount can't fix that and full umount/mount cycle may be impossible eg.
> for a root filesystem.
> 
> Allow writing strings to the sysfs checksum file that will trigger
> loading the crypto shash again and check if the found module is the
> desired one.
> 
> Possible values:
> 
> - default - select whatever is considered default by crypto subsystem,
>              either generic or accelerated
> - accel   - try loading an accelerated implementation (must not contain
>              "generic" in the name)
> - generic - load and select the generic implementation
> 
> A typical scenario, assuming sha256 is built-in:
> 
>    $ mkfs.btrfs --csum sha256
>    $ lsmod | grep sha256
>    $ mount /dev /mnt
>    $ cat ...FSID/checksum
>    sha256 (sha256-generic)
>    $ modprobe sha256
>    $ lsmod | grep sha256
>    sha256_ssse3
>    $ echo accel > ...FSID/checksum
>    sha256 (sha256-ni)
> 
> The crypto shash for all slots has the same lifetime as the mount, so
> it's not possible to unload the crypto module.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>   fs/btrfs/sysfs.c | 86 +++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 85 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/sysfs.c b/fs/btrfs/sysfs.c
> index cc943b236c92..0044644056ed 100644
> --- a/fs/btrfs/sysfs.c
> +++ b/fs/btrfs/sysfs.c
> @@ -1100,7 +1100,91 @@ static ssize_t btrfs_checksum_show(struct kobject *kobj,
>   			  crypto_shash_driver_name(fs_info->csum_shash[CSUM_DEFAULT]));
>   }
>   
> -BTRFS_ATTR(, checksum, btrfs_checksum_show);
> +static const char csum_impl[][8] = {
> +	[CSUM_DEFAULT]	= "default",
> +	[CSUM_GENERIC]	= "generic",
> +	[CSUM_ACCEL]	= "accel",
> +};
> +
> +static int select_checksum(struct btrfs_fs_info *fs_info, enum btrfs_csum_impl type)
> +{
> +	/* Already set */
> +	if (fs_info->csum_shash[CSUM_DEFAULT] == fs_info->csum_shash[type])
> +		return 0;
> +
> +	/* Allocate new if needed */
> +	if (!fs_info->csum_shash[type]) {
> +		const u16 csum_type = btrfs_super_csum_type(fs_info->super_copy);
> +		const char *csum_driver = btrfs_super_csum_driver(csum_type);
> +		struct crypto_shash *shash1, *tmp;
> +		char full_name[32];
> +		u32 mask = 0;
> +
> +		/*
> +		 * Generic must be requested explicitly, otherwise it could
> +		 * be accelerated implementation with highest priority.
> +		 */
> +		scnprintf(full_name, sizeof(full_name), "%s%s", csum_driver,
> +			  (type == CSUM_GENERIC ? "-generic" : ""));
> +
> +		shash1 = crypto_alloc_shash(full_name, 0, mask);
> +		if (IS_ERR(shash1))
> +			return PTR_ERR(shash1);
> +
> +		/* Accelerated requested but generic found */
> +		if (type != CSUM_GENERIC &&
> +		    strstr(crypto_shash_driver_name(shash1), "generic")) {
> +			crypto_free_shash(shash1);
> +			return -ENOENT;
> +		}
> +
> +		tmp = cmpxchg(&fs_info->csum_shash[type], NULL, shash1);
> +		if (tmp) {
> +			/* Something raced in */
> +			crypto_free_shash(shash1);
> +		}
> +	}
> +
> +	/* Select it */
> +	fs_info->csum_shash[CSUM_DEFAULT] = fs_info->csum_shash[type];
> +	return 0;
> +}
> +
> +static bool strmatch(const char *buffer, const char *string);
> +
> +static ssize_t btrfs_checksum_store(struct kobject *kobj,
> +				    struct kobj_attribute *a,
> +				    const char *buf, size_t len)
> +{
> +	struct btrfs_fs_info *fs_info = to_fs_info(kobj);
> +
> +	if (!fs_info)
> +		return -EPERM;
> +
> +	/* Pick the best available, generic or accelerated */
> +	if (strmatch(buf, csum_impl[CSUM_DEFAULT])) {
> +		if (fs_info->csum_shash[CSUM_ACCEL]) {
> +			fs_info->csum_shash[CSUM_DEFAULT] =
> +				fs_info->csum_shash[CSUM_ACCEL];
> +			return len;
> +		}
> +		ASSERT(fs_info->csum_shash[CSUM_GENERIC]);
> +		fs_info->csum_shash[CSUM_DEFAULT] = fs_info->csum_shash[CSUM_GENERIC];
> +		return len;
> +	}
> +
> +	for (int i = 1; i < CSUM_COUNT; i++) {
> +		if (strmatch(buf, csum_impl[i])) {
> +			int ret;
> +
> +			ret = select_checksum(fs_info, i);
> +			return ret < 0 ? ret : len;
> +		}
> +	}
> +
> +	return -EINVAL;
> +}
> +BTRFS_ATTR_RW(, checksum, btrfs_checksum_show, btrfs_checksum_store);
>   
>   static ssize_t btrfs_exclusive_operation_show(struct kobject *kobj,
>   		struct kobj_attribute *a, char *buf)