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From:   Andreas Dilger <adilger@dilger.ca>
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Subject: Re: [PATCH v2] ext4: Allow to change s_last_trim_minblks via sysfs
Date:   Tue, 2 Nov 2021 16:44:30 -0800
Message-Id: <5E30A795-F9C1-4865-8703-75940F918A88@dilger.ca>
References: <20211102141902.9808-1-lczerner@redhat.com>
Cc:     linux-ext4@vger.kernel.org, tytso@mit.edu,
        Laurent GUERBY <laurent@guerby.net>
In-Reply-To: <20211102141902.9808-1-lczerner@redhat.com>
To:     Lukas Czerner <lczerner@redhat.com>
Precedence: bulk

On Nov 2, 2021, at 06:19, Lukas Czerner <lczerner@redhat.com> wrote:
>=20
> =EF=BB=BFExt4 has an optimization mechanism for batched disacrd (FITRIM) t=
hat
> should help speed up subsequent calls of FITRIM ioctl by skipping the
> groups that were previously trimmed. However because the FITRIM allows
> to set the minimum size of an extent to trim, ext4 stores the last
> minimum extent size and only avoids trimming the group if it was
> previously trimmed with minimum extent size equal to, or smaller than
> the current call.
>=20
> There is currently no way to bypass the optimization without
> umount/mount cycle. This becomes a problem when the file system is
> live migrated to a different storage, because the optimization will
> prevent possibly useful discard calls to the storage.
>=20
> Fix it by exporting the s_last_trim_minblks via sysfs interface which
> will allow us to set the minimum size to the number of blocks larger
> than subsequent FITRIM call, effectively bypassing the optimization.
>=20
> By setting the s_last_trim_minblks to INT_MAX the optimization will be
> effectively cleared regardless of the previous state, or file system
> configuration.
>=20
> For example:
> echo 2147483647 > /sys/fs/ext4/dm-1/last_trim_minblks
>=20
> Signed-off-by: Lukas Czerner <lczerner@redhat.com>
> Reported-by: Laurent GUERBY <laurent@guerby.net>

The patch itself looks fine, but it isn't really clear why this field is ato=
mic_t?
There is no particular data integrity needed for this field between threads
and it is not harmful if it is set or read in a racy manner (at most it is s=
et at
the end of ext4_trim_fs() once per filesystem trim).

So it seems like this could be a __u32 (like s_clusters_per_group, which is
the largest possible value it can store)
and avoid the need to add a new type
to the sysfs attribute handling?

Cheers, Andreas


> ---
> v2: Remove unnecessary assignment
>=20
> fs/ext4/sysfs.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>=20
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index 2314f7446592..94c86eb8d3cc 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -187,6 +187,9 @@ static struct ext4_attr ext4_attr_##_name =3D {       =
     \
> #define EXT4_RO_ATTR_SBI_ATOMIC(_name,_elname)    \
>    EXT4_ATTR_OFFSET(_name, 0444, pointer_atomic, ext4_sb_info, _elname)
>=20
> +#define EXT4_RW_ATTR_SBI_ATOMIC(_name,_elname)    \
> +    EXT4_ATTR_OFFSET(_name, 0644, pointer_atomic, ext4_sb_info, _elname)
> +
> #define EXT4_ATTR_PTR(_name,_mode,_id,_ptr) \
> static struct ext4_attr ext4_attr_##_name =3D {            \
>    .attr =3D {.name =3D __stringify(_name), .mode =3D _mode },    \
> @@ -245,6 +248,7 @@ EXT4_ATTR(last_error_time, 0444, last_error_time);
> EXT4_ATTR(journal_task, 0444, journal_task);
> EXT4_RW_ATTR_SBI_UI(mb_prefetch, s_mb_prefetch);
> EXT4_RW_ATTR_SBI_UI(mb_prefetch_limit, s_mb_prefetch_limit);
> +EXT4_RW_ATTR_SBI_ATOMIC(last_trim_minblks, s_last_trim_minblks);
>=20
> static unsigned int old_bump_val =3D 128;
> EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
> @@ -295,6 +299,7 @@ static struct attribute *ext4_attrs[] =3D {
> #endif
>    ATTR_LIST(mb_prefetch),
>    ATTR_LIST(mb_prefetch_limit),
> +    ATTR_LIST(last_trim_minblks),
>    NULL,
> };
> ATTRIBUTE_GROUPS(ext4);
> @@ -474,6 +479,14 @@ static ssize_t ext4_attr_store(struct kobject *kobj,
>            return ret;
>        *((unsigned long *) ptr) =3D t;
>        return len;
> +    case attr_pointer_atomic:
> +        if (!ptr)
> +            return 0;
> +        ret =3D kstrtoint(skip_spaces(buf), 0, (int *)&t);
> +        if (ret)
> +            return ret;
> +        atomic_set((atomic_t *) ptr, t);
> +        return len;
>    case attr_inode_readahead:
>        return inode_readahead_blks_store(sbi, buf, len);
>    case attr_trigger_test_error:
> --=20
> 2.31.1
>=20