On Nov 2, 2021, at 06:19, Lukas Czerner <lczerner@xxxxxxxxxx> wrote:
>
> Ext4 has an optimization mechanism for batched disacrd (FITRIM) that
> 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.
>
> 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.
>
> 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.
>
> 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.
>
> For example:
> echo 2147483647 > /sys/fs/ext4/dm-1/last_trim_minblks
>
> Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
> Reported-by: Laurent GUERBY <laurent@xxxxxxxxxx>
The patch itself looks fine, but it isn't really clear why this field is atomic_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 set 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
>
> fs/ext4/sysfs.c | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
> 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 = { \
> #define EXT4_RO_ATTR_SBI_ATOMIC(_name,_elname) \
> EXT4_ATTR_OFFSET(_name, 0444, pointer_atomic, ext4_sb_info, _elname)
>
> +#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 = { \
> .attr = {.name = __stringify(_name), .mode = _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);
>
> static unsigned int old_bump_val = 128;
> EXT4_ATTR_PTR(max_writeback_mb_bump, 0444, pointer_ui, &old_bump_val);
> @@ -295,6 +299,7 @@ static struct attribute *ext4_attrs[] = {
> #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) = t;
> return len;
> + case attr_pointer_atomic:
> + if (!ptr)
> + return 0;
> + ret = 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:
> --
> 2.31.1
>
