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Date:   Tue, 2 Nov 2021 16:44:30 -0800
From:   Andreas Dilger <adilger@...ger.ca>
To:     Lukas Czerner <lczerner@...hat.com>
Cc:     linux-ext4@...r.kernel.org, tytso@....edu,
        Laurent GUERBY <laurent@...rby.net>
Subject: Re: [PATCH v2] ext4: Allow to change s_last_trim_minblks via sysfs

On Nov 2, 2021, at 06:19, Lukas Czerner <lczerner@...hat.com> 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@...hat.com>
> Reported-by: Laurent GUERBY <laurent@...rby.net>

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
> 

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