linux-ext4 - Re: [PATCH v2] ext4: Allow to change s_last_trim

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Date:   Wed, 3 Nov 2021 10:22:57 +0100
From:   Lukas Czerner <lczerner@...hat.com>
To:     Andreas Dilger <adilger@...ger.ca>
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 Tue, Nov 02, 2021 at 04:44:30PM -0800, Andreas Dilger wrote:
> 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).

Indeed I was asking myself the same question, but I didn't have the need
to change it. There does not seem to be any good reason for it to be
atomic_t.

> 
> 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?

Sure I can change it. Although the s_clusters_per_group in sbi structure
is unsigned long and that's what we check it against.

Thanks!
-Lukas


> 
> 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
> > 
>