Re: [PATCH 1/2] ext4: reduce computation of overhead during resize

["Kiselev, Oleg" <okiselev@amazon.com>]

Thanks for the review, Jan.

> On Jul 14, 2022, at 6:46 AM, Jan Kara <jack@suse.cz> wrote:
> 
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> 
> 
> 
> On Thu 30-06-22 02:17:21, Kiselev, Oleg wrote:
>> This patch avoids doing an O(n**2)-complexity walk through every flex group.
>> Instead, it uses the already computed overhead information for the newly
>> allocated space, and simply adds it to the previously calculated
>> overhead stored in the superblock.  This drastically reduces the time
>> taken to resize very large bigalloc filesystems (from 3+ hours for a
>> 64TB fs down to milliseconds).
>> 
>> Signed-off-by: Oleg Kiselev <okiselev@amazon.com>
>> ---
>> fs/ext4/resize.c | 20 +++++++++++++++++++-
>> 1 file changed, 19 insertions(+), 1 deletion(-)
> 
> Overall this looks fine, a few smaller comments below.
> 
>> 
>> diff --git a/fs/ext4/resize.c b/fs/ext4/resize.c
>> index 8b70a4701293..2acc9fca99ea 100644
>> --- a/fs/ext4/resize.c
>> +++ b/fs/ext4/resize.c
>> @@ -1380,6 +1380,16 @@ static int ext4_setup_new_descs(handle_t *handle, struct super_block *sb,
>>      return err;
>> }
>> 
>> +static void ext4_set_overhead(struct super_block *sb,
>> +                             const ext4_grpblk_t overhead)
>> +{
> 
> ext4_add_overhead() would be a better name I suppose. Also the 'overhead'
> should rather be ext4_fsblk_t to be on the safe side...

Done.

> 
>> +       struct ext4_sb_info *sbi = EXT4_SB(sb);
>> +       struct ext4_super_block *es = sbi->s_es;
> 
> Empty line between variable declarations and the code please.

Done

> 
>> +       sbi->s_overhead += overhead;
>> +       es->s_overhead_clusters = cpu_to_le32((unsigned long) sbi->s_overhead);
>                                                ^^^ the typecast looks
> bogus here...

This cast is the reverse of le32_to_cpu() cast done in fs/ext4/super.c:__ext4_fill_super():
        sbi->s_overhead = le32_to_cpu(es->s_overhead_clusters);
And follows the logic of casting done in fs/ext4/ioctl.c:set_overhead() and fs/ext4/ioctl.c:ext4_update_overhead(). 

> 
>> +       smp_wmb();
>> +}
> 
> The barrier without any comment makes me really wonder why it is here...
> But I get ext4_calculate_overhead() has is as well so you're just keeping
> it.

Yes, exactly.  I am aware of only one place where s_overhead and s_overhead_clusters have to be coherent, in ext4_update_overhead(), which potentially may be called concurrently with the resize.  Keeping the write barrier here seemed like a safe choice, since it’s not in the regular IO path, so the added expense is incurred only very rarely. 

> 
>> +
>> /*
>>  * ext4_update_super() updates the super block so that the newly added
>>  * groups can be seen by the filesystem.
>> @@ -1482,8 +1492,16 @@ static void ext4_update_super(struct super_block *sb,
>> 
>>      /*
>>       * Update the fs overhead information
>> +      *
>> +      * For bigalloc, if the superblock already has a properly calculated
>> +      * overhead, update it wth a value based on numbers already computed
>                                ^^ with
> 
>> +      * above for the newly allocated capacity.
>>       */
>> -     ext4_calculate_overhead(sb);
>> +     if (ext4_has_feature_bigalloc(sb) && (sbi->s_overhead != 0))
>> +             ext4_set_overhead(sb,
>> +                     EXT4_NUM_B2C(sbi, blocks_count - free_blocks));
>> +     else
>> +             ext4_calculate_overhead(sb);
>> 
>>      if (test_opt(sb, DEBUG))
>>              printk(KERN_DEBUG "EXT4-fs: added group %u:"
> 
>                                                                Honza
> --
> Jan Kara <jack@suse.com>
> SUSE Labs, CR

I’ll resubmit the updated patch.