linux-kernel - Re: [PATCH v5] fat: Batched discard support for fat

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Message-ID: <AANLkTinTzj_BurexWAHa+EOt+GvpMB3Xm=GdtwdAt8i2@mail.gmail.com>
Date:	Mon, 28 Mar 2011 15:56:42 +0900
From:	Kyungmin Park <kmpark@...radead.org>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Lukas Czerner <lczerner@...hat.com>
Subject: Re: [PATCH v5] fat: Batched discard support for fat

On Mon, Mar 28, 2011 at 3:21 PM, OGAWA Hirofumi
<hirofumi@...l.parknet.co.jp> wrote:
> Kyungmin Park <kmpark@...radead.org> writes:
>
>>>> +     start = range->start >> sb->s_blocksize_bits;
>>>> +     start = start / sbi->sec_per_clus;
>>>
>>> start is round-down, I think it's strange interface. E.g. user specified
>>> the range as "start=10 len=1024". So the range should be 10-1034,
>>> i.e. (assume cluster-size is 512) 512-1024, right?
>>
>> I don't know what's the correct way? If you're right. it's better to round-up.
>> If cluster-size is 32KiB and start sector is in the middle of cluster,
>> then which is better. round-down or round-up?
>
> It depends on the design of this interface though, I bet it's round-up,
> and should use same way with other FSes.
>
>>>> +     minlen = range->minlen >> sb->s_blocksize_bits;
>>>> +     minlen = minlen / sbi->sec_per_clus;
>>>> +     trimmed = 0;
>>>> +     count = 0;
>>>> +     err = -EINVAL;
>>>> +
>>>> +     lock_fat(sbi);
>>>> +     if (sbi->free_clusters != -1 && sbi->free_clus_valid)
>>>> +             goto out;
>>>
>>> free clusters count validation doesn't matter here. If you want to check
>>> free cluster count, you should check free_clusters==0 or not (after
>>> validation).
>>
>> I borrowed it from "fat_count_free_clusters()". anyway fill fix it.
>
> Yes. fat_count_free_clusters() needs to check free_clusters value,
> because it updates free_clusters value. If it's uptodate, does nothing.
>
>>>> +     if (start < FAT_START_ENT)
>>>> +             start = FAT_START_ENT;
>>>
>>> Valid data cluster is 2 - max_cluster, but it should be mapped to 0 -
>>> (max_cluster - FAT_START_ENT). Otherwise this interface's abstraction is
>>> useless, right?
>>
>> user program don't know the filesystem internals. The same program is
>> used for ext4 and fat. so it should be handled at filesystem.
>
> Yes. It's what I'm saying. If user wants to trim 0-2 then user will
> specify 0-2, but this trims only 2. It's not right.

There's similar code at ext4. it adjusts the start and len value if
given start is less than first_data_blk.

       if (start < first_data_blk) {
                len -= first_data_blk - start;
                start = first_data_blk;
        }

>
>>>> +     fatent_set_entry(&fatent, start);
>>>> +
>>>> +     while (count < sbi->max_cluster) {
>>>> +             if (fatent.entry >= sbi->max_cluster)
>>>> +                     fatent.entry = FAT_START_ENT;
>>>
>>> Why do we cyclic this?
>> If the start is middle and len is the whole disk size, then check the
>> all clusters.
>
> Strange design. If user wants to trim between middle and end, user have
> to know length from middle correctly? Ext4 really does it?

You're right, it only check the one-direction. else return -EINVAL.

       if (first_group > last_group)
                return -EINVAL;

>
> Thanks.
> --
> OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
>
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