Re: [PATCH v6] fat: additions to support fat_fallocate

[Namjae Jeon]

2013/9/23, OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>:
> Namjae Jeon <linkinjeon@xxxxxxxxx> writes:
>
>>>> +	if (MSDOS_I(inode)->mmu_private > round_up(i_size, sb->s_blocksize)
>>>> +			&& pos > i_size) {
>>>> +		err = fat_zero_falloc_area(file, mapping, pos);
>>>> +		if (err) {
>>>> +			fat_msg(sb, KERN_ERR,
>>>> +				"Error (%d) zeroing fallocated area", err);
>>>> +			return err;
>>>> +		}
>>>> +	}
>>>
>>> Again, I'm not fan of this way.
>>>
>>> Normally, get_block() returns with buffer_new(). Then, caller checks
>>> blockdev buffer with
>>>
>>> 	unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
>>>
>>> then, zeroed buffer. Do we really don't need to check this race?
>> We considered after your advice before. we reach for the conclusion
>> that use this method.
>> because, Cluster is already allocated in fat fallocate and
>> when we write with radom offset over i_size on fallocated region, It
>> will be hit by fat cache in fat_bmap of get_block, which mean buffer
>> is not set to new.
>
> Hm, how does it hit to fat cache? I think fat_alloc_clusters() and
> fat_chain_add() doesn't update fat cache, right? I.e. initial write
> after fallocate() should not hit fat cache over i_size?

Ah.. Sorry for wrong reply. old memory make me confusing.
By allocating cluster in fat fallocate, when write, fat_bmap of
get_block return physical sector number.
So buffer is not set to new in _fat_get_block.

When we fallocate with keep size on -> only clusters are added to the
fat chain calling fat_get_cluster(),and add the cluster to cluster
chain.
This doesnât call fat_get_block() .

Now when we try to write in the fallocated region in the
fat_write_begin() when it is called first time it checks that the
mismatch is present between the mmu_private and mmu_actual (i.e., the
file has pre-allocated blocks), and hence zero out the region ;
Since buffer_new() is not set for fallocated region by fat_get_block()
, we explicitly  zero out the lseek'ed region using
âfat_zero_falloc_areaâ and normal write occurs beyond that,and i_size
is updated accordingly.

Thanks :)
>
> Thanks.
> --
> OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
>
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