Re: [PATCH RESEND v5] fat: editions to support fat_fallocate

[OGAWA Hirofumi]

OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> writes:

> Namjae Jeon <linkinjeon@xxxxxxxxx> writes:
>
>>> Then, per-file discard fallocate space sounds like wrong. fallocate
>>> space probably is inode attribute.
>> Since, our preallocation will not be persistent after umount. So, we
>> need to free up the space at some point.
>> If we consider for normal pre-allocation in ext4, in that case also
>> the blocks are removed in ext4_release_file when the last writer
>> closes the file.
>>
>> ext4_release_file()
>> {
>> ...
>> /* if we are the last writer on the inode, drop the block reservation */
>> 	if ((filp->f_mode & FMODE_WRITE) &&
>> 			(atomic_read(&inode->i_writecount) == 1) &&
>> 		        !EXT4_I(inode)->i_reserved_data_blocks)
>> 	{
>> 		down_write(&EXT4_I(inode)->i_data_sem);
>> 		ext4_discard_preallocations(inode);
>> 		up_write(&EXT4_I(inode)->i_data_sem);
>> 	}
>>
>> So, we will need to have this per file . May be the condition for
>> checking is wrong which can be correct but the correctness points
>> should be same. We can give a thought on using "i_writecount" for
>> controlling the parallel write in FAT also.
>> how do you think ?
>
> AFAIK, preallocation != fallocate. ext*'s preallocation was there at
> before fallocation to optimize block allocation for user data blocks.
>
>>>>> I know. Question is, why do we need to initialize twice.
>>>>>
>>>>> 1) zeroed for uninitialized area, 2) then copy user data area. We need
>>>>> only either, right? This seems to be doing both for all fallocated area.
>>>> We did not initialize twice. We are using the ‘pos’ as the attribute
>>>> to define zeroing length in case of pre-allocation.
>>>> Zeroing out occurs till the ‘pos’ while actual write occur after ‘pos’.
>>>> If we file size is 100KB and we pre-allocated till 1MB. Next if we try
>>>> to write at 500KB,
>>>> Then zeroing out will occur only for 100KB->500KB, after that there
>>>> will be normal write. There is no duplication for the same space.
>>>
>>> Ah. Then write_begin() really initialize after i_size until page cache
>>> boudary for append write? I wonder if this patch works correctly for
>>> mmap.
>> Since you already provided me review comments to check truncate and
>> mmap, we checked all points for those cases.
>
> cluster size == 512b
>
> 1) create new file
> 2) fallocate 100MB
> 3) write(2) data for each 512b
>
> With this, write_begin() will be called for each 512b data. When we
> allocates new page for this file, write_begin() writes data 0-512. Then,
> we have to initialize 512-4096 by zero. Because mmap read maps 0-4096,
> even if i_size == 512.
>
> Who is initializing area for 512-4096?

>From other view, I guess fat_zero_falloc_area() is for filling zero for
0-10000, in the following case?

     1) create new file
     2) lseek(10000)
     3) write data by write(2)

This job is for cont_write_begin(). If example is correct, why
cont_write_begin() doesn't work? I guess, because get_block() doesn't
set buffer_new() for those area.

If above is correct, right implement to change get_block().

Thanks.
-- 
OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
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