linux-kernel - Re: [PATCH v3] fat: editions to support fat

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Date:	Tue, 12 Mar 2013 18:38:42 +0900
From:	Namjae Jeon <linkinjeon@...il.com>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, abartlet@...ba.org,
	Namjae Jeon <namjae.jeon@...sung.com>,
	Ravishankar N <ravi.n1@...sung.com>
Subject: Re: [PATCH v3] fat: editions to support fat_fallocate

2013/3/12, OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>:
> Namjae Jeon <linkinjeon@...il.com> writes:
>
>>> If so, probably, I didn't clear my opinion/suggestion, or misled
>>> you. Sorry about it.
>>>
>>> My opinion/suggestion is, "it should be before umount()".
>>> I.e. fallocate() doesn't have any affect to FAT on clean state (clean
>>> umount).
>>>
>>> To clear my state, I don't have strong opinion about implementation yet.
>>> For example, about ->release() or ->evict_inode().
>>>
>>> So, if you had reason to use ->release() over "we discussed", it would
>>> be good. (Or, if you still didn't have reasons, we would be better to
>>> think about it)
>> We considered all the possible points where we can release the
>> pre-allocated blocks.
>> As the "pre-allocation" is file based, we think it need to have
>> release mechanism.
>>
>> In case of evict, Eviction will either happen when the file is removed
>> or the inode is evicted from the cache by memory pressure when no
>> references are present for that file.
>> It is good that evict will be triggered in umount. but there is a risk
>> that umount time can be increased.
>> And It show users inconsistent result. e.g sometime user can not get
>> file discarded fallocated blocks by memory pressure.
>>
>> Let me know your opinion.
>
> Yes. My personal opinion is almost same, but I see some trade-off, and
> it is why I can't decide easily.
>
> Although I don't care about inconsistency on umount time. Because
> inconsistency window is opening while user is holding reference to
> inode, the window can already be enough big, and inconsistent only
> happens with crash.  And if crashed, after all, both of ->release and
> ->evict requires fsck to recover FS.
>
> Possibly longer umount time (batch modify) and longer close time (on
> demand modify) are simply trade-off.
>
> Predictable behavior would matter. Yes, evict time doesn't guarantee to
> fallocate() is still available, however evict can be able to say it just
> guarantees to available fallocated blocks between open() and
> close(). Users have to call fallocate() again after close(), but
> fallocate() may skip actual allocation if blocks still available.
>
> I see optimization window with this evict behavior, because it can do
> batch discard of fallocated blocks on multiple files, and update FAT at
> once.
>
> However, ->release() would be simpler, and would more predictable.
Yes, agreed so let’s prepare the solution with freeing blocks part of ->release.
This will also make solution align with the concept of normal default
reserved block allocation like ext2/XFS in which it allocates extra
blocks at initial write request and extra blocks are freed in
->release.
Once I will share the patch with all the review comments.

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