linux-kernel - Re: [FYI] tux3: Core changes

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Date:	Wed, 20 May 2015 15:51:11 -0700
From:	Daniel Phillips <daniel@...nq.net>
To:	Rik van Riel <riel@...hat.com>, Jan Kara <jack@...e.cz>,
	David Lang <david@...g.hm>
CC:	tux3@...3.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: Re: [FYI] tux3: Core changes



On 05/20/2015 12:53 PM, Rik van Riel wrote:
> On 05/20/2015 12:22 PM, Daniel Phillips wrote:
>> On 05/20/2015 07:44 AM, Jan Kara wrote:
>>> On Tue 19-05-15 13:33:31, David Lang wrote:
> 
>>>   Yeah, that's what I meant. If you create a function which manipulates
>>> page cache, you better make it work with other functions manipulating page
>>> cache. Otherwise it's a landmine waiting to be tripped by some unsuspecting
>>> developer. Sure you can document all the conditions under which the
>>> function is safe to use but a function that has several paragraphs in front
>>> of it explaning when it is safe to use isn't very good API...
>>
>> Violent agreement, of course. To put it in concrete terms, each of
>> the page fork support functions must be examined and determined
>> sane. They are:
>>
>>  * cow_replace_page_cache
>>  * cow_delete_from_page_cache
>>  * cow_clone_page
>>  * page_cow_one
>>  * page_cow_file
>>
>> Would it be useful to drill down into those, starting from the top
>> of the list?
> 
> How do these interact with other page cache functions, like
> find_get_page() ?

Nicely:

   https://github.com/OGAWAHirofumi/linux-tux3/blob/hirofumi/fs/tux3/filemap_mmap.c#L182

> How does tux3 prevent a user of find_get_page() from reading from
> or writing into the pre-COW page, instead of the current page?

Careful control of the dirty bits (we have two of them, one each
for front and back). That is what pagefork_for_blockdirty is about.

Regards,

Daniel

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