Re: [RFC,3/5] squashfs: remove cache for normal data page

[Minchan Kim]

On Fri, Oct 04, 2013 at 07:13:31AM +0100, Phillip Lougher wrote:
> Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >Sqsuashfs have used cache for normal data pages but it's pointless
> >because MM already has cache layer and squashfs adds extra pages
> >into MM's page cache when it reads a page from compressed block.
> >
> >This patch removes cache usage for normal data pages so it could
> >remove unnecessary one copy
> 
> 1. As I mentioned last week, the use of the "unnecessary" cache is there
> to prevent two or more processes simultaneously trying to read the same
> pages.  Without this, such racing processes will decompress the same
> blocks repeatedly.
> 
> It is easy to dismiss this as an rare event, but, when it happens it
> has a major impact on performance, because the racing processes
> can get stuck in a lock-step arrangement, repeatedly trying to access
> the same blocks until the eof.  If the file is many megabytes or
> gigabytes in size (such as when Squashfs is used as a container fs for
> cetain liveCDs, or virtual machine disk images) this will lead to
> a significant reduction in performance.
> 
> So I consider this a major regression.
> 
> 2. You patch also adds another regression, which is to reintroduce
> kmap() rather than kmap_atomic().
> 
> I was asked to remove this at my first submission attempt
> in 2005
> 
> http://lkml.indiana.edu/hypermail/linux/kernel/0503.2/0809.html
> 
> So I'm not particularly willing to reintroduce it now.
> 
> 3. Your patch potentially reintroduces this bug
> 
> http://www.spinics.net/lists/linux-fsdevel/msg02555.html
> http://zaitcev.livejournal.com/86954.html
> 
> 4. You patch is unconditional.  With such code changes as this
> it is always essential to make this a "buy in option", with the
> original behaviour retained as default.  Otherwise, lots of users
> potentially find their embedded/enterprise/mission critical
> system unexpectedly breaks when upgrading the kernel, and I get
> a lot of angry email.
> 
> Phillip

I will handle all your points in next patchset.

Thanks for the review!

-- 
Kind regards,
Minchan Kim
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