linux-kernel - Re: [PATCH] sysctl: Add a feature to drop caches selectively

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Date:	Wed, 25 Jun 2014 16:42:08 +0300
From:	Artem Bityutskiy <dedekind1@...il.com>
To:	Thomas Knauth <thomas.knauth@....de>
Cc:	David Rientjes <rientjes@...gle.com>,
	Maksym Planeta <mcsim.planeta@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: Add a feature to drop caches selectively

On Wed, 2014-06-25 at 15:23 +0200, Thomas Knauth wrote:
> On Wed, Jun 25, 2014 at 11:56 AM, Artem Bityutskiy <dedekind1@...il.com> wrote:
> > Thanks for the answer, although you forgot to comment on the question
> > about possibly extending the new interface to work with file ranges in
> > the future. For example, I have a 2 TiB file, and I am only interested
> > in dropping caches for the first couple of gigabytes. Would I extend
> > your interface, or would I come up with another one?
> 
> Ah, didn't quite understand what was meant with file ranges. Again, we
> had not considered this so far. I guess you could make a distinction
> between directories and files here. If the path points to a file, you
> can have an optional argument indicating the range of bytes you would
> like to drop. Something like
> 
> echo "my-file 0-1000,8000-1000" > /proc/sys/vm/sdrop_cache
> 
> If this is desirable, we can add it to the patch.

With a binary interface like an ioctl I can see how you could have extra
unused fields which you can ignore now and let people start adding extra
options like the range in the future.

With this kind of interface I am not sure how to do this.

Other questions I'd ask would be - how about the access control model?
Will only root be able to drop caches? Why can't I drop caches for my
own file?

I did not put much thinking into this, but it looks like ioctl could be
a better interface for the task you are trying to solve...

Sorry if I am a bit vague, I am mostly trying to make you guys give this
more thoughts, and come up with a deeper analysis. Interfaces are very
important to get right, or as right as possible...

-- 
Best Regards,
Artem Bityutskiy

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