linux-kernel - Re: [PATCH 2/2] mm: add overcommit

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Date:	Wed, 21 Aug 2013 09:23:07 -0700
From:	Dave Hansen <dave.hansen@...el.com>
To:	Jerome Marchand <jmarchan@...hat.com>
CC:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: add overcommit_kbytes sysctl variable

On 08/21/2013 08:22 AM, Jerome Marchand wrote:
>> > Instead of introducing yet another tunable, why don't we just make the
>> > ratio that comes in from the user more fine-grained?
>> > 
>> > 	sysctl overcommit_ratio=0.2
>> > 
>> > We change the internal 'sysctl_overcommit_ratio' to store tenths or
>> > hundreths of a percent (or whatever), then parse the input as two
>> > integers.  I don't think we need fully correct floating point parsing
>> > and rounding here, so it shouldn't be too much of a chore.  It'd
>> > probably end up being less code than you have as it stands.
>> > 
> Now that I think about it, that could break user space. Sure write access
> wouldn't be a problem (one can still write a plain integer), but a script
> that reads a fractional value when it expects an integer might not be able
> to cope with it.

You're right.  Something doing FOO=$(cat overcommit_ratio) and then
trying do do arithmetic would just fail loudly.  But, it would probably
fail silently if we create another tunable that all of a sudden returns
0 (when the kernel is not _behaving_ like it is set to 0).

I'm not sure there's a good way out of this without breakage (or at
least confusing) of _some_ old scripts/programs.  Either way has ups and
downs.

The existing dirty_ratio/bytes stuff just annoys me because I end up
having to check two places whenever I go looking for it.

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