linux-kernel - Re: [PATCH 0/5] fix up /proc/$pid/smaps to not split huge pages

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Message-ID: <20110215180026.GH5935@random.random>
Date:	Tue, 15 Feb 2011 19:00:26 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Eric B Munson <emunson@...bm.net>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Michael J Wolf <mjwolf@...ibm.com>
Subject: Re: [PATCH 0/5] fix up /proc/$pid/smaps to not split huge pages

On Tue, Feb 15, 2011 at 09:05:25AM -0800, Dave Hansen wrote:
> On Tue, 2011-02-15 at 18:01 +0100, Andrea Arcangeli wrote:
> > > The entire mapping is contained in a THP but the
> > > KernelPageSize shows 4kb.  For cases where the mapping might
> > > have mixed page sizes this may be okay, but for this
> > > particular mapping the 4kb page size is wrong.
> > 
> > I'm not sure this is a bug, if the mapping grows it may become 4096k
> > but the new pages may be 4k. There's no such thing as a
> > vma_mmu_pagesize in terms of hugepages because we support graceful
> > fallback and collapse/split on the fly without altering the vma. So I
> > think 4k is correct here
> 
> How about we bump MMUPageSize for mappings that are _entirely_ huge
> pages, but leave it at 4k for mixed mappings?  Anyone needing more
> detail than that can use the new AnonHugePages count.

Anyone needing the detail that you ask for, already can use the
AnonHugePages count.

> KernelPageSize is pretty ambiguous, and we could certainly make the
> argument that the kernel is or can still deal with things in 4k blocks.

That's my point. We could bring it to 2m whenever
AnonHugePages==Anonymous, that's a two liner change, but I'm not
really sure if it makes sense or it provides any meaningful info.

That is a slot specific to show hugetlbfs presence and to
differentiate between 2m/1g mappings. I think remaining mutually
exclusive between AnonHugePages > 0 and MMUPageSize >4k is actually
cleaner than a two liner magic returning 2m if AnonHugePages ==
Anonymous.
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