linux-kernel - Re: [PATCH] mm: per-thread vma caching

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Date:	Mon, 24 Feb 2014 17:50:51 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Michel Lespinasse <walken@...gle.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: per-thread vma caching

On Mon, 2014-02-24 at 17:42 -0800, Linus Torvalds wrote:
> On Mon, Feb 24, 2014 at 5:16 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> >
> > If we add the two missing bits to the shifting and use PAGE_SHIFT (x86
> > at least) we get just as good results as with 10. So we would probably
> > prefer hashing based on the page number and not some offset within the
> > page.
> 
> So just
> 
>     int idx = (addr >> PAGE_SHIFT) & 3;
> 
> works fine?

Yep.

> 
> That makes me think it all just wants to be maximally spread out to
> approximate some NRU when adding an entry.
> 
>  Also, as far as I can tell, "vmacache_update()" should then become
> just a simple unconditional
> 
>     int idx = (addr >> PAGE_SHIFT) & 3;
>     current->vmacache[idx] = newvma;
> 

Yes, my thoughts exactly!

> because your original code did
> 
> +       if (curr->vmacache[idx] != newvma)
> +               curr->vmacache[idx] = newvma;
> 
> and that doesn't seem to make sense, since if "newvma" was already in
> the cache, then we would have found it when looking up, and we
> wouldn't be here updating it after doing the rb-walk? 

I noticed this as well but kept my fingers shut and was planning on
fixing it in v2.

> And with the
> per-mm cache removed, all that should remain is that simple version,
> no? 

Yes. 

Although I am planning on keeping the current way of doing things for
nommu configs as there's no dup_mmap. I'm not sure if that's the best
idea though, it makes things less straightforward.

> You don't even need the "check the vmcache sequence number and
> clear if bogus", because the rule should be that you have always done
> a "vmcache_find()" first, which should have done that..

Makes sense, noted.

Thanks,
Davidlohr

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