linux-kernel - Re: [PATCH 00/11] V4: rwsem changes + down_read

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Date:	Wed, 26 May 2010 10:23:59 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Michel Lespinasse <walken@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 00/11] V4: rwsem changes + down_read_critical() proposal

On Tue, 25 May 2010 11:27:55 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2010-05-25 at 02:12 -0700, Michel Lespinasse wrote:
> > On Tue, May 25, 2010 at 1:47 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > > So what happened to those patches that dropped mmap_sem during I/O?
> > 
> > Yes, we do have patches trying to release the mmap_sem when a page
> > fault for a file backed VMA blocks on accessing the corresponding
> > file. We have not given up on these, and we intend to try submitting
> > them again. However, these patches do *not* address the case of a page
> > fault blocking while trying to get a free page (i.e. when you get
> > under high memory pressure).
> 
> But I guess they could, right? Simply make the allocation under mmap_sem
> be __GFP_HARDWALL|__GFP_HIGHMEM|__GFP_MOVABLE__GFP_NOWARN or
> (GFP_HUGHUSER_MOVABLE & ~(__GFP_WAIT|__GFP_IO|__GFP_FS))|__GFP_NOWARN
> 
> and drop the mmap_sem when that fails.
> 
> > > I really don't like people tinkering with the lock implementations like
> > > this. Nor do I like the naming, stats are in no way _critical_.
> > 
> > Critical here refers to the fact that you're not allowed to block
> > while holding the unfairly acquired rwsem.
> 
> We usually call that atomic, your 0/n patch didn't explain any of that.
> 
> Also, do you really think doing something like:
> 
>         /*
>          * Check the vma index is within the range and do
>          * sequential scan until m_index.
>          */
>         vma = NULL;
>         if ((unsigned long)l < mm->map_count) {
>                 vma = mm->mmap;
>                 while (l-- && vma)
>                         vma = vma->vm_next;
>                 goto out;
>         }
> 
> with preemption disabled is a _good_ thing?
> 
> People were talking about raising our vma limit of 64k...

Yes, at least, my customers want over 64k vmas ;)

Hmm..can't we use something speculative lookup for reading maps ?
(as we played in several months ago...)

Thanks,
-Kame


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