linux-kernel - Re: [RFC PATCH 1/2] zap_pte_range: update addr when forcing flush after TLB batching faiure

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Message-ID: <20141107165003.GG29148@e104818-lin.cambridge.arm.com>
Date:	Fri, 7 Nov 2014 16:50:04 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Will Deacon <Will.Deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH 1/2] zap_pte_range: update addr when forcing flush
 after TLB batching faiure

On Thu, Nov 06, 2014 at 09:29:54PM +0000, Linus Torvalds wrote:
> On Thu, Nov 6, 2014 at 10:38 AM, Catalin Marinas
> <catalin.marinas@....com> wrote:
> > On Thu, Nov 06, 2014 at 05:53:58PM +0000, Linus Torvalds wrote:
> >
> > Sorry, I wasn't clear enough about the "increments" part. I agreed with
> > not using end = start + PMD_SIZE/PAGE_SIZE from your previous email
> > already.
> 
> Ahh, I misunderstood. You're really just after the granularity of tlb flushes.

Yes. The granularity would also help when tearing down page tables as
the granule would be PMD_SIZE.

> That's fine. That makes sense. In fact, how about adding "granularity"
> to the mmu_gather structure, and then doing:\
> 
>  - in __tlb_reset_range(), setting it to ~0ul
> 
>  - add "granularity" to __tlb_adjust_range(), and make it do something like
> 
>        if (!tlb->fullmm) {
>                tlb->granularity = min(tlb->granularity, granularity);
>                tlb->start = min(tlb->start, address);
>                tlb->end = max(tlb->end, address+1);
>        }
> 
> and then the TLB flush logic would basically do
> 
>    address = tlb->start;
>    do {
>         flush(address);
>         if (address + tlb->granularity < address)
>                 break;
>         address = address + tlb->granularity;
>    } while (address < tlb->end);
> 
> or something like that.

Indeed. We'll come up with a patch after Will's clean-up.

> Now, if you unmap mixed ranges of large-pages and regular pages, you'd
> still have that granularity of one page, but quite frankly, if you do
> that, you probably deserve it. The common case is almost certainly
> going to be just "unmap large pages" or "unmap normal pages".

I think this could only happen with transparent huge pages that replaced
small pages in an anonymous mapping. I don't think munmap'ing them
happens very often.

Thanks.

-- 
Catalin
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