linux-kernel - Re: [PATCH] mm: mmu_gather: remove __tlb_reset

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Message-ID: <20190513112712.GO2623@hirez.programming.kicks-ass.net>
Date:   Mon, 13 May 2019 13:27:12 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Yang Shi <yang.shi@...ux.alibaba.com>,
        "jstancek@...hat.com" <jstancek@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.vnet.ibm.com>,
        Nick Piggin <npiggin@...il.com>,
        Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] mm: mmu_gather: remove __tlb_reset_range() for force
 flush

On Mon, May 13, 2019 at 09:21:01AM +0000, Nadav Amit wrote:
> > On May 13, 2019, at 2:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:

> >> The other thing I was thinking of is trying to detect overlap through
> >> the page-tables themselves, but we have a distinct lack of storage
> >> there.
> > 
> > We might just use some state in the pmd, there's still 2 _pt_pad_[12] in
> > struct page to 'use'. So we could come up with some tlb generation
> > scheme that would detect conflict.
> 
> It is rather easy to come up with a scheme (and I did similar things) if you
> flush the table while you hold the page-tables lock. But if you batch across
> page-tables it becomes harder.

Yeah; finding that out now. I keep finding holes :/

> Thinking about it while typing, perhaps it is simpler than I think - if you
> need to flush range that runs across more than a single table, you are very
> likely to flush a range of more than 33 entries, so anyhow you are likely to
> do a full TLB flush.

We can't rely on the 33, that x86 specific. Other architectures can have
another (or no) limit on that.

> So perhaps just avoiding the batching if only entries from a single table
> are flushed would be enough.

That's near to what Will suggested initially, just flush the entire
thing when there's a conflict.