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

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Date:   Mon, 13 May 2019 11:12:05 +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 10:36:06AM +0200, Peter Zijlstra wrote:
> On Thu, May 09, 2019 at 09:21:35PM +0000, Nadav Amit wrote:
> > It may be possible to avoid false-positive nesting indications (when the
> > flushes do not overlap) by creating a new struct mmu_gather_pending, with
> > something like:
> > 
> >   struct mmu_gather_pending {
> >  	u64 start;
> > 	u64 end;
> > 	struct mmu_gather_pending *next;
> >   }
> > 
> > tlb_finish_mmu() would then iterate over the mm->mmu_gather_pending
> > (pointing to the linked list) and find whether there is any overlap. This
> > would still require synchronization (acquiring a lock when allocating and
> > deallocating or something fancier).
> 
> We have an interval_tree for this, and yes, that's how far I got :/
> 
> 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.