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

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Date:   Mon, 13 May 2019 09:21:01 +0000
From:   Nadav Amit <namit@...are.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 May 13, 2019, at 2:12 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> 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.

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.

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.

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