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Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for
 RCU_TABLE_FREE
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20180824084717.GK24124@hirez.programming.kicks-ass.net>
Date:   Fri, 24 Aug 2018 10:26:50 -0700
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Benjamin Herrenschmidt <benh@au1.ibm.com>,
        Nick Piggin <npiggin@gmail.com>,
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        Adin Scannell <ascannell@google.com>,
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To:     Peter Zijlstra <peterz@infradead.org>,
        Will Deacon <will.deacon@arm.com>
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at 1:47 AM, Peter Zijlstra <peterz@infradead.org> wrote:

> On Thu, Aug 23, 2018 at 02:39:59PM +0100, Will Deacon wrote:
>> The only problem with this approach is that we've lost track of the =
granule
>> size by the point we get to the tlb_flush(), so we can't adjust the =
stride of
>> the TLB invalidations for huge mappings, which actually works nicely =
in the
>> synchronous case (e.g. we perform a single invalidation for a 2MB =
mapping,
>> rather than iterating over it at a 4k granule).
>>=20
>> One thing we could do is switch to synchronous mode if we detect a =
change in
>> granule (i.e. treat it like a batch failure).
>=20
> We could use tlb_start_vma() to track that, I think. Shouldn't be too
> hard.

Somewhat unrelated, but I use this opportunity that TLB got your =
attention
for something that bothers me for some time. clear_fixmap(), which is =
used
in various places (e.g., text_poke()), ends up in doing only a local TLB
flush (in __set_pte_vaddr()).

Is that sufficient?