linux-kernel - Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE

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Message-Id: <D74A89DF-0D89-4AB6-8A6B-93BEC9A83595@gmail.com>
Date:   Fri, 24 Aug 2018 10:26:50 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Benjamin Herrenschmidt <benh@....ibm.com>,
        Nick Piggin <npiggin@...il.com>,
        Andrew Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Rik van Riel <riel@...riel.com>,
        Jann Horn <jannh@...gle.com>,
        Adin Scannell <ascannell@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        David Miller <davem@...emloft.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for
 RCU_TABLE_FREE

at 1:47 AM, Peter Zijlstra <peterz@...radead.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).
>> 
>> 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).
> 
> 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?