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

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Message-ID: <20180824180438.GS24124@hirez.programming.kicks-ass.net>
Date:   Fri, 24 Aug 2018 20:04:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Will Deacon <will.deacon@....com>,
        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

On Fri, Aug 24, 2018 at 10:26:50AM -0700, Nadav Amit wrote:
> 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?

Urgh.. weren't the fixmaps per cpu? Bah, I remember looking at this
during PTI, but I seem to have forgotten everything again.