Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE

From: Nadav Amit <nadav.amit@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>, Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Benjamin Herrenschmidt <benh@au1.ibm.com>,
	Nick Piggin <npiggin@gmail.com>,
	Andrew Lutomirski <luto@kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	Borislav Petkov <bp@alien8.de>, Rik van Riel <riel@surriel.com>,
	Jann Horn <jannh@google.com>,
	Adin Scannell <ascannell@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-mm <linux-mm@kvack.org>, David Miller <davem@davemloft.net>,
	Martin Schwidefsky <schwidefsky@de.ibm.com>,
	Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH 3/4] mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE
Date: Fri, 24 Aug 2018 10:26:50 -0700	[thread overview]
Message-ID: <D74A89DF-0D89-4AB6-8A6B-93BEC9A83595@gmail.com> (raw)
In-Reply-To: <20180824084717.GK24124@hirez.programming.kicks-ass.net>

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).
>> 
>> 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?