linux-kernel - Re: [RFC PATCH 1/2] mm/mmu_gather: Invalidate TLB correctly on batch allocation failure and flush

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Date:   Tue, 17 Dec 2019 10:09:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     akpm@...ux-foundation.org, npiggin@...il.com, mpe@...erman.id.au,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC PATCH 1/2] mm/mmu_gather: Invalidate TLB correctly on batch
 allocation failure and flush

On Tue, Dec 17, 2019 at 12:47:12PM +0530, Aneesh Kumar K.V wrote:
> Architectures for which we have hardware walkers of Linux page table should
> flush TLB on mmu gather batch allocation failures and batch flush. Some
> architectures like POWER supports multiple translation modes (hash and radix)
> and in the case of POWER only radix translation mode needs the above TLBI.
> This is because for hash translation mode kernel wants to avoid this extra
> flush since there are no hardware walkers of linux page table. With radix
> translation, the hardware also walks linux page table and with that, kernel
> needs to make sure to TLB invalidate page walk cache before page table pages are
> freed.

> Based on changes from Peter Zijlstra <peterz@...radead.org>

AFAICT it is all my patch ;-)

Anyway, this commit:

> More details in
> commit: d86564a2f085 ("mm/tlb, x86/mm: Support invalidating TLB caches for RCU_TABLE_FREE")

states that you do an explicit invalidate in __p*_free_tlb(), which, if
I'm not mistaken is still there:

  arch/powerpc/include/asm/nohash/pgalloc.h:      tlb_flush_pgtable(tlb, address);

Or am I reading this wrong? I'm thinking you can remove that now.

> diff --git a/arch/powerpc/include/asm/tlb.h b/arch/powerpc/include/asm/tlb.h
> index b2c0be93929d..feea1a09bbce 100644
> --- a/arch/powerpc/include/asm/tlb.h
> +++ b/arch/powerpc/include/asm/tlb.h
> @@ -27,6 +27,10 @@
>  #define tlb_flush tlb_flush
>  extern void tlb_flush(struct mmu_gather *tlb);
>  
> +#ifdef CONFIG_HAVE_RCU_TABLE_FREE
/*
 * PPC-Hash does not use the linux page-tables, so we can avoid
 * the TLBI for page-table freeing, PPC-Radix otoh does use the
 * page-tables and needs the TLBI.
 */
> +#define tlb_needs_table_invalidate()	radix_enabled()
> +#endif

Also, are you really sure about the !SMP case? Esp. on Radix I'm
thinking that the PWC (page-walk-cache) can give trouble even on UP,
when we get preempted in the middle of mmu_gather. Hmm?

>  /* Get the generic bits... */
>  #include <asm-generic/tlb.h>