linux-kernel - Re: [PATCH v2] x86/mm/tlb: ignore f->new_tlb

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Date:   Tue, 19 Jul 2022 09:13:23 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <nadav.amit@...il.com>, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Linux MM <linux-mm@...ck.org>,
        Nadav Amit <namit@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v2] x86/mm/tlb: ignore f->new_tlb_gen when zero

On 7/10/22 16:28, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> Commit aa44284960d5 ("x86/mm/tlb: Avoid reading mm_tlb_gen when
> possible") introduced an optimization of skipping the flush if the TLB
> generation that is flushed (as provided in flush_tlb_info) was already
> flushed.
> 
> However, arch_tlbbatch_flush() does not provide any generation in
> flush_tlb_info. As a result, try_to_unmap_one() would not perform any
> TLB flushes.
> 
> Fix it by checking whether f->new_tlb_gen is nonzero. Zero value is
> anyhow is an invalid generation value. To avoid future confusions,
> introduce TLB_GENERATION_INVALID constant and use it properly. Add some
> assertions to check no partial flushes are done with
> TLB_GENERATION_INVALID or when f->mm is NULL, since this does not make
> any sense.
> 
> In addition, add the missing unlikely().

I've applied this:

> https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?h=x86/mm&id=8f1d56f64f8d6b80dea2d1978d10071132a695c5

Please double-check that my rewording of the commit message looks good
to you.  I also replaced the VM_BUG_ON()'s with warnings.  Screwing up
TLB flushing isn't great, but it's also not worth killing the system.