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

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Date:   Fri, 8 Jul 2022 20:02:05 +0000
From:   Nadav Amit <namit@...are.com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "x86@...nel.org" <x86@...nel.org>, Linux MM <linux-mm@...ck.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86/mm/tlb: ignore f->new_tlb_gen when zero

On Jul 8, 2022, at 12:21 PM, Hugh Dickins <hughd@...gle.com> wrote:

> ⚠ External Email
> 
> On Thu, 7 Jul 2022, 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.
>> 
>> In addition, add the missing unlikely() and jump to get tracing right.
>> 
>> Fixes: aa44284960d5 ("x86/mm/tlb: Avoid reading mm_tlb_gen when possible")
>> Reported-by: Hugh Dickins <hughd@...gle.com>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
>> Cc: Andy Lutomirski <luto@...nel.org>
>> Signed-off-by: Nadav Amit <namit@...are.com>
> 
> Thanks a lot for your rapid response and thinking it through
> (before I got around to any "nopcid" or "nopti" experiments).
> 
> I've been testing this one for a few hours now, and no problems seen.
> I expect you'll be sending another version, maybe next week, meeting
> Dave's concerns; but wanted to reassure that you have correctly
> identified the issue and fixed it with this - thanks.

Thanks, Hugh. Sorry again for my mistake.

Can I please have your “Tested-by”?