Re: [PATCH] x86/mm/tlb: ignore f->new_tlb_gen when zero

[Hugh Dickins]

On Thu, 7 Jul 2022, Nadav Amit wrote:

> From: Nadav Amit <namit@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>

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.

Hugh

> ---
>  arch/x86/mm/tlb.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
> index d9314cc8b81f..d81b4084bb8a 100644
> --- a/arch/x86/mm/tlb.c
> +++ b/arch/x86/mm/tlb.c
> @@ -771,14 +771,14 @@ static void flush_tlb_func(void *info)
>  		return;
>  	}
>  
> -	if (f->new_tlb_gen <= local_tlb_gen) {
> +	if (unlikely(f->new_tlb_gen != 0 && f->new_tlb_gen <= local_tlb_gen)) {
>  		/*
>  		 * The TLB is already up to date in respect to f->new_tlb_gen.
>  		 * While the core might be still behind mm_tlb_gen, checking
>  		 * mm_tlb_gen unnecessarily would have negative caching effects
>  		 * so avoid it.
>  		 */
> -		return;
> +		goto done;
>  	}
>  
>  	/*
> -- 
> 2.25.1