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

[Hugh Dickins]

On Fri, 8 Jul 2022, Nadav Amit wrote:
> On Jul 8, 2022, at 12:21 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> 
> > ⚠ External Email
> > 
> > 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.
> 
> Thanks, Hugh. Sorry again for my mistake.
> 
> Can I please have your “Tested-by”?

Sure, let me scrabble around in my box of tags, yes, here's one:

Tested-by: Hugh Dickins <hughd@xxxxxxxxxx>