Messages in this thread | | | Date | Fri, 8 Jul 2022 12:21:51 -0700 (PDT) | From | Hugh Dickins <> | Subject | Re: [PATCH] x86/mm/tlb: ignore f->new_tlb_gen when zero |
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On Thu, 7 Jul 2022, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.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@google.com> > Cc: Dave Hansen <dave.hansen@linux.intel.com> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org> > Cc: Andy Lutomirski <luto@kernel.org> > Signed-off-by: Nadav Amit <namit@vmware.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.
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
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