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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id v123-20020a379381000000b006a6c230f5e0sm27158280qkd.31.2022.07.08.12.22.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 08 Jul 2022 12:22:02 -0700 (PDT) Date: Fri, 8 Jul 2022 12:21:51 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Nadav Amit cc: linux-kernel@vger.kernel.org, Hugh Dickins , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, Linux MM , Nadav Amit , Dave Hansen , Peter Zijlstra , Andy Lutomirski , Andrew Morton Subject: Re: [PATCH] x86/mm/tlb: ignore f->new_tlb_gen when zero In-Reply-To: <20220708003053.158480-1-namit@vmware.com> Message-ID: <4eccbb3a-2b5a-039-e934-da451e469929@google.com> References: <20220708003053.158480-1-namit@vmware.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 7 Jul 2022, Nadav Amit wrote: > From: Nadav Amit > > 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 > Cc: Dave Hansen > Cc: Peter Zijlstra (Intel) > Cc: Andy Lutomirski > Signed-off-by: Nadav Amit 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