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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 ARC-Authentication-Results: i=1; imf21.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=k9VsfveF; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf21.hostedemail.com: domain of hughd@google.com designates 209.85.222.177 as permitted sender) smtp.mailfrom=hughd@google.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1657308124; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=aaWVjRt+AHG/IqpoV2z89+r6hmRW47qVhgIz9FsXZ7c=; b=ipKQwR3XpRl5HJBhKOYyJ9DDYQrKpT6Mfjp2ZqmWfbjDN332Cil8ExkWSfItNMJayVlSpk 6AMUHd9+jkg2xk6JQefoi8ur6Nyqa7gwIR4/9jCkjKTE5y7L/3RRe6quBMJrr1KlkqpA/X ONN9RVdpLNaQZCxFw90xM1CvlaypT8s= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1657308124; a=rsa-sha256; cv=none; b=TBRA15KdRcHxIlzsWzh5/8NLBAx9TUaiW+nC3fseg0hOy7074wYiVHN/qZYr0Rixo4kqu1 skIQ73Ll5ME8yCf9udA7HBky6mgjiq5mcKknTGoqYcyRP+N4/6DZD/PR2wd5rJ8l8jmtCH CQ+CeY3FAmGkO5yrn2i63Qz8zCg6wp4= X-Stat-Signature: rnows4ufsaix6k5d67n9nuf9ejqsh4ff X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 50BBD1C0055 Authentication-Results: imf21.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=k9VsfveF; dmarc=pass (policy=reject) header.from=google.com; spf=pass (imf21.hostedemail.com: domain of hughd@google.com designates 209.85.222.177 as permitted sender) smtp.mailfrom=hughd@google.com X-Rspam-User: X-HE-Tag: 1657308124-771227 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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