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08 Jul 2022 11:54:16 -0700 X-IronPort-AV: E=Sophos;i="5.92,256,1650956400"; d="scan'208";a="626796171" Received: from yyan2-mobl2.amr.corp.intel.com (HELO [10.212.242.16]) ([10.212.242.16]) by orsmga001-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 08 Jul 2022 11:54:16 -0700 Message-ID: <2e4d0193-1c5a-ec30-53a1-8009370cde36@intel.com> Date: Fri, 8 Jul 2022 11:54:07 -0700 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.1 Subject: Re: [PATCH] x86/mm/tlb: ignore f->new_tlb_gen when zero Content-Language: en-US To: Nadav Amit Cc: "linux-kernel@vger.kernel.org" , Hugh Dickins , Thomas Gleixner , Ingo Molnar , Borislav Petkov , "x86@kernel.org" , Linux MM , Dave Hansen , Peter Zijlstra , Andy Lutomirski References: <20220708003053.158480-1-namit@vmware.com> <4F7D1BBF-9695-4DE2-A40E-2D2546B2BAAE@vmware.com> From: Dave Hansen In-Reply-To: <4F7D1BBF-9695-4DE2-A40E-2D2546B2BAAE@vmware.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit ARC-Message-Signature: i=1; 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dkim=pass header.d=intel.com header.s=Intel header.b="NHfU/9pQ"; dmarc=pass (policy=none) header.from=intel.com; spf=none (imf01.hostedemail.com: domain of dave.hansen@intel.com has no SPF policy when checking 192.55.52.43) smtp.mailfrom=dave.hansen@intel.com X-Rspam-User: X-Rspamd-Server: rspam05 X-HE-Tag: 1657306458-393028 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 7/8/22 10:04, Nadav Amit wrote: > On Jul 8, 2022, at 7:49 AM, Dave Hansen wrote: >> On 7/7/22 17:30, Nadav Amit wrote: >> You might want to fix the clock on the system from which you sent this. >> I was really scratching my head trying to figure out how you got this >> patch out before Hugh's bug report. >> >>> 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. >> >> It is, but the check below uses 'f->end == TLB_FLUSH_ALL' as the marker >> for f->new_tlb_gen being invalid. Being consistent seems like a good >> idea on this stuff. > > If we get a request to do a flush, regardless whether full or partial, > that logically we have already done, there is not reason to do it. > > I therefore do not see a reason to look on f->end. I think that looking > at the generation is very intuitive. If you want, I can add a constant > such as TLB_GENERATION_INVALID. That's a good point. But, _my_ point was that there was only really one read site of f->new_tlb_gen in flush_tlb_func(). That site is guarded by the "f->end != TLB_FLUSH_ALL" check which prevented it from making the same error that your patch did. Whatever we do, it would be nice to have a *single* way to check for "does f->new_tlb_gen have an actual, valid bit of tlb gen data in it?" Using something like TLB_GENERATION_INVALID seems reasonable to me.