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Thu, 25 Mar 2021 19:13:39 +0100 (CET) Authentication-Results: pio-pvt-msa1.bahnhof.se; dkim=pass (1024-bit key; unprotected) header.d=shipmail.org header.i=@shipmail.org header.b="bOXPgDuL"; dkim-atps=neutral X-Virus-Scanned: Debian amavisd-new at bahnhof.se Received: from pio-pvt-msa1.bahnhof.se ([127.0.0.1]) by localhost (pio-pvt-msa1.bahnhof.se [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 36X5fsGQR3cs; Thu, 25 Mar 2021 19:13:38 +0100 (CET) Received: by pio-pvt-msa1.bahnhof.se (Postfix) with ESMTPA id D7A783F377; Thu, 25 Mar 2021 19:13:36 +0100 (CET) Received: from [10.249.254.165] (unknown [192.198.151.44]) by mail1.shipmail.org (Postfix) with ESMTPSA id 880B636059E; Thu, 25 Mar 2021 19:13:35 +0100 (CET) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=shipmail.org; s=mail; t=1616696016; bh=ccthR8+3Hk5Ox8PeQv8Va4F118+/tBSCEPzJg3VAxZs=; h=Subject:To:Cc:References:From:Date:In-Reply-To:From; b=bOXPgDuLyt0NMxRDWcFUkrudDGfPZJamxR9X+vliH4e3m8vKYTc82qLVR8olQEqM8 hcUxNVANfDq+5Nctk1NCiDllhxuzOm4XGIHFqcLQXqRHDFnlY2Vx6P+zBjxrbpuxD8 EqoNkJCXBXHo0CgJvGJiCHN4wBh83heMYUFA76xg= Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages To: Jason Gunthorpe Cc: Dave Hansen , "Williams, Dan J" , "dri-devel@lists.freedesktop.org" , "christian.koenig@amd.com" , "airlied@linux.ie" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , "akpm@linux-foundation.org" References: <20210321184529.59006-1-thomas_os@shipmail.org> <20210321184529.59006-2-thomas_os@shipmail.org> <75423f64-adef-a2c4-8e7d-2cb814127b18@intel.com> <6b0de827-738d-b3c5-fc79-8ca9047bad35@intel.com> <9f789d64-940f-c728-8d5e-aab74d562fb6@shipmail.org> <20210325175504.GH2356281@nvidia.com> From: =?UTF-8?Q?Thomas_Hellstr=c3=b6m_=28Intel=29?= Message-ID: <1ed48d99-1cd9-d87b-41dd-4169afc77f70@shipmail.org> Date: Thu, 25 Mar 2021 19:13:33 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210325175504.GH2356281@nvidia.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: AAA382000253 X-Stat-Signature: 6rjpbbr7mybdko4ocm8bsikhh4pt9csx Received-SPF: none (shipmail.org>: No applicable sender policy available) receiver=imf18; identity=mailfrom; envelope-from=""; helo=pio-pvt-msa1.bahnhof.se; client-ip=79.136.2.40 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1616696020-617528 Content-Transfer-Encoding: quoted-printable 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 3/25/21 6:55 PM, Jason Gunthorpe wrote: > On Thu, Mar 25, 2021 at 06:51:26PM +0100, Thomas Hellstr=C3=B6m (Intel)= wrote: >> On 3/24/21 9:25 PM, Dave Hansen wrote: >>> On 3/24/21 1:22 PM, Thomas Hellstr=C3=B6m (Intel) wrote: >>>>> We also have not been careful at *all* about how _PAGE_BIT_SOFTW* a= re >>>>> used.=C2=A0 It's quite possible we can encode another use even in t= he >>>>> existing bits. >>>>> >>>>> Personally, I'd just try: >>>>> >>>>> #define _PAGE_BIT_SOFTW5=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 = 57=C2=A0=C2=A0=C2=A0=C2=A0=C2=A0 /* available for programmer */ >>>>> >>>> OK, I'll follow your advise here. FWIW I grepped for SW1 and it seem= s >>>> used in a selftest, but only for PTEs AFAICT. >>>> >>>> Oh, and we don't care about 32-bit much anymore? >>> On x86, we have 64-bit PTEs when running 32-bit kernels if PAE is >>> enabled. IOW, we can handle the majority of 32-bit CPUs out there. >>> >>> But, yeah, we don't care about 32-bit. :) >> Hmm, >> >> Actually it makes some sense to use SW1, to make it end up in the same= dword >> as the PSE bit, as from what I can tell, reading of a 64-bit pmd_t on = 32-bit >> PAE is not atomic, so in theory a huge pmd could be modified while rea= ding >> the pmd_t making the dwords inconsistent.... How does that work with f= ast >> gup anyway? > It loops to get an atomic 64 bit value if the arch can't provide an > atomic 64 bit load Hmm, ok, I see a READ_ONCE() in gup_pmd_range(), and then the resulting=20 pmd is dereferenced either in try_grab_compound_head() or=20 __gup_device_huge(), before the pmd is compared to the value the pointer=20 is currently pointing to. Couldn't those dereferences be on invalid=20 pointers? /Thomas > > Jason