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Thu, 23 Oct 2025 07:09:14 -0400 (EDT) Date: Thu, 23 Oct 2025 12:09:12 +0100 From: Kiryl Shutsemau To: David Hildenbrand Cc: Andrew Morton , Matthew Wilcox , Linus Torvalds , Alexander Viro , Christian Brauner , Jan Kara , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/filemap: Implement fast short reads Message-ID: <55p3kov54tjf5cr2sm3h3z7hv5cq6nlcqihlyxa427pz5mtkuv@nddjx4onw6mq> References: <20251017141536.577466-1-kirill@shutemov.name> <06333766-fb79-4deb-9b53-5d1230b9d88d@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <06333766-fb79-4deb-9b53-5d1230b9d88d@redhat.com> X-Rspamd-Queue-Id: 731A34000C X-Rspamd-Server: rspam11 X-Rspam-User: X-Stat-Signature: 8oqe9pfp4iwwu9wnn8b9z6aoz74gfgt6 X-HE-Tag: 1761217756-530879 X-HE-Meta: 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 iXIM8maw nI5MC/r6Kw8G+CA2YdrK/xYK0kH4AtSkkkdF7sdKTgHKsG5EJHgRZKEUL+TlTZ1c5d44w/Qsli1la+9NFu3FC+VnvPEHEX7y2kM7pRVA1nZgCz8sxXTySe0BTB6b/ushWH2kSj+Ohq6BMm733JOwipzS62i9W41Y1BU0CiZrU5oXZrPCr8SGHrWwmRLW+KdkA8rDhZkrOrk5yO150FSJBjyw67zciqEsk1BT+rYeD3LWEMya7CoRPqtsz681cvn7cENFBdd9I84faeprws+8F+cwxfPRza6rQYZb91k6KfehDBWm8RcTDGSMvP85RC5plFPEAKVgCXsiI/ZOEZX1NZH0J8l+JMbKheuLsca29dwYhJrvQIs11L6QgQee1ueTjS65TGaskRXxohZjWM/UkYKdVs0pi6ar1EmgMW/MCkMpf/2FaJUB16bVF9Q== 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: List-Subscribe: List-Unsubscribe: On Thu, Oct 23, 2025 at 12:54:59PM +0200, David Hildenbrand wrote: > On 23.10.25 12:31, Kiryl Shutsemau wrote: > > On Wed, Oct 22, 2025 at 07:28:27PM +0200, David Hildenbrand wrote: > > > "garbage" as in pointing at something without a direct map, something that's > > > protected differently (MTE? weird CoCo protection?) or even worse MMIO with > > > undesired read-effects. > > > > Pedro already points to the problem with missing direct mapping. > > _nofault() copy should help with this. > > Yeah, we do something similar when reading the kcore for that reason. > > > > > Can direct mapping ever be converted to MMIO? It can be converted to DMA > > buffer (which is fine), but MMIO? I have not seen it even in virtualized > > environments. > > I recall discussions in the context of PAT and the adjustment of caching > attributes of the direct map for MMIO purposes: so I suspect there are ways > that can happen, but I am not 100% sure. > > > Thinking about it, in VMs we have the direct map set on balloon inflated > pages that should not be touched, not even read, otherwise your hypervisor > might get very angry. That case we could likely handle by checking whether > the source page actually exists and doesn't have PageOffline() set, before > accessing it. A bit nasty. > > A more obscure cases would probably be reading a page that was poisoned by > hardware and is not expected to be used anymore. Could also be checked by > checking the page. I don't think we can check the page. Since the page is not stabilized with a reference, it is TOCTOU race. If there's some side effect that we cannot suppress on read (like _nofault() does) we are screwed. -- Kiryl Shutsemau / Kirill A. Shutemov