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Thu, 23 Oct 2025 06:31:34 -0400 (EDT) Date: Thu, 23 Oct 2025 11:31:32 +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: References: <20251017141536.577466-1-kirill@shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Rspamd-Queue-Id: B126F18000A X-Rspamd-Server: rspam03 X-Stat-Signature: pwmo953hipr93kg9fecto9znd51odceb X-HE-Tag: 1761215497-193207 X-HE-Meta: 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 WxXMpK1L 7POtHRIVqOL6RI/9h/WXftUrK5v1T8RJEaqZlK/6QhfrUjQ4MkB260h1rP3xETlzJg0jRLe+FzrZs9ja+L4d/ZRdvXsBDYKLr73Kxv0IlmoOUx8e7lD7dkwRvnxCcWhRe5N+3CiPfYXGqsQhTUvf+5v4Y5mFLqMUFj6xWd0OdhZOuwrERI8yTGeaxn+rbD2n9wQt3cY6KoLr3HjjGH3IwfgTfMMR37cSXBLEbnegzXbXGDVLHyAI5gQONavHTRMq+vz7pqLoLXqcd9nHitJaPW/ixDJE0+GFWaR/ZEaXipgALtAPhRKIUIfMbQgUHX9IlE0qcPu+EiyLJ9r0jNcGcYOf+hxkHnEWMfI7xa6kFI8K3RB6d5ksPpKCCegO5viRyQfwPD62F9LEJuZBIpTQVjJtUnQ== 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 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. 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 cannot say for all CoCo protections, but TDX guest shared<->private should be fine. I am not sure about MTE. Is there a way to bypass MTE check for a load? And how does it deal with stray reads from load_unaligned_zeropad()? -- Kiryl Shutsemau / Kirill A. Shutemov