linux-kernel - Re: [PATCH] mm/filemap: Implement fast short reads

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Message-ID: <CAHbLzkpx7iv40Tt+CDpbSsOupkGXKcix0wfiF6cVGrLFe0dvRQ@mail.gmail.com>
Date: Thu, 23 Oct 2025 10:42:37 -0700
From: Yang Shi <shy828301@...il.com>
To: Kiryl Shutsemau <kirill@...temov.name>
Cc: David Hildenbrand <david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Matthew Wilcox <willy@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/filemap: Implement fast short reads

On Thu, Oct 23, 2025 at 3:34 AM Kiryl Shutsemau <kirill@...temov.name> 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.
>
> 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()?

If I remember correctly, _nofault() copy should skip tag check too.

Thanks,
Yang

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov
>