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

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Message-ID: <hb54gc3iezwzpe2j6ssgqtwcnba4pnnffzlh3eb46preujhnoa@272dqbjakaiy>
Date: Thu, 23 Oct 2025 11:31:32 +0100
From: Kiryl Shutsemau <kirill@...temov.name>
To: David Hildenbrand <david@...hat.com>
Cc: 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 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