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

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Message-ID: <55p3kov54tjf5cr2sm3h3z7hv5cq6nlcqihlyxa427pz5mtkuv@nddjx4onw6mq>
Date: Thu, 23 Oct 2025 12:09:12 +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 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