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

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Message-ID: <06333766-fb79-4deb-9b53-5d1230b9d88d@redhat.com>
Date: Thu, 23 Oct 2025 12:54:59 +0200
From: David Hildenbrand <david@...hat.com>
To: Kiryl Shutsemau <kirill@...temov.name>
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 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.

Essentially all cases where we try to avoid reading ordinary memory 
already when creating memory dumps that might have a direct map.

Regarding MTE and load_unaligned_zeropad(): I don't know unfortunately.

On s390x, reading a "protected" page of a CoCo Vm will trigger an 
interrupt, I'd assume _nofault would take care of this.

-- 
Cheers

David / dhildenb