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

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Message-ID: <56d9f1d9-fc20-4be8-b64a-07beac3c64d0@redhat.com>
Date: Thu, 23 Oct 2025 13:10:53 +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 12:54, 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.
> 
> 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.

Looking into this, I'd assume the exception handler will take care of it.

load_unaligned_zeropad() is interesting if there is a direct map but the 
memory should not be touched (especially regarding PageOffline and 
memory errors).

I read drivers/firmware/efi/unaccepted_memory.c where we there is a 
lengthy discussion about guard pages and how that works for unaccepted 
memory.

While it works for unaccepted memory, it wouldn't work for other random 
accesses as I suspect we could produce in this patch.

-- 
Cheers

David / dhildenb