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

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Message-ID: <5b33b587-ffd1-4a25-95e5-5f803a935a57@redhat.com>
Date: Thu, 23 Oct 2025 13:11:43 +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 13:10, David Hildenbrand wrote:
> 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

Sorry I meant here "while that works for load_unaligned_zeropad()".

-- 
Cheers

David / dhildenb