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

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Message-ID: <fqexozfjp3l6vj643lecky4nndmwblvee7u5f5ejcqkpsij3wp@3yb4y4lyd3dn>
Date: Thu, 23 Oct 2025 13:41:52 +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 01:49:22PM +0200, David Hildenbrand wrote:
> On 23.10.25 13:40, Kiryl Shutsemau wrote:
> > On Thu, Oct 23, 2025 at 01:11:43PM +0200, David Hildenbrand wrote:
> > > 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()".
> > 
> > Do we have other random reads?
> > 
> > For unaccepted memory, we care about touching memory that was never
> > allocated because accepting memory is one way road.
> 
> Right, but I suspect if you get a random read (as the unaccepted memory doc
> states) you'd be in trouble as well.

Yes. Random read of unaccepted memory is unrecoverable exit to host for
TDX guest.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov