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

[David Hildenbrand]

On 23.10.25 13:09, Kiryl Shutsemau wrote:
> 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.

Indeed :(

--
Cheers

David / dhildenb