Re: [PATCH v2] mm/pagewalk: split walk_page_range_novma() into kernel/user parts

[Jann Horn <jannh@google.com>]

On Thu, Jun 5, 2025 at 10:23 PM David Hildenbrand <david@redhat.com> wrote:
> On 05.06.25 21:19, Jann Horn wrote:
> > On Wed, Jun 4, 2025 at 4:21 PM Lorenzo Stoakes
> > <lorenzo.stoakes@oracle.com> wrote:
> >> The walk_page_range_novma() function is rather confusing - it supports two
> >> modes, one used often, the other used only for debugging.
> >>
> >> The first mode is the common case of traversal of kernel page tables, which
> >> is what nearly all callers use this for.
> >>
> >> Secondly it provides an unusual debugging interface that allows for the
> >> traversal of page tables in a userland range of memory even for that memory
> >> which is not described by a VMA.
> >>
> >> It is far from certain that such page tables should even exist, but perhaps
> >> this is precisely why it is useful as a debugging mechanism.
> >>
> >> As a result, this is utilised by ptdump only. Historically, things were
> >> reversed - ptdump was the only user, and other parts of the kernel evolved
> >> to use the kernel page table walking here.
> >
> > Just for the record, copy-pasting my comment on v1 that was
> > accidentally sent off-list:
> > ```
> > Sort of a tangential comment: I wonder if it would make sense to give
> > ptdump a different page table walker that uses roughly the same safety
> > contract as gup_fast() - turn off IRQs and then walk the page tables
> > locklessly. We'd need basically no locking and no special cases
> > (regarding userspace mappings at least), at the cost of having to
> > write the walker code such that we periodically restart the walk from
> > scratch and not being able to inspect referenced pages. (That might
> > also be nicer for debugging, since it wouldn't block on locks...)
> > ```
>
> I assume we don't have to dump more than pte values etc? So
> pte_special() and friends are not relevant to get it right.
>
> GUP-fast depend on CONFIG_HAVE_GUP_FAST, not sure if that would be a
> concern for now.

Ah, good point, that's annoying... maaaybe we should just gate this
entire feature on CONFIG_HAVE_GUP_FAST to make sure the userspace
mappings are designed to be walkable in this way? It's in debugfs,
which _theoretically_
(https://docs.kernel.org/filesystems/debugfs.html) means there are no
stability guarantees, and I think it is normally used on architectures
that define CONFIG_HAVE_GUP_FAST...