On 05.06.25 21:19, Jann Horn wrote:
On Wed, Jun 4, 2025 at 4:21 PM Lorenzo Stoakes
<[email protected]> 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.
--
Cheers,
David / dhildenb
