On Fri, Jun 06, 2025 at 12:59:20PM +0200, Jann Horn wrote: > On Thu, Jun 5, 2025 at 10:23 PM David Hildenbrand <da...@redhat.com> wrote: > > On 05.06.25 21:19, Jann Horn wrote: > > > On Wed, Jun 4, 2025 at 4:21 PM Lorenzo Stoakes > > > <lorenzo.stoa...@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...
Hm, it's a nice idea, but I wonder if it's worthwhile just for ptdump? I really hate how we're just arbitrarily using init_mm.mmap_lock as a mutex here though. Could we GUP fast walkers here in general I wonder...? Or optionally maybe for more general page table walking? I mean of course gated on availability. We sorely need a truly generalised page walker :) though of course it's a matter of people having time :P
