Re: [PATCH v18 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

["Michał Mirosław" <emmir@google.com>]

On Thu, 15 Jun 2023 at 16:52, Michał Mirosław <emmir@google.com> wrote:
> On Thu, 15 Jun 2023 at 15:58, Muhammad Usama Anjum
> <usama.anjum@collabora.com> wrote:
> > I'll send next revision now.
> > On 6/14/23 11:00 PM, Michał Mirosław wrote:
> > > (A quick reply to answer open questions in case they help the next version.)
> > >
> > > On Wed, 14 Jun 2023 at 19:10, Muhammad Usama Anjum
> > > <usama.anjum@collabora.com> wrote:
> > >> On 6/14/23 8:14 PM, Michał Mirosław wrote:
> > >>> On Wed, 14 Jun 2023 at 15:46, Muhammad Usama Anjum
> > >>> <usama.anjum@collabora.com> wrote:
> > >>>>
> > >>>> On 6/14/23 3:36 AM, Michał Mirosław wrote:
> > >>>>> On Tue, 13 Jun 2023 at 12:29, Muhammad Usama Anjum
> > >>>>> <usama.anjum@collabora.com> wrote:
> > >>>>> For flags name: PM_REQUIRE_WRITE_ACCESS?
> > >>>>> Or Is it intended to be checked only if doing WP (as the current name
> > >>>>> suggests) and so it would be redundant as WP currently requires
> > >>>>> `p->required_mask = PAGE_IS_WRITTEN`?
> > >>>> This is intended to indicate that if userfaultfd is needed. If
> > >>>> PAGE_IS_WRITTEN is mentioned in any of mask, we need to check if
> > >>>> userfaultfd has been initialized for this memory. I'll rename to
> > >>>> PM_SCAN_REQUIRE_UFFD.
> > >>>
> > >>> Why do we need that check? Wouldn't `is_written = false` work for vmas
> > >>> not registered via uffd?
> > >> UFFD_FEATURE_WP_ASYNC and UNPOPULATED needs to be set on the memory region
> > >> for it to report correct written values on the memory region. Without UFFD
> > >> WP ASYNC and UNPOUPULATED defined on the memory, we consider UFFD_WP state
> > >> undefined. If user hasn't initialized memory with UFFD, he has no right to
> > >> set is_written = false.
> > >
> > > How about calculating `is_written = is_uffd_registered() &&
> > > is_uffd_wp()`? This would enable a user to apply GET+WP for the whole
> > > address space of a process regardless of whether all of it is
> > > registered.
> > I wouldn't want to check if uffd is registered again and again. This is why
> > we are doing it only once every walk in pagemap_scan_test_walk().
>
> There is no need to do the checks repeatedly. If I understand the code
> correctly, uffd registration is per-vma, so it can be communicated
> from test_walk to entry/hole callbacks via a field in
> pagemap_scan_private.

Actually... this could be exposed as a page category for the filter
(e.g. PAGE_USES_UFFD_WP) and then you could just make the ioctl() to
work for your usecase without tracking the ranges at the userspace
side.

Best Regards
Michał Mirosław