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

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Date:   Thu, 15 Jun 2023 16:58:02 +0200
From:   Michał Mirosław <emmir@...gle.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Peter Xu <peterx@...hat.com>, David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Matthew Wilcox <willy@...radead.org>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>, kernel@...labora.com
Subject: Re: [PATCH v18 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Thu, 15 Jun 2023 at 16:52, Michał Mirosław <emmir@...gle.com> wrote:
> On Thu, 15 Jun 2023 at 15:58, Muhammad Usama Anjum
> <usama.anjum@...labora.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@...labora.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@...labora.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@...labora.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