linux-kernel - Re: [v3] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning

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Date:   Wed, 26 Jul 2023 23:10:33 +0200
From:   Michał Mirosław <emmir@...gle.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Alex Sierra <alex.sierra@....com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Christian Brauner <brauner@...nel.org>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Rapoport <rppt@...nel.org>, Nadav Amit <namit@...are.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Peter Xu <peterx@...hat.com>, Shuah Khan <shuah@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Yang Shi <shy828301@...il.com>,
        Yun Zhou <yun.zhou@...driver.com>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kselftest@...r.kernel.org,
        kernel@...labora.com
Subject: Re: [v3] fs/proc/task_mmu: Implement IOCTL for efficient page table scanning

On Wed, 26 Jul 2023 at 10:34, Muhammad Usama Anjum
<usama.anjum@...labora.com> wrote:
> On 7/25/23 11:05 PM, Michał Mirosław wrote:
> > On Tue, 25 Jul 2023 at 11:11, Muhammad Usama Anjum
> > <usama.anjum@...labora.com> wrote:
> >>
> >> ----
> >> Michal please post your thoughts before I post this as v26.
> >> ----
> > [...]
> >
> > Looks ok - minor things below.
> >
> > 1. I'd change the _WPASYNC things to something better, if this can
> > also work with "normal" UFFD WP.
> Yeah, but we don't have any use case where UFFD WP is required. It can be
> easily added later when user case arrives. Also UFFD WP sends messages to
> userspace. User can easily do the bookkeeping in userspace as performance
> isn't a concern there.

We shouldn't name the flags based on the use case but based on what
they actually do. So if this checks UFFD registration for WP, then
maybe PAGE_IS_WPALLOWED or something better describing the trait it
matches?

> > 2. For the address tagging part I'd prefer someone who knows how this
> > is used take a look. We're ignoring the tag (but clear it on return in
> > ->start) - so it doesn't matter for the ioctl() itself.
> I've added Kirill if he can give his thoughts about tagged memory.
>
> Right now we are removing the tags from all 3 pointers (start, end, vec)
> before using the pointers on kernel side. But we are overwriting and
> writing the walk ending address in start which user can read/use.
>
> I think we shouldn't over-write the start (and its tag) and instead return
> the ending walk address in new variable, walk_end.

The overwrite of `start` is making the ioctl restart (continuation)
easier to handle. I prefer the current way, but it's not a strong
opinion.

> > 3. BTW, One of the uses is the GetWriteWatch and I wonder how it
> > behaves on HugeTLB (MEM_LARGE_PAGES allocation)? Shouldn't it return a
> > list of huge pages and write *lpdwGranularity = HPAGE_SIZE?
> Wine/Proton doesn't used hugetlb by default. Hugetlb isn't enabled by
> default on Debian as well. For GetWriteWatch() we don't care about the
> hugetlb at all. We have added hugetlb's implementation to complete the
> feature and leave out something.

How is GetWriteWatch() working when passed a VirtualAlloc(...,
MEM_LARGE_PAGES|MEM_WRITE_WATCH...)-allocated range? Does it still
report 4K pages?
This is only a problem when using max_pages: a hugetlb range might
need counting and reporting huge pages and not 4K parts.

Best Regards
Michał Mirosław