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From:   =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= <emmir@google.com>
Date:   Tue, 21 Feb 2023 13:42:29 +0100
Message-ID: <CABb0KFGWi0dtgXZ-AeUuHb55EgnwTu3JfJ9cW3ftCqezKi8dAQ@mail.gmail.com>
Subject: Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or
 the clear info about PTEs
To:     Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc:     Andrei Vagin <avagin@gmail.com>, Mike Rapoport <rppt@kernel.org>,
        Nadav Amit <namit@vmware.com>,
        David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Paul Gofman <pgofman@codeweavers.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Shuah Khan <shuah@kernel.org>,
        Christian Brauner <brauner@kernel.org>,
        Yang Shi <shy828301@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        "Liam R . Howlett" <Liam.Howlett@oracle.com>,
        Yun Zhou <yun.zhou@windriver.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Alex Sierra <alex.sierra@amd.com>,
        Peter Xu <peterx@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        "Gustavo A . R . Silva" <gustavoars@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
        Greg KH <gregkh@linuxfoundation.org>, kernel@collabora.com,
        Danylo Mocherniuk <mdanylo@google.com>
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On Tue, 21 Feb 2023 at 11:28, Muhammad Usama Anjum
<usama.anjum@collabora.com> wrote:
>
> Hi Micha=C5=82,
>
> Thank you so much for comment!
>
> On 2/17/23 8:18=E2=80=AFPM, Micha=C5=82 Miros=C5=82aw wrote:
[...]
> > For the page-selection mechanism, currently required_mask and
> > excluded_mask have conflicting
> They are opposite of each other:
> All the set bits in required_mask must be set for the page to be selected=
.
> All the set bits in excluded_mask must _not_ be set for the page to be
> selected.
>
> > responsibilities. I suggest to rework that to:
> > 1. negated_flags: page flags which are to be negated before applying
> > the page selection using following masks;
> Sorry I'm unable to understand the negation (which is XOR?). Lets look at
> the truth table:
> Page Flag       negated_flags
> 0               0                       0
> 0               1                       1
> 1               0                       1
> 1               1                       0
>
> If a page flag is 0 and negated_flag is 1, the result would be 1 which ha=
s
> changed the page flag. It isn't making sense to me. Why the page flag bit
> is being fliped?
>
> When Anrdei had proposed these masks, they seemed like a fancy way of
> filtering inside kernel and it was straight forward to understand. These
> masks would help his use cases for CRIU. So I'd included it. Please can y=
ou
> elaborate what is the purpose of negation?

The XOR is a way to invert the tested value of a flag (from positive
to negative and the other way) without having the API with invalid
values (with required_flags and excluded_flags you need to define a
rule about what happens if a flag is present in both of the masks -
either prioritise one mask over the other or reject the call).
(Note: the XOR is applied only to the value of the flags for the
purpose of testing page-selection criteria.)

So:
1. if a flag is not set in negated_flags, but set in required_flags,
then it means "this flag must be one" - equivalent to it being set in
required_flag (in your current version of the API).
2. if a flag is set in negated_flags and also in required_flags, then
it means "this flag must be zero" - equivalent to it being set in
excluded_flags.

The same thing goes for anyof_flags: if a flag is set in anyof_flags,
then for it to be considered matched:
1. it must have a value of 1 if it is not set in negated_flags
2. it must have a value of 0 if it is set in negated_flags

BTW, I think I assumed that both conditions (all flags in
required_flags and at least one in anyof_flags is present) need to be
true for the page to be selected - is this your intention? The example
code has a bug though, in that if anyof_flags is zero it will never
match. Let me fix the selection part:

// calc. a mask of flags that have expected ("active") values
tested_flags =3D page_flags ^ negated_flags;
// are all required flags in "active" state? [=3D=3D all zero when negated]
if (~tested_flags & required_mask)
  skip page;
// is any extra flag "active"?
if (anyof_flags && !(tested_flags & anyof_flags))
  skip page;


Best Regards
Micha=C5=82 Miros=C5=82aw