linux-kernel - Re: [PATCH v19 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Message-ID: <CABb0KFHpE+jJH0MmxZTFaQ9FNFNUnJcnnv7sSGDYqDqqB_FRqw@mail.gmail.com>
Date:   Wed, 21 Jun 2023 15:29:20 +0200
From:   Michał Mirosław <emmir@...gle.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Andrei Vagin <avagin@...il.com>, Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        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 v19 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Wed, 21 Jun 2023 at 08:35, Muhammad Usama Anjum
<usama.anjum@...labora.com> wrote:
> On 6/20/23 11:03 PM, Andrei Vagin wrote:
> ...
> >> +struct pagemap_scan_private {
> >> +    struct page_region *vec_buf, cur_buf;
> >> +    unsigned long long vec_buf_len, vec_buf_index, max_pages, found_pages, flags;
> >
> > should it be just unsigned long?
> These internal values are storing data coming from user in struct
> pm_scan_arg in which all variables are 64 bit(__u64) explicitly. This is
> why we have unsigned long long here. It is absolutely necessary.

vec_buf_len and vec_buf_index can only have values in 0..512 range.
flags has only a few lower bits defined (this is checked on ioctl
entry) and max_pages can be limited to ULONG_MAX. Actually putting `if
(!max_pages || max_pages > ULONG_MAX) max_pages = ULONG_MAX` would
avoid having to check !max_pages during the walk.

Best Regards
Michał Mirosław