linux-kernel - Re: [PATCH v26 2/5] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

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Date:   Thu, 27 Jul 2023 13:40:30 +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,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>
Subject: Re: [PATCH v26 2/5] fs/proc/task_mmu: Implement IOCTL to get and
 optionally clear info about PTEs

On Thu, 27 Jul 2023 at 11:37, Muhammad Usama Anjum
<usama.anjum@...labora.com> wrote:
> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
> the info about page table entries. The following operations are supported
> in this ioctl:
> - Get the information if the pages have Async Write-Protection enabled
>   (``PAGE_IS_WPALLOWED``), have been written to (``PAGE_IS_WRITTEN``), file
>   mapped (``PAGE_IS_FILE``), present (``PAGE_IS_PRESENT``), swapped
>   (``PAGE_IS_SWAPPED``) or page has pfn zero (``PAGE_IS_PFNZERO``).
> - Find pages which have been written to and/or write protect
>   (atomic ``PM_SCAN_WP_MATCHING + PM_SCAN_CHECK_WPASYNC``) the pages
>   atomically. The (``PM_SCAN_WP_MATCHING``) is used to WP the matched
>   pages. The (``PM_SCAN_CHECK_WPASYNC``) aborts the operation if
>   non-Async-Write-Protected pages are found. Get is automatically performed
>   if output buffer is specified.
>
> This IOCTL can be extended to get information about more PTE bits. The
> entire address range passed by user [start, end) is scanned until either
> the user provided buffer is full or max_pages have been found.
>
> Reviewed-by: Andrei Vagin <avagin@...il.com>
> Reviewed-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> Signed-off-by: Michał Mirosław <mirq-linux@...e.qmqm.pl>
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@...labora.com>

Thanks for all the work!

Small request below.

> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
[...]
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +static unsigned long pagemap_thp_category(pmd_t pmd)
> +{
> +       unsigned long categories = 0;
> +
> +       if (pmd_present(pmd)) {
> +               categories |= PAGE_IS_PRESENT;
> +               if (!pmd_uffd_wp(pmd))
> +                       categories |= PAGE_IS_WRITTEN;
> +               if (is_zero_pfn(pmd_pfn(pmd)))
> +                       categories |= PAGE_IS_PFNZERO;
> +       } else if (is_swap_pmd(pmd)) {
> +               categories |= PAGE_IS_SWAPPED;
> +               if (!pmd_swp_uffd_wp(pmd))
> +                       categories |= PAGE_IS_WRITTEN;
> +       }
> +
> +       return categories;
> +}
[...]
> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> +
> +#ifdef CONFIG_HUGETLB_PAGE
> +static unsigned long pagemap_hugetlb_category(pte_t pte)
> +{
> +       unsigned long categories = 0;
> +
> +       if (pte_present(pte)) {
> +               categories |= PAGE_IS_PRESENT;
> +               if (!huge_pte_uffd_wp(pte))
> +                       categories |= PAGE_IS_WRITTEN;
> +               if (!PageAnon(pte_page(pte)))
> +                       categories |= PAGE_IS_FILE;
> +               if (is_zero_pfn(pte_pfn(pte)))
> +                       categories |= PAGE_IS_PFNZERO;
> +       } else if (is_swap_pte(pte)) {
> +               categories |= PAGE_IS_SWAPPED;
> +               if (!pte_swp_uffd_wp_any(pte))
> +                       categories |= PAGE_IS_WRITTEN;
> +       }
> +
> +       return categories;
> +}

Could you add PAGE_IS_HUGE for THP and HugeTLB pages? This would help
maintaining checkpointed process'es page sizes by CRIU when THP is
used.

Best Regards
Michał Mirosław