linux-kernel - Re: [PATCH v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

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Date:   Mon, 20 Feb 2023 15:26:42 +0200
From:   Mike Rapoport <rppt@...nel.org>
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>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Cyrill Gorcunov <gorcunov@...il.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>,
        Nadav Amit <namit@...are.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 v10 3/6] fs/proc/task_mmu: Implement IOCTL to get and/or
 the clear info about PTEs

Hi,

On Thu, Feb 02, 2023 at 04:29:12PM +0500, Muhammad Usama Anjum 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 been written-to (PAGE_IS_WRITTEN),
>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
>   (PAGE_IS_SWAPPED).
> - Write-protect the pages (PAGEMAP_WP_ENGAGE) to start finding which
>   pages have been written-to.
> - Find pages which have been written-to and write protect the pages
>   (atomic PAGE_IS_WRITTEN + PAGEMAP_WP_ENGAGE)
> 
> +/*
> + * struct pagemap_scan_arg - Pagemap ioctl argument
> + * @start:		Starting address of the region
> + * @len:		Length of the region (All the pages in this length are included)
> + * @vec:		Address of page_region struct array for output
> + * @vec_len:		Length of the page_region struct array
> + * @max_pages:		Optional max return pages
> + * @flags:		Flags for the IOCTL
> + * @required_mask:	Required mask - All of these bits have to be set in the PTE
> + * @anyof_mask:		Any mask - Any of these bits are set in the PTE
> + * @excluded_mask:	Exclude mask - None of these bits are set in the PTE
> + * @return_mask:	Bits that are to be reported in page_region
> + */
> +struct pagemap_scan_arg {
> +	__u64 start;
> +	__u64 len;
> +	__u64 vec;
> +	__u64 vec_len;
> +	__u32 max_pages;
> +	__u32 flags;
> +	__u64 required_mask;
> +	__u64 anyof_mask;
> +	__u64 excluded_mask;
> +	__u64 return_mask;
> +};

After Nadav's comment I've realized I missed the API part :)

A few quick notes for now:
* The arg struct is fixed, so it would be impossible to extend the API
later. Following the clone3() example, I'd add 'size' field to the
pagemam_scan_arg so that it would be possible to add new fields afterwards.
* Please make flags __u64, just in case
* Put size and flags at the beginning of the struct, e.g.

strucr pagemap_scan_arg {
	size_t size;
	__u64 flags;
	/* all the rest */
};

> +
> +/* Special flags */
> +#define PAGEMAP_WP_ENGAGE	(1 << 0)
> +
>  #endif /* _UAPI_LINUX_FS_H */
> -- 
> 2.30.2
> 

-- 
Sincerely yours,
Mike.