linux-kernel - Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE

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Message-ID: <CABb0KFGCm=K2X3-O=y3BJN85sT2C-y+XZRtLxnuabuOg+OrHwQ@mail.gmail.com>
Date:   Tue, 18 Oct 2022 13:11:25 +0200
From:   Michał Mirosław <emmir@...gle.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc:     Danylo Mocherniuk <mdanylo@...gle.com>, avagin@...il.com,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        gregkh@...uxfoundation.org, corbet@....net, david@...hat.com,
        kernel@...labora.com, krisman@...labora.com,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        peter.enderborg@...y.com, shuah@...nel.org,
        viro@...iv.linux.org.uk, willy@...radead.org, figiel@...gle.com,
        kyurtsever@...gle.com, Paul Gofman <pgofman@...eweavers.com>,
        surenb@...gle.com
Subject: Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE

On Tue, 18 Oct 2022 at 12:36, Muhammad Usama Anjum
<usama.anjum@...labora.com> wrote:
[...]
> I've included the masks which the CRIU developers have specified.
> max_out_page is another new optional variable which is needed to
> terminate the operation without visiting all the pages after finding the
> max_out_page number of desired pages. There is no way to terminate the
> operation without this variable.
>
> How does the interface looks now? Please comment.
>
> /* PAGEMAP IOCTL */
> #define PAGEMAP_GET             _IOWR('f', 16, struct pagemap_sd_args)
> #define PAGEMAP_CLEAR           _IOWR('f', 17, struct pagemap_sd_args)
> #define PAGEMAP_GET_AND_CLEAR   _IOWR('f', 18, struct pagemap_sd_args)

Why are three IOCTLs needed? Could CLEAR be a flag (like the
PAGEMAP_NO_REUSED_REGIONS) or 'cmask' and GET be implied when vec !=
NULL?

> /* Bits are set in the bitmap of the page_region and masks in
> pagemap_sd_args */
> #define PAGE_IS_SD      1 << 0
> #define PAGE_IS_FILE    1 << 1
> #define PAGE_IS_PRESENT 1 << 2
> #define PAGE_IS_SWAPED  1 << 3
>
> /**
>   * struct page_region - Page region with bitmap flags
>   * @start:     Start of the region
>   * @len:       Length of the region
>   * bitmap:     Bits sets for the region
>   */
> struct page_region {
>         __u64 start;
>         __u64 len;
>         __u64 bitmap;
> };

Could you explain what units start and len are using? Are they bytes
or pages (what size)?

> /**
>   * struct pagemap_sd_args - Soft-dirty IOCTL argument

Nit: it's not soft-dirty-specific anymore. Maybe "pagemap_scan_args"?

>   * @start:             Starting address
>   * @len:               Length of the region
>   * @vec:               Output page_region struct array
>   * @vec_len:           Length of the page_region struct array
>   * @max_out_page:      Optional max output pages (It must be less than
> vec_len if specified)

Why is it required to be less than vec_len? vec_len effectively
specifies max number of ranges to find, and this new additional field
counts pages, I suppose?
BTW, if we count pages, then what size of them? Maybe using bytes
(matching start/len fields) would be more consistent?

>   * @flags:             Special flags for the IOCTL
>   * @rmask:             Special flags for the IOCTL
>   * @amask:             Special flags for the IOCTL
>   * @emask:             Special flags for the IOCTL
>   * @__reserved:                Reserved member to preserve data alignment. Must be 0.
>   */
> struct pagemap_sd_args {
>         __u64 __user start;
>         __u64 len;
>         __u64 __user vec; // page_region
>         __u64 vec_len;    // sizeof(page_region)
>         __u32 flags;      // special flags
>         __u32 rmask;
>         __u32 amask;
>         __u32 emask;
>         __u32 max_out_page;
>         __u32 __reserved;
> };
>
> /* Special flags */
> #define PAGEMAP_NO_REUSED_REGIONS       0x1

What does this flag do?

Best Regards
Michał Mirosław