linux-kernel - Re: [PATCH v4 1/4] userfaultfd.2: Add UFFD_FEATURE_THREAD

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Date:   Tue, 23 Mar 2021 10:27:34 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-man@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Alejandro Colomar <alx.manpages@...il.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 1/4] userfaultfd.2: Add UFFD_FEATURE_THREAD_ID docs

On Mon, Mar 22, 2021 at 06:08:45PM -0400, Peter Xu wrote:
> UFFD_FEATURE_THREAD_ID is supported since Linux 4.14.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  man2/userfaultfd.2 | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index e7dc9f813..555e37409 100644
> --- a/man2/userfaultfd.2
> +++ b/man2/userfaultfd.2
> @@ -77,6 +77,13 @@ When the last file descriptor referring to a userfaultfd object is closed,
>  all memory ranges that were registered with the object are unregistered
>  and unread events are flushed.
>  .\"
> +.PP
> +Since Linux 4.14, userfaultfd page fault message can selectively embed faulting
> +thread ID information into the fault message.
> +One needs to enable this feature explicitly using the
> +.BR UFFD_FEATURE_THREAD_ID
> +feature bit when initializing the userfaultfd context.
> +By default, thread ID reporting is diabled.

				     ^ disabled :)
>  .SS Usage
>  The userfaultfd mechanism is designed to allow a thread in a multithreaded
>  program to perform user-space paging for the other threads in the process.
> @@ -229,6 +236,9 @@ struct uffd_msg {
>          struct {
>              __u64 flags;    /* Flags describing fault */
>              __u64 address;  /* Faulting address */
> +            union {
> +                __u32 ptid; /* Thread ID of the fault */
> +            } feat;
>          } pagefault;
> 
>          struct {            /* Since Linux 4.11 */
> @@ -358,6 +368,9 @@ otherwise it is a read fault.
>  .\" UFFD_PAGEFAULT_FLAG_WP is not yet supported.
>  .RE
>  .TP
> +.I pagefault.feat.pid
> +The thread ID that triggered the page fault.
> +.TP
>  .I fork.ufd
>  The file descriptor associated with the userfault object
>  created for the child created by
> -- 
> 2.26.2
> 

-- 
Sincerely yours,
Mike.