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([2001:a61:2553:a401:cfc6:2039:a9ec:21ff]) by smtp.gmail.com with ESMTPSA id v189sm726455wme.39.2021.04.05.14.14.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 05 Apr 2021 14:14:11 -0700 (PDT) Cc: mtk.manpages@gmail.com, Peter Xu , linux-man@vger.kernel.org, Mike Rapoport Subject: Re: [PATCH 5/5] ioctl_userfaultfd.2: Add write-protect mode docs To: Alejandro Colomar References: <20210405131307.2892-1-alx.manpages@gmail.com> <20210405131307.2892-6-alx.manpages@gmail.com> From: "Michael Kerrisk (man-pages)" Message-ID: Date: Mon, 5 Apr 2021 23:14:10 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0 MIME-Version: 1.0 In-Reply-To: <20210405131307.2892-6-alx.manpages@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org Hello Alex, Peter, I've applied the patch below, but I have some questions. On 4/5/21 3:13 PM, Alejandro Colomar wrote: > From: Peter Xu > > Userfaultfd write-protect mode is supported starting from Linux 5.7. > > Acked-by: Mike Rapoport > Signed-off-by: Peter Xu > [alx: ffix + srcfix] > Signed-off-by: Alejandro Colomar > --- > man2/ioctl_userfaultfd.2 | 86 ++++++++++++++++++++++++++++++++++++++-- > 1 file changed, 83 insertions(+), 3 deletions(-) > > diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2 > index 014c0b5c1..c29e0bb6a 100644 > --- a/man2/ioctl_userfaultfd.2 > +++ b/man2/ioctl_userfaultfd.2 > @@ -234,6 +234,11 @@ operation is supported. > The > .B UFFDIO_UNREGISTER > operation is supported. > +.TP > +.B 1 << _UFFDIO_WRITEPROTECT > +The > +.B UFFDIO_WRITEPROTECT > +operation is supported. > .PP > This > .BR ioctl (2) > @@ -322,9 +327,6 @@ Track page faults on missing pages. > .B UFFDIO_REGISTER_MODE_WP > Track page faults on write-protected pages. > .PP > -Currently, the only supported mode is > -.BR UFFDIO_REGISTER_MODE_MISSING . > -.PP > If the operation is successful, the kernel modifies the > .I ioctls > bit-mask field to indicate which > @@ -443,6 +445,16 @@ operation: > .TP > .B UFFDIO_COPY_MODE_DONTWAKE > Do not wake up the thread that waits for page-fault resolution > +.TP > +.B UFFDIO_COPY_MODE_WP > +Copy the page with read-only permission. > +This allows the user to trap the next write to the page, What is "the user" in this context? I think you really mean something like "the fault-handling thread"" or something like that? > +which will block and generate another write-protect userfault message. > +This is only used when both > +.B UFFDIO_REGISTER_MODE_MISSING > +and > +.B UFFDIO_REGISTER_MODE_WP > +modes are enabled for the registered range. > .PP > The > .I copy > @@ -654,6 +666,74 @@ field of the > structure was not a multiple of the system page size; or > .I len > was zero; or the specified range was otherwise invalid. > +.SS UFFDIO_WRITEPROTECT (Since Linux 5.7) > +Write-protect or write-unprotect an userfaultfd registered memory range > +registered with mode > +.BR UFFDIO_REGISTER_MODE_WP . > +.PP > +The > +.I argp > +argument is a pointer to a > +.I uffdio_range > +structure as shown below: > +.PP > +.in +4n > +.EX > +struct uffdio_writeprotect { > + struct uffdio_range range; /* Range to change write permission*/ > + __u64 mode; /* Mode to change write permission */ > +}; > +.EE > +.in > +.PP > +There're two mode bits that are supported in this structure: > +.TP > +.B UFFDIO_WRITEPROTECT_MODE_WP > +When this mode bit is set, > +the ioctl will be a write-protect operation upon the memory range specified by > +.IR range . > +Otherwise it'll be a write-unprotect operation upon the specified range, > +which can be used to resolve an userfaultfd write-protect page fault. > +.TP > +.B UFFDIO_WRITEPROTECT_MODE_DONTWAKE > +When this mode bit is set, > +do not wake up any thread that waits for > +page-fault resolution after the operation. > +This could only be specified if > +.B UFFDIO_WRITEPROTECT_MODE_WP > +is not specified. > +.PP > +This > +.BR ioctl (2) > +operation returns 0 on success. > +On error, \-1 is returned and > +.I errno > +is set to indicate the error. > +Possible errors include: > +.TP > +.B EINVAL > +The > +.I start > +or the > +.I len > +field of the > +.I ufdio_range > +structure was not a multiple of the system page size; or > +.I len > +was zero; or the specified range was otherwise invalid. > +.TP > +.B EAGAIN > +The process was interrupted; retry this call. > +.TP > +.B ENOENT > +The range specified in > +.I range > +is not valid. > +For example, the virtual address does not exist, > +or not registered with userfaultfd write-protect mode. > +.TP > +.B EFAULT > +Encountered a generic fault during processing. > .SH RETURN VALUE > See descriptions of the individual operations, above. > .SH ERRORS Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/