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([2001:a61:2482:101:3351:6160:8173:cc31]) by smtp.gmail.com with ESMTPSA id 91sm8073740wra.37.2020.04.14.23.41.01 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 14 Apr 2020 23:41:02 -0700 (PDT) Cc: mtk.manpages@gmail.com, "Michael S . Tsirkin" , Arnd Bergmann , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-api@vger.kernel.org, Andy Lutomirski , Will Deacon , Andrea Arcangeli , Sonny Rao , Minchan Kim , Joel Fernandes , Yu Zhao , Jesse Barnes , Florian Weimer , "Kirill A . Shutemov" , linux-man@vger.kernel.org Subject: Re: [PATCH v7] mremap.2: Add information for MREMAP_DONTUNMAP. To: Brian Geffon , Andrew Morton References: <20200221174248.244748-1-bgeffon@google.com> <20200221174248.244748-3-bgeffon@google.com> From: "Michael Kerrisk (man-pages)" Message-ID: Date: Wed, 15 Apr 2020 08:40:59 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200221174248.244748-3-bgeffon@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hello Brian, I see that MREMAP_DONTUNMAP has been merged. Thanks for the patch below. In addition to Vlastimil's comments, could you please take a look at my comments below. On 2/21/20 6:42 PM, Brian Geffon wrote: > Signed-off-by: Brian Geffon > --- > man2/mremap.2 | 30 +++++++++++++++++++++++++++++- > 1 file changed, 29 insertions(+), 1 deletion(-) > > diff --git a/man2/mremap.2 b/man2/mremap.2 > index d73fb64fa..54ec67b20 100644 > --- a/man2/mremap.2 > +++ b/man2/mremap.2 > @@ -26,7 +26,8 @@ > .\" 1996-04-12 Tom Bjorkholm > .\" Update for Linux 1.3.87 and later > .\" 2005-10-11 mtk: Added NOTES for MREMAP_FIXED; revised EINVAL text. > -.\" > +.\" 2020-02-05 Brian Geffon > +.\" Update for MREMAP_DONTUNMAP. No need to add this piece. This info is maintained via the Git log these days. > .TH MREMAP 2 2019-03-06 "Linux" "Linux Programmer's Manual" > .SH NAME > mremap \- remap a virtual memory address > @@ -129,6 +130,13 @@ If > is specified, then > .B MREMAP_MAYMOVE > must also be specified. > +.TP > +.BR MREMAP_DONTUNMAP " (since Linux 5.7)" > +This flag which must be used in conjuction with s/conjuction/conjunction/ > +.B MREMAP_MAYMOVE > +remaps a mapping to a new address and it does not unmap the mapping at \fIold_address\fP. This flag can only be used with private anonymous mappings. Any access to the range specified at \fIold_address\fP after completion will result in a page fault. If a Please wrap source lines to no more than about 75 columns. Also, always start new sentences on new lines ("Semantic newlines"). As a general rule, I prefer formatting to be done like this: .BR old_address . rather than: \fIold_address\fP. (Yes, I know there's plenty of existing text that goes the other way, but I try to avoid the \fX...\fP style for new text. Re the "Any access to the range ... will result in a page fault", I think it would be helpful to be more explicit. I presume that if we access the range at old_address the mapping is repopulated with zero-filled pages, right? It would be good to note that explicitly, > +.BR userfaultfd (2) > +was registered on the mapping specified by \fIold_address\fP it will continue to watch that mapping for faults. (See comments above re wrapping and formatting.) Perhaps it would be nice to have a short paragraph on use cases? > .PP > If the memory segment specified by > .I old_address > @@ -176,6 +184,8 @@ a value other than > .B MREMAP_MAYMOVE > or > .B MREMAP_FIXED > +or > +.B MREMAP_DONTUNMAP > was specified in > .IR flags ; > .IP * > @@ -197,9 +207,17 @@ and > .IR old_size ; > .IP * > .B MREMAP_FIXED > +or > +.B MREMAP_DONTUNMAP > was specified without also specifying > .BR MREMAP_MAYMOVE ; > .IP * > +.B MREMAP_DONTUNMAP > +was specified with an \fIold_address\fP that was not private anonymous; > +.IP * > +.B MREMAP_DONTUNMAP > +was specified and \fIold_size\fP was not equal to \fInew_size\fP; > +.IP * > \fIold_size\fP was zero and \fIold_address\fP does not refer to a > shareable mapping (but see BUGS); > .IP * > @@ -209,10 +227,20 @@ flag was not specified. > .RE > .TP > .B ENOMEM > +Not enough memory was available to complete the operation. > +Possible causes are: > +.RS > +.IP * 3 > The memory area cannot be expanded at the current virtual address, and the > .B MREMAP_MAYMOVE > flag is not set in \fIflags\fP. > Or, there is not enough (virtual) memory available. > +.IP * > +.B MREMAP_DONTUNMAP > +was used without > +.B MREMAP_FIXED > +causing a new mapping to be created that would exceed the virtual memory available or it would exceed the maximum number of allowed mappings. (See comments above re wrapping.) > +.RE > .SH CONFORMING TO > This call is Linux-specific, and should not be used in programs > intended to be portable. Thanks, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/