linux-ext4 - [PATCH] mmap.2: Add description of MAP_SHARED_VALIDATE and MAP

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Date:   Wed,  1 Nov 2017 16:36:48 +0100
From:   Jan Kara <jack@...e.cz>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Christoph Hellwig <hch@...radead.org>,
        <linux-fsdevel@...r.kernel.org>, linux-nvdimm@...ts.01.org,
        <linux-mm@...ck.org>, linux-api@...r.kernel.org,
        <linux-ext4@...r.kernel.org>, <linux-xfs@...r.kernel.org>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        Jan Kara <jack@...e.cz>
Subject: [PATCH] mmap.2: Add description of MAP_SHARED_VALIDATE and MAP_SYNC

Reviewed-by: Ross Zwisler <ross.zwisler@...ux.intel.com>
Signed-off-by: Jan Kara <jack@...e.cz>
---
 man2/mmap.2 | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/man2/mmap.2 b/man2/mmap.2
index 47c3148653be..b38ee6809327 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -125,6 +125,21 @@ are carried through to the underlying file.
 to the underlying file requires the use of
 .BR msync (2).)
 .TP
+.BR MAP_SHARED_VALIDATE " (since Linux 4.15)"
+The same as
+.B MAP_SHARED
+except that
+.B MAP_SHARED
+mappings ignore unknown flags in
+.IR flags .
+In contrast when creating mapping of
+.B MAP_SHARED_VALIDATE
+mapping type, the kernel verifies all passed flags are known and fails the
+mapping with
+.BR EOPNOTSUPP
+otherwise. This mapping type is also required to be able to use some mapping
+flags.
+.TP
 .B MAP_PRIVATE
 Create a private copy-on-write mapping.
 Updates to the mapping are not visible to other processes
@@ -134,7 +149,10 @@ It is unspecified whether changes made to the file after the
 .BR mmap ()
 call are visible in the mapped region.
 .PP
-Both of these flags are described in POSIX.1-2001 and POSIX.1-2008.
+.B MAP_SHARED
+and
+.B MAP_PRIVATE
+are described in POSIX.1-2001 and POSIX.1-2008.
 .PP
 In addition, zero or more of the following values can be ORed in
 .IR flags :
@@ -352,6 +370,21 @@ option.
 Because of the security implications,
 that option is normally enabled only on embedded devices
 (i.e., devices where one has complete control of the contents of user memory).
+.TP
+.BR MAP_SYNC " (since Linux 4.15)"
+This flags is available only with
+.B MAP_SHARED_VALIDATE
+mapping type. Mappings of
+.B MAP_SHARED
+type will silently ignore this flag.
+This flag is supported only for files supporting DAX (direct mapping of persistent
+memory). For other files, creating mapping with this flag results in
+.B EOPNOTSUPP
+error. Shared file mappings with this flag provide the guarantee that while
+some memory is writeably mapped in the address space of the process, it will
+be visible in the same file at the same offset even after the system crashes or
+is rebooted. This allows users of such mappings to make data modifications
+persistent in a more efficient way using appropriate CPU instructions.
 .PP
 Of the above flags, only
 .B MAP_FIXED
-- 
2.12.3