I would like to add a new MAP_ flag to mmap() prior to the 4.x freeze.
The purpose of the flag is to prevent the syncer from syncing the
file underlying the map. The VM system will still page to the file as
needed and, of course, the data will remain consistent with the file.
This will allow normal files to be used to back (for example) shared
memory without incuring the extra overhead that sync causes every 30
seconds. Currently it is not possible to use normal files in this
fashion (verses using MAP_ANON, for which sharability is limited, or
SysV shared memory which often has unexpected limitations) and still
have an efficient system.
The flag will be called MAP_NOSYNC
Operationally, the syncer will not sync the file while any mmap()'s
exist with that flag set. Once such flagged maps go away, the syncer
will be able to sync the file (if it still exists).
Comments are welcome.
-Matt
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