I received two patches from SUSE against the 2.4.21 Linux kernel that implement xip2fs, or Execute In Place ext2 file system. The commentary I received was this: "Xip2fs is a file system driver that allows you to mount an ext2-compatible file system from a DCSS (VM Discontiguous Shared Segment). The benefit of using xip2fs instead of mounting the file system as ext2 using the DCSS block device driver is that xip2fs provides execute-in-place capability.
"This means that memory-mapping a file residing on xip2fs will simply result in the user process' page table entries pointing directly to the DCSS pages, as opposed to reading file pages into the page cache (as a typical ext2 mounted file system would do). "Since all executables and shared libraries are memory-mapped in order to execute, running those from an xip2 mounted file system will mean that all processes across all VM guests in the system that use these files will share the same physical pages of memory to hold the executable code, potentially resulting in significant overall memory savings." This looks like a nice way to reduce the storage footprint for Linux/390 guests that are using a shared read-only file system in a DCSS. My understanding is that documentation for this new driver will be arriving in the next week or so. When I hear anything definite, I'll send another note to the list. The patches are at http://linuxvm.org/Patches, as usual, but there appears to be a problem with the web server when trying to retrieve patch #1. So, instead of using file:///d:/webpages/linuxvm.org/Patches/S390/xip2fs_part1.diff.gz as the URL, try file:///d:/webpages/linuxvm.org/Patches/S390/xip2fs1.gz and see if that works better. Mark Post
