On (15/07/08 12:13), David Gibson didst pronounce: > On Mon, Jul 14, 2008 at 10:17:00PM +0100, Mel Gorman wrote: > > When HUGETLB_SHARE=1, read-only text segments are copied to a file on > > a hugetlbfs mount and shared between multiple processes. However with > > kernel 2.6.27 and later, the expectation is that pages will be reserved for > > MAP_PRIVATE mappings as well as MAP_SHARED. This will render HUGETLB_SHARE > > ineffective as huge pages will be reserved that already exist in the page > > cache. This patch disables reservations for read-only mappings when sharing > > is enabled on the assumption mprotect() will never be called for remapped > > read-only segments. > > Sorry, I don't 100% follow what the problem is here. Are you saying > the pages get over-reserved, once for the SHARED mapping and once for > the PRIVATE? If that's the case, isn't that a kernel bug? >
The excessive reservation is due to how libhugetlbfs does the shared backing. It first maps MAP_SHARED (taking a reserve), copies the data and unmaps the file. The reserves are now consumed and in page cache. It later maps that file MAP_PRIVATE (again taking a reserve) but in this case we know that the page is already in page cache and a COW will never be necessary. At this point, the reserve pages are wasted memory as they will never be used. The MAP_NORESERVE is used in this case because we know the MAP_PRIVATE mapping will be safe for the lifetime of the process and the pages already exist. The kernel is forced to take a reservation at mmap() time as mprotect() could be called making a read-only mapping read-write. At that point, the application could fail. In theory, the kernel could force the reservation to take place at mprotect() and fail if the reservation could not be made but it would be fairly complex and I expect mprotect() failing to be a big surprise to a developer and the condition not handled properly. -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab ------------------------------------------------------------------------- This SF.Net email is sponsored by the Moblin Your Move Developer's challenge Build the coolest Linux based applications with Moblin SDK & win great prizes Grand prize is a trip for two to an Open Source event anywhere in the world http://moblin-contest.org/redirect.php?banner_id=100&url=/ _______________________________________________ Libhugetlbfs-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/libhugetlbfs-devel
