From: Hugh Dickins Date: Sat, 23 Feb 2013 00:36:06 +0000 (-0800) Subject: ksm: shrink 32-bit rmap_item back to 32 bytes X-Git-Tag: v3.9-rc1~99^2~4 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bc56620b493496b8a6962080b644ccc537f4d526;p=pandora-kernel.git ksm: shrink 32-bit rmap_item back to 32 bytes Think of struct rmap_item as an extension of struct page (restricted to MADV_MERGEABLE areas): there may be a lot of them, we need to keep them small, especially on 32-bit architectures of limited lowmem. Siting "int nid" after "unsigned int checksum" works nicely on 64-bit, making no change to its 64-byte struct rmap_item; but bloats the 32-bit struct rmap_item from (nicely cache-aligned) 32 bytes to 36 bytes, which rounds up to 40 bytes once allocated from slab. We'd better avoid that. Hey, I only just remembered that the anon_vma pointer in struct rmap_item has no purpose until the rmap_item is hung from a stable tree node (which has its own nid field); and rmap_item's nid field no purpose than to say which tree root to tell rb_erase() when unlinking from an unstable tree. Double them up in a union. There's just one place where we set anon_vma early (when we already hold mmap_sem): now we must remove tree_rmap_item from its unstable tree there, before overwriting nid. No need to spatter BUG()s around: we'd be seeing oopses if this were wrong. Signed-off-by: Hugh Dickins Cc: Mel Gorman Cc: Petr Holasek Cc: Andrea Arcangeli Cc: Izik Eidus Cc: Johannes Weiner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Reading git-diff-tree failed