From bb451fdf3d058dfecee1c0b965342d344e7d8317 Mon Sep 17 00:00:00 2001 From: Greg Thelen Date: Fri, 17 Aug 2018 15:45:19 -0700 Subject: [PATCH] mm/vmscan.c: condense scan_control Use smaller scan_control fields for order, priority, and reclaim_idx. Convert fields from int => s8. All easily fit within a byte: - allocation order range: 0..MAX_ORDER(64?) - priority range: 0..12(DEF_PRIORITY) - reclaim_idx range: 0..6(__MAX_NR_ZONES) Since 6538b8ea886e ("x86_64: expand kernel stack to 16K") x86_64 stack overflows are not an issue. But it's inefficient to use ints. Use s8 (signed byte) rather than u8 to allow for loops like: do { ... } while (--sc.priority >= 0); Add BUILD_BUG_ON to verify that s8 is capable of storing max values. This reduces sizeof(struct scan_control): - 96 => 80 bytes (x86_64) - 68 => 56 bytes (i386) scan_control structure field order is changed to utilize padding. After this patch there is 1 bit of scan_control padding. akpm: makes my vmscan.o's .text 572 bytes smaller as well. Link: http://lkml.kernel.org/r/20180530061212.84915-1-gthelen@google.com Signed-off-by: Greg Thelen Suggested-by: Matthew Wilcox Reviewed-by: Andrew Morton Cc: Michal Hocko Cc: Johannes Weiner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/vmscan.c | 32 ++++++++++++++++++++------------ 1 file changed, 20 insertions(+), 12 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index 03822f86f288..a00d94530e57 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -65,12 +65,6 @@ struct scan_control { /* How many pages shrink_list() should reclaim */ unsigned long nr_to_reclaim; - /* This context's GFP mask */ - gfp_t gfp_mask; - - /* Allocation order */ - int order; - /* * Nodemask of nodes allowed by the caller. If NULL, all nodes * are scanned. @@ -83,12 +77,6 @@ struct scan_control { */ struct mem_cgroup *target_mem_cgroup; - /* Scan (total_size >> priority) pages at once */ - int priority; - - /* The highest zone to isolate pages for reclaim from */ - enum zone_type reclaim_idx; - /* Writepage batching in laptop mode; RECLAIM_WRITE */ unsigned int may_writepage:1; @@ -111,6 +99,18 @@ struct scan_control { /* One of the zones is ready for compaction */ unsigned int compaction_ready:1; + /* Allocation order */ + s8 order; + + /* Scan (total_size >> priority) pages at once */ + s8 priority; + + /* The highest zone to isolate pages for reclaim from */ + s8 reclaim_idx; + + /* This context's GFP mask */ + gfp_t gfp_mask; + /* Incremented by the number of inactive pages that were scanned */ unsigned long nr_scanned; @@ -3063,6 +3063,14 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order, .may_swap = 1, }; + /* + * scan_control uses s8 fields for order, priority, and reclaim_idx. + * Confirm they are large enough for max values. + */ + BUILD_BUG_ON(MAX_ORDER > S8_MAX); + BUILD_BUG_ON(DEF_PRIORITY > S8_MAX); + BUILD_BUG_ON(MAX_NR_ZONES > S8_MAX); + /* * Do not enter reclaim if fatal signal was delivered while throttled. * 1 is returned so that the page allocator does not OOM kill at this