From 036e7aa49fb29e0b49b99a56fa5611d4a5b99fb1 Mon Sep 17 00:00:00 2001 From: Matthew Wilcox Date: Wed, 31 Jan 2018 16:19:02 -0800 Subject: [PATCH] mm: store compound_dtor / compound_order as bytes Neither of these values get even close to 256; compound_dtor is currently at a maximum of 3, and compound_order can't be over 64. No machine has inefficient access to bytes since EV5, and while those are still supported, we don't optimise for them any more. This does not shrink struct page, but it removes an ifdef and frees up 2-6 bytes for future use. diff of pahole output: struct callback_head callback_head; /* 32 16 */ struct { long unsigned int compound_head; /* 32 8 */ - unsigned int compound_dtor; /* 40 4 */ - unsigned int compound_order; /* 44 4 */ + unsigned char compound_dtor; /* 40 1 */ + unsigned char compound_order; /* 41 1 */ }; /* 32 16 */ }; /* 32 16 */ union { [mawilcox@microsoft.com: add comment] Link: http://lkml.kernel.org/r/20171221000144.GB2980@bombadil.infradead.org Link: http://lkml.kernel.org/r/20171220155552.15884-7-willy@infradead.org Signed-off-by: Matthew Wilcox Acked-by: Kirill A. Shutemov Acked-by: Michal Hocko Cc: Christoph Lameter Cc: Randy Dunlap Signed-off-by: Matthew Wilcox Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mm_types.h | 16 +++------------- 1 file changed, 3 insertions(+), 13 deletions(-) diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 5521c9799c50..3e7e99784656 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -136,19 +136,9 @@ struct page { unsigned long compound_head; /* If bit zero is set */ /* First tail page only */ -#ifdef CONFIG_64BIT - /* - * On 64 bit system we have enough space in struct page - * to encode compound_dtor and compound_order with - * unsigned int. It can help compiler generate better or - * smaller code on some archtectures. - */ - unsigned int compound_dtor; - unsigned int compound_order; -#else - unsigned short int compound_dtor; - unsigned short int compound_order; -#endif + unsigned char compound_dtor; + unsigned char compound_order; + /* two/six bytes available here */ }; #if defined(CONFIG_TRANSPARENT_HUGEPAGE) && USE_SPLIT_PMD_PTLOCKS -- 2.30.2