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authorMatthew Wilcox <mawilcox@microsoft.com>2018-01-31 16:19:02 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2018-01-31 17:18:38 -0800
commit036e7aa49fb29e0b49b99a56fa5611d4a5b99fb1 (patch)
tree23f670d823c98fe81823c43c24f015518072bcc9
parent0dd4da5b110c6915d4244b8ed87a1c8d3945224b (diff)
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 <mawilcox@microsoft.com> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Acked-by: Michal Hocko <mhocko@suse.com> Cc: Christoph Lameter <cl@linux.com> Cc: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/mm_types.h16
1 files 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