path: root/mm/Kconfig
diff options
authorChristoph Lameter <>2008-04-28 02:12:55 -0700
committerLinus Torvalds <>2008-04-28 08:58:22 -0700
commite20b8cca760ed2a6abcfe37ef56f2306790db648 (patch)
tree85e5610364f73da193ee781be61710fddfbe045f /mm/Kconfig
parent97965478a66fbdf0f4ad5e4ecc4828f0cb548a45 (diff)
PAGEFLAGS_EXTENDED and separate page flags for Head and Tail
Having separate page flags for the head and the tail of a compound page allows the compiler to use bitops instead of operations on a word to check for a tail page. That is f.e. important for virt_to_head_page() which is used in various critical code paths (kfree for example): Code for PageTail(page) Before: mov (%rdi),%rdx page->flags mov %rdx,%rax 3 bytes and $0x12000,%eax 5 bytes cmp $0x12000,%rax 6 bytes je 897 <kfree+0xa7> After: mov (%rdi),%rax test $0x40,%ah (3 bytes) jne 887 <kfree+0x97> So we go from 14 bytes to 3 bytes and from 3 instructions to one. From the use of 2 registers we go to none. We can only use page flags for this if we have page flags available. This patch introduces CONFIG_PAGEFLAGS_EXTENDED that is set if pageflags are not scarce due to SPARSEMEM using page flags for its sectionid on 32 bit NUMA platforms. Additional page flag definitions can be added to the CONFIG_PAGEFLAGS_EXTENDED section in page-flags.h if the functionality depends on PAGEFLAGS_EXTENDED or if more page flag overlapping tricks are used for the !PAGEFLAGS_EXTENDED fallback (the upcoming virtual compound patch may hook in here and Rik's/Lee's additional page flags to solve the reclaim issues could also be added there [hint... hint... where are these patchsets?]). Avoiding the overlaying of Pg_reclaim also clears the way for possible use of compound pages for the pagecache or on the LRU. Signed-off-by: Christoph Lameter <> Cc: Nick Piggin <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'mm/Kconfig')
1 files changed, 12 insertions, 0 deletions
diff --git a/mm/Kconfig b/mm/Kconfig
index 0016ebd4dcb..3aa819d628c 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -143,6 +143,18 @@ config MEMORY_HOTREMOVE
depends on MIGRATION
+# If we have space for more page flags then we can enable additional
+# optimizations and functionality.
+# Regular Sparsemem takes page flag bits for the sectionid if it does not
+# use a virtual memmap. Disable extended page flags for 32 bit platforms
+# that require the use of a sectionid in the page flags.
+ def_bool y
# Heavily threaded applications may benefit from splitting the mm-wide
# page_table_lock, so that faults on different parts of the user address
# space can be handled with less contention: split it at this NR_CPUS.