From 505ecd7b30fca1dc296205ff3c0dc3aff9d76e12 Mon Sep 17 00:00:00 2001
From: Sergey Bugaev <bugaevc@gmail.com>
Date: Mon, 26 Jun 2023 14:26:50 +0300
Subject: Shrink struct vm_page size
struct vm_page is supposed to be a "small structure", but it takes up 96
bytes on x86_64 (to represent a 4k page). By utilizing bitfields and
strategically reordering members to avoid excessive padding, it can be
shrunk to 80 bytes.
- page_lock and unlock_request only need to store a bitmask of
VM_PROT_READ, VM_PROT_WRITE, and VM_PROT_EXECUTE. Even though the
special values VM_PROT_NO_CHANGE and VM_PROT_NOTIFY are defined, they
are not used for the two struct vm_page members.
- type and seg_index both need to store one of the four possible values
in the range from 0 to 3. Two bits are sufficient for this.
- order needs to store a number from 0 to VM_PAGE_NR_FREE_LISTS (which
is 11), or a special value VM_PAGE_ORDER_UNLISTED. Four bits are
sufficient for this.
No functional change.
Message-Id: <20230626112656.435622-2-bugaevc@gmail.com>
---
vm/vm_page.c | 4 ++--
vm/vm_page.h | 18 ++++++++++++------
vm/vm_resident.c | 2 +-
3 files changed, 15 insertions(+), 9 deletions(-)
(limited to 'vm')
diff --git a/vm/vm_page.c b/vm/vm_page.c
index 50916b74..ce37c8aa 100644
--- a/vm/vm_page.c
+++ b/vm/vm_page.c
@@ -94,7 +94,7 @@ struct vm_page_cpu_pool {
* Special order value for pages that aren't in a free list. Such pages are
* either allocated, or part of a free block of pages but not the head page.
*/
-#define VM_PAGE_ORDER_UNLISTED ((unsigned short)-1)
+#define VM_PAGE_ORDER_UNLISTED (VM_PAGE_NR_FREE_LISTS + 1)
/*
* Doubly-linked list of free blocks.
@@ -1016,7 +1016,7 @@ vm_page_seg_balance_page(struct vm_page_seg *seg,
vm_page_set_type(dest, 0, src->type);
memcpy(&dest->vm_page_header, &src->vm_page_header,
- sizeof(*dest) - VM_PAGE_HEADER_SIZE);
+ VM_PAGE_BODY_SIZE);
vm_page_copy(src, dest);
if (!src->dirty) {
diff --git a/vm/vm_page.h b/vm/vm_page.h
index d457f9a2..b2581d9e 100644
--- a/vm/vm_page.h
+++ b/vm/vm_page.h
@@ -79,9 +79,6 @@
struct vm_page {
struct list node; /* page queues or free list (P) */
- unsigned short type;
- unsigned short seg_index;
- unsigned short order;
void *priv;
/*
@@ -95,7 +92,6 @@ struct vm_page {
/* We use an empty struct as the delimiter. */
struct {} vm_page_header;
-#define VM_PAGE_HEADER_SIZE offsetof(struct vm_page, vm_page_header)
vm_object_t object; /* which object am I in (O,P) */
vm_offset_t offset; /* offset into that object (O,P) */
@@ -126,10 +122,20 @@ struct vm_page {
* without having data. (O)
* [See vm_object_overwrite] */
- vm_prot_t page_lock; /* Uses prohibited by data manager (O) */
- vm_prot_t unlock_request; /* Outstanding unlock request (O) */
+ vm_prot_t page_lock:3; /* Uses prohibited by data manager (O) */
+ vm_prot_t unlock_request:3; /* Outstanding unlock request (O) */
+
+ struct {} vm_page_footer;
+
+ unsigned short type:2;
+ unsigned short seg_index:2;
+ unsigned short order:4;
};
+#define VM_PAGE_BODY_SIZE \
+ (offsetof(struct vm_page, vm_page_footer) \
+ - offsetof(struct vm_page, vm_page_header))
+
/*
* For debugging, this macro can be defined to perform
* some useful check on a page structure.
diff --git a/vm/vm_resident.c b/vm/vm_resident.c
index e0a03bf5..d4777e70 100644
--- a/vm/vm_resident.c
+++ b/vm/vm_resident.c
@@ -746,7 +746,7 @@ boolean_t vm_page_convert(struct vm_page **mp)
memcpy(&real_m->vm_page_header,
&fict_m->vm_page_header,
- sizeof(*fict_m) - VM_PAGE_HEADER_SIZE);
+ VM_PAGE_BODY_SIZE);
real_m->fictitious = FALSE;
vm_page_insert(real_m, object, offset);
--
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