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mm: support only one page_type per page
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By using a few values in the top byte, users of page_type can store up to
24 bits of additional data in page_type.  It also reduces the code size as
(with replacement of READ_ONCE() with data_race()), the kernel can check
just a single byte.  eg:

ffffffff811e3a79:       8b 47 30                mov    0x30(%rdi),%eax
ffffffff811e3a7c:       55                      push   %rbp
ffffffff811e3a7d:       48 89 e5                mov    %rsp,%rbp
ffffffff811e3a80:       25 00 00 00 82          and    $0x82000000,%eax
ffffffff811e3a85:       3d 00 00 00 80          cmp    $0x80000000,%eax
ffffffff811e3a8a:       74 4d                   je     ffffffff811e3ad9 <folio_mapping+0x69>

becomes:

ffffffff811e3a69:       80 7f 33 f5             cmpb   $0xf5,0x33(%rdi)
ffffffff811e3a6d:       55                      push   %rbp
ffffffff811e3a6e:       48 89 e5                mov    %rsp,%rbp
ffffffff811e3a71:       74 4d                   je     ffffffff811e3ac0 <folio_mapping+0x60>

replacing three instructions with one.

[[email protected]: fix ubsan warnings]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Hyeonggon Yoo <[email protected]>
Cc: Kent Overstreet <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
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@akpm00
Matthew Wilcox (Oracle) authored and akpm00 committed Sep 4, 2024
1 parent e880034 commit 4ffca5a
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Showing 3 changed files with 56 additions and 51 deletions.
68 changes: 28 additions & 40 deletions include/linux/page-flags.h
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Original file line number Diff line number Diff line change
Expand Up @@ -923,42 +923,29 @@ PAGEFLAG_FALSE(HasHWPoisoned, has_hwpoisoned)
#endif

/*
* For pages that are never mapped to userspace,
* page_type may be used. Because it is initialised to -1, we invert the
* sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
* __ClearPageFoo *sets* the bit used for PageFoo. We reserve a few high and
* low bits so that an underflow or overflow of _mapcount won't be
* mistaken for a page type value.
* For pages that do not use mapcount, page_type may be used.
* The low 24 bits of pagetype may be used for your own purposes, as long
* as you are careful to not affect the top 8 bits. The low bits of
* pagetype will be overwritten when you clear the page_type from the page.
*/

enum pagetype {
PG_buddy = 0x40000000,
PG_offline = 0x20000000,
PG_table = 0x10000000,
PG_guard = 0x08000000,
PG_hugetlb = 0x04000000,
PG_slab = 0x02000000,
PG_zsmalloc = 0x01000000,
PG_unaccepted = 0x00800000,

PAGE_TYPE_BASE = 0x80000000,

/*
* Reserve 0xffff0000 - 0xfffffffe to catch _mapcount underflows and
* allow owners that set a type to reuse the lower 16 bit for their own
* purposes.
*/
PAGE_MAPCOUNT_RESERVE = ~0x0000ffff,
/* 0x00-0x7f are positive numbers, ie mapcount */
/* Reserve 0x80-0xef for mapcount overflow. */
PGTY_buddy = 0xf0,
PGTY_offline = 0xf1,
PGTY_table = 0xf2,
PGTY_guard = 0xf3,
PGTY_hugetlb = 0xf4,
PGTY_slab = 0xf5,
PGTY_zsmalloc = 0xf6,
PGTY_unaccepted = 0xf7,

PGTY_mapcount_underflow = 0xff
};

#define PageType(page, flag) \
((READ_ONCE(page->page_type) & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)
#define folio_test_type(folio, flag) \
((READ_ONCE(folio->page.page_type) & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE)

static inline bool page_type_has_type(int page_type)
{
return page_type < PAGE_MAPCOUNT_RESERVE;
return page_type < (PGTY_mapcount_underflow << 24);
}

/* This takes a mapcount which is one more than page->_mapcount */
Expand All @@ -969,40 +956,41 @@ static inline bool page_mapcount_is_type(unsigned int mapcount)

static inline bool page_has_type(const struct page *page)
{
return page_type_has_type(READ_ONCE(page->page_type));
return page_mapcount_is_type(data_race(page->page_type));
}

#define FOLIO_TYPE_OPS(lname, fname) \
static __always_inline bool folio_test_##fname(const struct folio *folio)\
static __always_inline bool folio_test_##fname(const struct folio *folio) \
{ \
return folio_test_type(folio, PG_##lname); \
return data_race(folio->page.page_type >> 24) == PGTY_##lname; \
} \
static __always_inline void __folio_set_##fname(struct folio *folio) \
{ \
VM_BUG_ON_FOLIO(!folio_test_type(folio, 0), folio); \
folio->page.page_type &= ~PG_##lname; \
VM_BUG_ON_FOLIO(data_race(folio->page.page_type) != UINT_MAX, \
folio); \
folio->page.page_type = (unsigned int)PGTY_##lname << 24; \
} \
static __always_inline void __folio_clear_##fname(struct folio *folio) \
{ \
VM_BUG_ON_FOLIO(!folio_test_##fname(folio), folio); \
folio->page.page_type |= PG_##lname; \
folio->page.page_type = UINT_MAX; \
}

#define PAGE_TYPE_OPS(uname, lname, fname) \
FOLIO_TYPE_OPS(lname, fname) \
static __always_inline int Page##uname(const struct page *page) \
{ \
return PageType(page, PG_##lname); \
return data_race(page->page_type >> 24) == PGTY_##lname; \
} \
static __always_inline void __SetPage##uname(struct page *page) \
{ \
VM_BUG_ON_PAGE(!PageType(page, 0), page); \
page->page_type &= ~PG_##lname; \
VM_BUG_ON_PAGE(data_race(page->page_type) != UINT_MAX, page); \
page->page_type = (unsigned int)PGTY_##lname << 24; \
} \
static __always_inline void __ClearPage##uname(struct page *page) \
{ \
VM_BUG_ON_PAGE(!Page##uname(page), page); \
page->page_type |= PG_##lname; \
page->page_type = UINT_MAX; \
}

/*
Expand Down
8 changes: 4 additions & 4 deletions kernel/vmcore_info.c
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Original file line number Diff line number Diff line change
Expand Up @@ -198,17 +198,17 @@ static int __init crash_save_vmcoreinfo_init(void)
VMCOREINFO_NUMBER(PG_private);
VMCOREINFO_NUMBER(PG_swapcache);
VMCOREINFO_NUMBER(PG_swapbacked);
#define PAGE_SLAB_MAPCOUNT_VALUE (~PG_slab)
#define PAGE_SLAB_MAPCOUNT_VALUE (PGTY_slab << 24)
VMCOREINFO_NUMBER(PAGE_SLAB_MAPCOUNT_VALUE);
#ifdef CONFIG_MEMORY_FAILURE
VMCOREINFO_NUMBER(PG_hwpoison);
#endif
VMCOREINFO_NUMBER(PG_head_mask);
#define PAGE_BUDDY_MAPCOUNT_VALUE (~PG_buddy)
#define PAGE_BUDDY_MAPCOUNT_VALUE (PGTY_buddy << 24)
VMCOREINFO_NUMBER(PAGE_BUDDY_MAPCOUNT_VALUE);
#define PAGE_HUGETLB_MAPCOUNT_VALUE (~PG_hugetlb)
#define PAGE_HUGETLB_MAPCOUNT_VALUE (PGTY_hugetlb << 24)
VMCOREINFO_NUMBER(PAGE_HUGETLB_MAPCOUNT_VALUE);
#define PAGE_OFFLINE_MAPCOUNT_VALUE (~PG_offline)
#define PAGE_OFFLINE_MAPCOUNT_VALUE (PGTY_offline << 24)
VMCOREINFO_NUMBER(PAGE_OFFLINE_MAPCOUNT_VALUE);

#ifdef CONFIG_KALLSYMS
Expand Down
31 changes: 24 additions & 7 deletions mm/debug.c
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Original file line number Diff line number Diff line change
Expand Up @@ -36,11 +36,6 @@ const struct trace_print_flags pageflag_names[] = {
{0, NULL}
};

const struct trace_print_flags pagetype_names[] = {
__def_pagetype_names,
{0, NULL}
};

const struct trace_print_flags gfpflag_names[] = {
__def_gfpflag_names,
{0, NULL}
Expand All @@ -51,14 +46,35 @@ const struct trace_print_flags vmaflag_names[] = {
{0, NULL}
};

#define DEF_PAGETYPE_NAME(_name) [PGTY_##_name - 0xf0] = __stringify(_name)

static const char *page_type_names[] = {
DEF_PAGETYPE_NAME(slab),
DEF_PAGETYPE_NAME(hugetlb),
DEF_PAGETYPE_NAME(offline),
DEF_PAGETYPE_NAME(guard),
DEF_PAGETYPE_NAME(table),
DEF_PAGETYPE_NAME(buddy),
DEF_PAGETYPE_NAME(unaccepted),
};

static const char *page_type_name(unsigned int page_type)
{
unsigned i = (page_type >> 24) - 0xf0;

if (i >= ARRAY_SIZE(page_type_names))
return "unknown";
return page_type_names[i];
}

static void __dump_folio(struct folio *folio, struct page *page,
unsigned long pfn, unsigned long idx)
{
struct address_space *mapping = folio_mapping(folio);
int mapcount = atomic_read(&page->_mapcount);
char *type = "";

mapcount = page_type_has_type(mapcount) ? 0 : mapcount + 1;
mapcount = page_mapcount_is_type(mapcount) ? 0 : mapcount + 1;
pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n",
folio_ref_count(folio), mapcount, mapping,
folio->index + idx, pfn);
Expand Down Expand Up @@ -92,7 +108,8 @@ static void __dump_folio(struct folio *folio, struct page *page,
pr_warn("%sflags: %pGp%s\n", type, &folio->flags,
is_migrate_cma_folio(folio, pfn) ? " CMA" : "");
if (page_has_type(&folio->page))
pr_warn("page_type: %x\n", folio->page.page_type);
pr_warn("page_type: %x(%s)\n", folio->page.page_type >> 24,
page_type_name(folio->page.page_type));

print_hex_dump(KERN_WARNING, "raw: ", DUMP_PREFIX_NONE, 32,
sizeof(unsigned long), page,
Expand Down

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