folio大概是5.16引入的，看起来像是page的封装，这里有一篇讲解folio很好的博客，论好名字的重要性： Linux内核page到folio的变迁-CSDN博客
struct folio {
/* private: don't document the anon union */
union {
struct {
/* public: */
unsigned long flags;
union {
struct list_head lru;
/* private: avoid cluttering the output */
struct {
void *__filler;
/* public: */
unsigned int mlock_count;
/* private: */
};
/* public: */
};
struct address_space *mapping;
pgoff_t index;
union {
void *private;
swp_entry_t swap;
};
atomic_t _mapcount;
atomic_t _refcount;
#ifdef CONFIG_MEMCG
unsigned long memcg_data;
#endif
#if defined(WANT_PAGE_VIRTUAL)
void *virtual;
#endif
#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
int _last_cpupid;
#endif
/* private: the union with struct page is transitional */
};
struct page page;
};
union {
struct {
unsigned long _flags_1;
unsigned long _head_1;
unsigned long _folio_avail;
/* public: */
atomic_t _entire_mapcount;
atomic_t _nr_pages_mapped;
atomic_t _pincount;
#ifdef CONFIG_64BIT
unsigned int _folio_nr_pages;
#endif
/* private: the union with struct page is transitional */
};
struct page __page_1;
};
union {
struct {
unsigned long _flags_2;
unsigned long _head_2;
/* public: */
void *_hugetlb_subpool;
void *_hugetlb_cgroup;
void *_hugetlb_cgroup_rsvd;
void *_hugetlb_hwpoison;
/* private: the union with struct page is transitional */
};
struct {
unsigned long _flags_2a;
unsigned long _head_2a;
/* public: */
struct list_head _deferred_list;
/* private: the union with struct page is transitional */
};
struct page __page_2;
};
};
简单来看它似乎是三个page结构的组合。与第一个page union的结构跟page结构几乎一致。引入folio是为了解决长久以来page混乱的语义。page除了可以代表单页也可以代表连续多个页面，甚至大页。page在内核中应用广泛，这种混乱增加了写代码和理解代码的难度，人为的增加混乱。folio代表一个或多个page，本身就可以代表page所有的语义。在新的内核代码中folio在很多场合完成了page的替代，但是page依然存在。
比如compound_order的实现。在folio之前是这样的。
static inline unsigned int compound_order(struct page *page)
{
if (!PageHead(page))
return 0;
return page[1].compound_order;
}
先检查page是不是单页，如果是单页直接返回0，对于复合页order保存在后一个page的compound_order成员中。也即是单个page是表示不了多页的，但是folio可以。
static inline unsigned int compound_order(struct page *page)
{
struct folio *folio = (struct folio *)page;
if (!test_bit(PG_head, &folio->flags))
return 0;
return folio->_flags_1 & 0xff;
}
新的代码中首先将page强转为folio，判断其是否为复合页，如果是复合页order保存在_flags_1中。看起来也没简化，反而更复杂一点，但是可以在一个folio结构中解决问题，不再依赖于tail page。
看一下page结构
struct page {
unsigned long flags; /* Atomic flags, some possibly
* updated asynchronously */
/*
* Five words (20/40 bytes) are available in this union.
* WARNING: bit 0 of the first word is used for PageTail(). That
* means the other users of this union MUST NOT use the bit to
* avoid collision and false-positive PageTail().
*/
union {
struct { /* Page cache and anonymous pages */
/**
* @lru: Pageout list, eg. active_list protected by
* lruvec->lru_lock. Sometimes used as a generic list
* by the page owner.
*/
union {
struct list_head lru;
/* Or, for the Unevictable "LRU list" slot */
struct {
/* Always even, to negate PageTail */
void *__filler;
/* Count page's or folio's mlocks */
unsigned int mlock_count;
};
/* Or, free page */
struct list_head buddy_list;
struct list_head pcp_list;
};
/* See page-flags.h for PAGE_MAPPING_FLAGS */
struct address_space *mapping;
union {
pgoff_t index; /* Our offset within mapping. */
unsigned long share; /* share count for fsdax */
};
/**
* @private: Mapping-private opaque data.
* Usually used for buffer_heads if PagePrivate.
* Used for swp_entry_t if PageSwapCache.
* Indicates order in the buddy system if PageBuddy.
*/
unsigned long private;
};
struct { /* page_pool used by netstack */
/**
* @pp_magic: magic value to avoid recycling non
* page_pool allocated pages.
*/
unsigned long pp_magic;
struct page_pool *pp;
unsigned long _pp_mapping_pad;
unsigned long dma_addr;
atomic_long_t pp_ref_count;
};
struct { /* Tail pages of compound page */
unsigned long compound_head; /* Bit zero is set */
};
struct { /* ZONE_DEVICE pages */
/** @pgmap: Points to the hosting device page map. */
struct dev_pagemap *pgmap;
void *zone_device_data;
/*
* ZONE_DEVICE private pages are counted as being
* mapped so the next 3 words hold the mapping, index,
* and private fields from the source anonymous or
* page cache page while the page is migrated to device
* private memory.
* ZONE_DEVICE MEMORY_DEVICE_FS_DAX pages also
* use the mapping, index, and private fields when
* pmem backed DAX files are mapped.
*/
};
/** @rcu_head: You can use this to free a page by RCU. */
struct rcu_head rcu_head;
};
union { /* This union is 4 bytes in size. */
/*
* If the page can be mapped to userspace, encodes the number
* of times this page is referenced by a page table.
*/
atomic_t _mapcount;
/*
* If the page is neither PageSlab nor mappable to userspace,
* the value stored here may help determine what this page
* is used for. See page-flags.h for a list of page types
* which are currently stored here.
*/
unsigned int page_type;
};
/* Usage count. *DO NOT USE DIRECTLY*. See page_ref.h */
atomic_t _refcount;
#ifdef CONFIG_MEMCG
unsigned long memcg_data;
#endif
/*
* On machines where all RAM is mapped into kernel address space,
* we can simply calculate the virtual address. On machines with
* highmem some memory is mapped into kernel virtual memory
* dynamically, so we need a place to store that address.
* Note that this field could be 16 bits on x86 ... ;)
*
* Architectures with slow multiplication can define
* WANT_PAGE_VIRTUAL in asm/page.h
*/
#if defined(WANT_PAGE_VIRTUAL)
void *virtual; /* Kernel virtual address (NULL if
not kmapped, ie. highmem) */
#endif /* WANT_PAGE_VIRTUAL */
#ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
int _last_cpupid;
#endif
#ifdef CONFIG_KMSAN
/*
* KMSAN metadata for this page:
* - shadow page: every bit indicates whether the corresponding
* bit of the original page is initialized (0) or not (1);
* - origin page: every 4 bytes contain an id of the stack trace
* where the uninitialized value was created.
*/
struct page *kmsan_shadow;
struct page *kmsan_origin;
#endif
} _struct_page_alignment;
1. flags
enum pageflags {
PG_locked, /* Page is locked. Don't touch. */
PG_writeback, /* Page is under writeback */
PG_referenced,
PG_uptodate,
PG_dirty,
PG_lru,
PG_head, /* Must be in bit 6 */
PG_waiters, /* Page has waiters, check its waitqueue. Must be bit #7 and in the same byte as "PG_locked" */
PG_active,
PG_workingset,
PG_error,
PG_slab,
PG_owner_priv_1, /* Owner use. If pagecache, fs may use*/
PG_arch_1,
PG_reserved,
PG_private, /* If pagecache, has fs-private data */
PG_private_2, /* If pagecache, has fs aux data */
PG_mappedtodisk, /* Has blocks allocated on-disk */
PG_reclaim, /* To be reclaimed asap */
PG_swapbacked, /* Page is backed by RAM/swap */
PG_unevictable,
...
flags由四部分构成，|node|zone|last_cpuid|flags|
2. mapping
最低两个bits可以用来判断是否为匿名映射或ksm映射。对于匿名映射指向anon_vma. 对于file映射指向address_space结构。
#define PAGE_MAPPING_ANON 0x1 #define PAGE_MAPPING_MOVABLE 0x2 #define PAGE_MAPPING_KSM (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE) #define PAGE_MAPPING_FLAGS (PAGE_MAPPING_ANON | PAGE_MAPPING_MOVABLE)
3. _refcount
表示页面在内核中的引用次数。大于0代表正在使用。
static inline void get_page(struct page *page)
{
folio_get(page_folio(page));
}
static inline void folio_get(struct folio *folio)
{
VM_BUG_ON_FOLIO(folio_ref_zero_or_close_to_overflow(folio), folio);
folio_ref_inc(folio);
}
static inline void folio_ref_inc(struct folio *folio)
{
page_ref_inc(&folio->page);
}
static inline void page_ref_inc(struct page *page)
{
atomic_inc(&page->_refcount);
if (page_ref_tracepoint_active(page_ref_mod))
__page_ref_mod(page, 1);
}
folio让page操作变得非常繁琐，这样真的好吗？
分配内存时_refcount + 1, 加入lru链表时+1等。
4. _mapcount
表示这个页面被进程映射的次数，用做反向映射。-1代表没有页表映射。
page相关的API
static inline struct zone *page_zone(const struct page *page)
{
return &NODE_DATA(page_to_nid(page))->node_zones[page_zonenum(page)];
}
static inline int page_zone_id(struct page *page)
{
return (page->flags >> ZONEID_PGSHIFT) & ZONEID_MASK;
}
mapping相关
struct address_space *page_mapping(struct page *page)
{
return folio_mapping(page_folio(page));
}
struct address_space *folio_mapping(struct folio *folio)
{
struct address_space *mapping;
/* This happens if someone calls flush_dcache_page on slab page */
if (unlikely(folio_test_slab(folio)))
return NULL;
if (unlikely(folio_test_swapcache(folio)))
return swap_address_space(folio->swap);
mapping = folio->mapping;
//如果是匿名页或ksm页
if ((unsigned long)mapping & PAGE_MAPPING_FLAGS)
return NULL;
return mapping;
}
page_mapped
static inline bool page_mapped(struct page *page)
{
if (likely(!PageCompound(page)))
return atomic_read(&page->_mapcount) >= 0;
return folio_large_is_mapped(page_folio(page));
}
对于普通页面只需判断_mapcount值。
标签：folio,struct,unsigned,long,PG,linux,page From： https://www.cnblogs.com/banshanjushi/p/17995379