【page cache】简介
目录
当前内容基于 Linux Kernel v5.4.121
page cache
由于磁盘 HDD 甚至于现在广泛使用的固态硬盘 SSD 的读写速度都远小于内存 DRAM 的读写速度，为了避免每次读取数据都要直接访问这些低速的底层存储设备，Linux 在利用 DRAM 实现了一个缓存层，缓存的粒度是 page，因此也叫 page cache，中文一般称为页（面）缓存
经过这层 page cache 的作用，IO 的性能得到了显著的提升。不过由于 DRAM 具有易失性，在掉电后数据会丢失，因此内核中的 回写机制 定时将 page cache 中的数据下刷到设备上，保证数据的持久化。此外内核还在 page cache 中实现了巧妙的 预读机制 提升了顺序读性能
直接 IO 与 缓存 IO
在拥有 page cache 这一层缓存后，写数据就有了三种不同的策略：
- 不经过缓存，直接写底层存储设备，但同时要使缓存中数据失效，也叫不缓存（nowrite）
- 只写缓存，缓存中数据定期刷到底层存储设备上，也叫写回（write back）
- 同时写缓存和底层存储设备，也叫写穿（write through）
前两种就是 Linux 在 IO 栈中支持的直接 IO（direct IO）和缓存 IO（buffer IO）
第三种策略虽然能非常简单保证缓存和底层设备的一致性，不过基于时间局部性原理，page cache 中的数据可能只是中间态，会被频繁修改，每次写穿会产生大量的开销
Linux IO 栈
详细的 Linux IO 栈图如下（来源于 Thomas-Krenn-Wiki）：
其实简化一下，可以分为文件系统、块层和设备驱动层这三层
Linux 中的具体实现
这里介绍和 page cache 相关的主要结构体和一些常用的函数
相关结构体
超级块 super_block
每个文件系统都有 super_block 结构体，用于存储该文件系统的特定信息
其定义在 include/linux/fs.h 中
struct super_block {
struct list_head s_list; /* Keep this first */
dev_t s_dev; /* search index; _not_ kdev_t */
unsigned char s_blocksize_bits;
unsigned long s_blocksize;
loff_t s_maxbytes; /* Max file size */
struct file_system_type *s_type;
const struct super_operations *s_op;
const struct dquot_operations *dq_op;
const struct quotactl_ops *s_qcop;
const struct export_operations *s_export_op;
unsigned long s_flags;
unsigned long s_iflags; /* internal SB_I_* flags */
unsigned long s_magic;
struct dentry *s_root;
struct rw_semaphore s_umount;
int s_count;
atomic_t s_active;
#ifdef CONFIG_SECURITY
void *s_security;
#endif
const struct xattr_handler **s_xattr;
#ifdef CONFIG_FS_ENCRYPTION
const struct fscrypt_operations *s_cop;
struct key *s_master_keys; /* master crypto keys in use */
#endif
#ifdef CONFIG_FS_VERITY
const struct fsverity_operations *s_vop;
#endif
struct hlist_bl_head s_roots; /* alternate root dentries for NFS */
struct list_head s_mounts; /* list of mounts; _not_ for fs use */
struct block_device *s_bdev;
struct backing_dev_info *s_bdi;
struct mtd_info *s_mtd;
struct hlist_node s_instances;
unsigned int s_quota_types; /* Bitmask of supported quota types */
struct quota_info s_dquot; /* Diskquota specific options */
struct sb_writers s_writers;
/*
* Keep s_fs_info, s_time_gran, s_fsnotify_mask, and
* s_fsnotify_marks together for cache efficiency. They are frequently
* accessed and rarely modified.
*/
void *s_fs_info; /* Filesystem private info */
/* Granularity of c/m/atime in ns (cannot be worse than a second) */
u32 s_time_gran;
/* Time limits for c/m/atime in seconds */
time64_t s_time_min;
time64_t s_time_max;
#ifdef CONFIG_FSNOTIFY
__u32 s_fsnotify_mask;
struct fsnotify_mark_connector __rcu *s_fsnotify_marks;
#endif
char s_id[32]; /* Informational name */
uuid_t s_uuid; /* UUID */
unsigned int s_max_links;
fmode_t s_mode;
/*
* The next field is for VFS *only*. No filesystems have any business
* even looking at it. You had been warned.
*/
struct mutex s_vfs_rename_mutex; /* Kludge */
/*
* Filesystem subtype. If non-empty the filesystem type field
* in /proc/mounts will be "type.subtype"
*/
const char *s_subtype;
const struct dentry_operations *s_d_op; /* default d_op for dentries */
/*
* Saved pool identifier for cleancache (-1 means none)
*/
int cleancache_poolid;
struct shrinker s_shrink; /* per-sb shrinker handle */
/* Number of inodes with nlink == 0 but still referenced */
atomic_long_t s_remove_count;
/* Pending fsnotify inode refs */
atomic_long_t s_fsnotify_inode_refs;
/* Being remounted read-only */
int s_readonly_remount;
/* AIO completions deferred from interrupt context */
struct workqueue_struct *s_dio_done_wq;
struct hlist_head s_pins;
/*
* Owning user namespace and default context in which to
* interpret filesystem uids, gids, quotas, device nodes,
* xattrs and security labels.
*/
struct user_namespace *s_user_ns;
/*
* The list_lru structure is essentially just a pointer to a table
* of per-node lru lists, each of which has its own spinlock.
* There is no need to put them into separate cachelines.
*/
struct list_lru s_dentry_lru;
struct list_lru s_inode_lru;
struct rcu_head rcu;
struct work_struct destroy_work;
struct mutex s_sync_lock; /* sync serialisation lock */
/*
* Indicates how deep in a filesystem stack this SB is
*/
int s_stack_depth;
/* s_inode_list_lock protects s_inodes */
spinlock_t s_inode_list_lock ____cacheline_aligned_in_smp;
struct list_head s_inodes; /* all inodes */
spinlock_t s_inode_wblist_lock;
struct list_head s_inodes_wb; /* writeback inodes */
} __randomize_layout;
super_block 通常在挂载文件系统时会从底层存储设备上读取并构建，并且需要同步回底层存储设备
索引节点 inode
而 inode 则是文件系统中最重要的一个结构体，用于保存一个文件的元数据以及其在底层设备上的位置信息等（在 Linux 下一切皆是文件，目录也是一种文件）
其定义也在 include/linux/fs.h 中
/*
* Keep mostly read-only and often accessed (especially for
* the RCU path lookup and 'stat' data) fields at the beginning
* of the 'struct inode'
*/
struct inode {
// inode 类型
umode_t i_mode;
unsigned short i_opflags;
kuid_t i_uid;
kgid_t i_gid;
unsigned int i_flags;
#ifdef CONFIG_FS_POSIX_ACL
struct posix_acl *i_acl;
struct posix_acl *i_default_acl;
#endif
const struct inode_operations *i_op;
struct super_block *i_sb;
// page cache 相关
struct address_space *i_mapping;
#ifdef CONFIG_SECURITY
void *i_security;
#endif
/* Stat data, not accessed from path walking */
unsigned long i_ino;
/*
* Filesystems may only read i_nlink directly. They shall use the
* following functions for modification:
*
* (set|clear|inc|drop)_nlink
* inode_(inc|dec)_link_count
*/
union {
const unsigned int i_nlink;
unsigned int __i_nlink;
};
dev_t i_rdev;
loff_t i_size;
struct timespec64 i_atime;
struct timespec64 i_mtime;
struct timespec64 i_ctime;
spinlock_t i_lock; /* i_blocks, i_bytes, maybe i_size */
unsigned short i_bytes;
u8 i_blkbits;
u8 i_write_hint;
blkcnt_t i_blocks;
#ifdef __NEED_I_SIZE_ORDERED
seqcount_t i_size_seqcount;
#endif
/* Misc */
unsigned long i_state;
struct rw_semaphore i_rwsem;
unsigned long dirtied_when; /* jiffies of first dirtying */
unsigned long dirtied_time_when;
struct hlist_node i_hash;
struct list_head i_io_list; /* backing dev IO list */
#ifdef CONFIG_CGROUP_WRITEBACK
struct bdi_writeback *i_wb; /* the associated cgroup wb */
/* foreign inode detection, see wbc_detach_inode() */
int i_wb_frn_winner;
u16 i_wb_frn_avg_time;
u16 i_wb_frn_history;
#endif
struct list_head i_lru; /* inode LRU list */
struct list_head i_sb_list;
struct list_head i_wb_list; /* backing dev writeback list */
union {
struct hlist_head i_dentry;
struct rcu_head i_rcu;
};
atomic64_t i_version;
atomic64_t i_sequence; /* see futex */
atomic_t i_count;
atomic_t i_dio_count;
atomic_t i_writecount;
#if defined(CONFIG_IMA) || defined(CONFIG_FILE_LOCKING)
atomic_t i_readcount; /* struct files open RO */
#endif
union {
const struct file_operations *i_fop; /* former ->i_op->default_file_ops */
void (*free_inode)(struct inode *);
};
struct file_lock_context *i_flctx;
struct address_space i_data;
struct list_head i_devices;
union {
struct pipe_inode_info *i_pipe;
struct block_device *i_bdev;
struct cdev *i_cdev;
char *i_link;
unsigned i_dir_seq;
};
__u32 i_generation;
#ifdef CONFIG_FSNOTIFY
__u32 i_fsnotify_mask; /* all events this inode cares about */
struct fsnotify_mark_connector __rcu *i_fsnotify_marks;
#endif
#ifdef CONFIG_FS_ENCRYPTION
struct fscrypt_info *i_crypt_info;
#endif
#ifdef CONFIG_FS_VERITY
struct fsverity_info *i_verity_info;
#endif
void *i_private; /* fs or device private pointer */
} __randomize_layout;
与 super_block 一样，inode 也是从底层存储设备上读取并构建的，并且也需要同步回底层存储设备
文件 file
file 其实是已经打开的底层存储设备上的文件在内存中的表示。与前面两个结构体有所不同，底层存储设备上并不会存储，该结构体只是内核抽象出来的，仅仅存在于内存中，方便管理
其定义也在 include/linux/fs.h 中
struct file {
union {
struct llist_node fu_llist;
struct rcu_head fu_rcuhead;
} f_u;
struct path f_path;
struct inode *f_inode; /* cached value */
const struct file_operations *f_op;
/*
* Protects f_ep_links, f_flags.
* Must not be taken from IRQ context.
*/
spinlock_t f_lock;
enum rw_hint f_write_hint;
atomic_long_t f_count;
unsigned int f_flags;
fmode_t f_mode;
struct mutex f_pos_lock;
loff_t f_pos;
struct fown_struct f_owner;
const struct cred *f_cred;
struct file_ra_state f_ra;
u64 f_version;
#ifdef CONFIG_SECURITY
void *f_security;
#endif
/* needed for tty driver, and maybe others */
void *private_data;
#ifdef CONFIG_EPOLL
/* Used by fs/eventpoll.c to link all the hooks to this file */
struct list_head f_ep_links;
struct list_head f_tfile_llink;
#endif /* #ifdef CONFIG_EPOLL */
struct address_space *f_mapping;
errseq_t f_wb_err;
} __randomize_layout
__attribute__((aligned(4))); /* lest something weird decides that 2 is OK */
每个 file 都与一个 inode 对应
目录项 dentry
dentry 与 file 类似，底层设备上并不会存储，该结构体也是内核抽象出来的，针对目录文件进行特殊的管理
目录项相对于文件具有更高的热度，同时为了加速打开文件时的路径解析，内核中还实现了目录项缓存 dcache 用于缓存 dentry
其定义在 include/linux/dcache.h 中
struct dentry {
/* RCU lookup touched fields */
unsigned int d_flags; /* protected by d_lock */
seqcount_t d_seq; /* per dentry seqlock */
struct hlist_bl_node d_hash; /* lookup hash list */
struct dentry *d_parent; /* parent directory */
struct qstr d_name;
struct inode *d_inode; /* Where the name belongs to - NULL is
* negative */
unsigned char d_iname[DNAME_INLINE_LEN]; /* small names */
/* Ref lookup also touches following */
struct lockref d_lockref; /* per-dentry lock and refcount */
const struct dentry_operations *d_op;
struct super_block *d_sb; /* The root of the dentry tree */
unsigned long d_time; /* used by d_revalidate */
void *d_fsdata; /* fs-specific data */
union {
struct list_head d_lru; /* LRU list */
wait_queue_head_t *d_wait; /* in-lookup ones only */
};
struct list_head d_child; /* child of parent list */
struct list_head d_subdirs; /* our children */
/*
* d_alias and d_rcu can share memory
*/
union {
struct hlist_node d_alias; /* inode alias list */
struct hlist_bl_node d_in_lookup_hash; /* only for in-lookup ones */
struct rcu_head d_rcu;
} d_u;
} __randomize_layout;
因为目录也是文件，所以每个 dentry 也会与一个 inode 对应
页缓存 address_space
在 inode 结构体中可以看见一个类型为 address_space 结构体指针的 i_mapping 字段，其实它就是 page cache 的核心结构体
其定义也在 include/linux/fs.h 中
/**
* struct address_space - Contents of a cacheable, mappable object.
* @host: Owner, either the inode or the block_device.
* @i_pages: Cached pages.
* @gfp_mask: Memory allocation flags to use for allocating pages.
* @i_mmap_writable: Number of VM_SHARED mappings.
* @nr_thps: Number of THPs in the pagecache (non-shmem only).
* @i_mmap: Tree of private and shared mappings.
* @i_mmap_rwsem: Protects @i_mmap and @i_mmap_writable.
* @nrpages: Number of page entries, protected by the i_pages lock.
* @nrexceptional: Shadow or DAX entries, protected by the i_pages lock.
* @writeback_index: Writeback starts here.
* @a_ops: Methods.
* @flags: Error bits and flags (AS_*).
* @wb_err: The most recent error which has occurred.
* @private_lock: For use by the owner of the address_space.
* @private_list: For use by the owner of the address_space.
* @private_data: For use by the owner of the address_space.
*/
struct address_space {
// 指向文件 inode
struct inode *host;
// 当前文件缓存的所有 page
struct xarray i_pages;
gfp_t gfp_mask;
atomic_t i_mmap_writable;
#ifdef CONFIG_READ_ONLY_THP_FOR_FS
/* number of thp, only for non-shmem files */
atomic_t nr_thps;
#endif
struct rb_root_cached i_mmap;
struct rw_semaphore i_mmap_rwsem;
unsigned long nrpages;
unsigned long nrexceptional;
pgoff_t writeback_index;
const struct address_space_operations *a_ops;
unsigned long flags;
errseq_t wb_err;
spinlock_t private_lock;
struct list_head private_list;
void *private_data;
} __attribute__((aligned(sizeof(long)))) __randomize_layout;
/*
* On most architectures that alignment is already the case; but
* must be enforced here for CRIS, to let the least significant bit
* of struct page's "mapping" pointer be used for PAGE_MAPPING_ANON.
*/
struct request_queue;
struct block_device {
dev_t bd_dev; /* not a kdev_t - it's a search key */
int bd_openers;
struct inode * bd_inode; /* will die */
struct super_block * bd_super;
struct mutex bd_mutex; /* open/close mutex */
void * bd_claiming;
void * bd_holder;
int bd_holders;
bool bd_write_holder;
#ifdef CONFIG_SYSFS
struct list_head bd_holder_disks;
#endif
struct block_device * bd_contains;
unsigned bd_block_size;
u8 bd_partno;
struct hd_struct * bd_part;
/* number of times partitions within this device have been opened. */
unsigned bd_part_count;
int bd_invalidated;
struct gendisk * bd_disk;
struct request_queue * bd_queue;
struct backing_dev_info *bd_bdi;
struct list_head bd_list;
/*
* Private data. You must have bd_claim'ed the block_device
* to use this. NOTE: bd_claim allows an owner to claim
* the same device multiple times, the owner must take special
* care to not mess up bd_private for that case.
*/
unsigned long bd_private;
/* The counter of freeze processes */
int bd_fsfreeze_count;
/* Mutex for freeze */
struct mutex bd_fsfreeze_mutex;
} __randomize_layout;
每一个 address_space 与一个 inode 对应，同时 file 中的 f_mapping 字段通常由该文件的 inode 中 i_mapping 赋值，也就是说每个文件都会有独自的 file、inode 以及 address_space 结构体
address_space 结构体 struct xarray i_pages 就是该文件的 page cache 中缓存的所有物理页。它是通过基数树这一结构进行管理的，而 xarray 只是在基数树上进行了一层封装
常用函数
通常 address_space 上会挂载一个 address_space_operations 结构，用于自定义对 page cache 中的页面操作的函数
address_space_operations 结构定义也在 include/linux/fs.h 中
struct address_space_operations {
int (*writepage)(struct page *page, struct writeback_control *wbc);
int (*readpage)(struct file *, struct page *);
/* Write back some dirty pages from this mapping. */
int (*writepages)(struct address_space *, struct writeback_control *);
/* Set a page dirty. Return true if this dirtied it */
int (*set_page_dirty)(struct page *page);
/*
* Reads in the requested pages. Unlike ->readpage(), this is
* PURELY used for read-ahead!.
*/
int (*readpages)(struct file *filp, struct address_space *mapping,
struct list_head *pages, unsigned nr_pages);
int (*write_begin)(struct file *, struct address_space *mapping,
loff_t pos, unsigned len, unsigned flags,
struct page **pagep, void **fsdata);
int (*write_end)(struct file *, struct address_space *mapping,
loff_t pos, unsigned len, unsigned copied,
struct page *page, void *fsdata);
/* Unfortunately this kludge is needed for FIBMAP. Don't use it */
sector_t (*bmap)(struct address_space *, sector_t);
void (*invalidatepage) (struct page *, unsigned int, unsigned int);
int (*releasepage) (struct page *, gfp_t);
void (*freepage)(struct page *);
ssize_t (*direct_IO)(struct kiocb *, struct iov_iter *iter);
/*
* migrate the contents of a page to the specified target. If
* migrate_mode is MIGRATE_ASYNC, it must not block.
*/
int (*migratepage) (struct address_space *,
struct page *, struct page *, enum migrate_mode);
bool (*isolate_page)(struct page *, isolate_mode_t);
void (*putback_page)(struct page *);
int (*launder_page) (struct page *);
int (*is_partially_uptodate) (struct page *, unsigned long,
unsigned long);
void (*is_dirty_writeback) (struct page *, bool *, bool *);
int (*error_remove_page)(struct address_space *, struct page *);
/* swapfile support */
int (*swap_activate)(struct swap_info_struct *sis, struct file *file,
sector_t *span);
void (*swap_deactivate)(struct file *file);
};
这里简要介绍一下其中通用的部分常用函数
从底层填充
当打开一个文件后，page cache 不会立即缓存这个文件的所有数据页，而是随着对文件的读写来逐渐填充的
readpage 和 readpages 就是将底层存储设备上的一个或者多个页的数据读到 page cache 中
写入修改
page cache 的写入较为复杂，主要分为三个阶段：
write_begin主要负责查找、或者分配新的物理页，并将其锁定，有时还需要先从底层读取最新的数据页writepage或者writepages就是负责对这些物理页的实际写入过程write_end主要负责解锁这些物理页，并且更新inode中的元数据信息，例如i_size
其他
direct_IO 则是负责不经过 page cache 的直接 IO 的实现
需要注意的是，当需要读的数据在 page cache 中缓存的和底层存储数据不一致时，也就是 page 为 dirty 状态时，通常需要调用 filemap_write_and_wait 或者 filemap_write_and_wait_range 先将这部分脏数据写到底层设备之后，才能执行 direct read
参考资料
- 【Thomas-Krenn-Wiki】Linux Storage Stack Diagram
- 【CSDN】浅墨: 聊聊 Linux IO (中) —— Linux 内核中的 IO 栈
- 【LKD】Linux Kernel Development (3rd Edition)
- 【阿里云技术博客】The Xarray Data Structure
- 【LWN】Some VFS address space operations changes
本文作者： ywang_wnlo
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