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标题: PG_Readahead的疑问 [打印本页]

作者: 王非文 时间: 2014-07-24 11:44
标题: PG_Readahead的疑问
在readahead.c 中有如下注释，觉得PG_readahead应该是一个独立的标志位。
/*
* On-demand readahead design.
*
* The fields in struct file_ra_state represent the most-recently-executed
* readahead attempt:
*
*                      |<----- async_size ---------|
*    |------------------- size -------------------->|
*    |==================#===========================|
*    ^start          ^page marked with PG_readahead

而在include/linux/page_flags.h 中的pageflags的枚举变量中并没有 PG_Readahead 的成员。

SetPageReadahead() 展开之后其实是置位PG_reclaim
Static inline void SetPageReadahead(struct page *page)
{
Set_bit(PG_reclaim, &page->flags);
}

疑惑啊，为什么PG_Readahead 不是pageflags中的一员，有知道的解释一下吗？

（1）
enum pageflags {
PG_locked,    /* Page is locked. Don't touch. */
PG_error,
PG_referenced,
PG_uptodate,
PG_dirty,
PG_lru,
PG_active,
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_writeback,    /* Page is under writeback */
#ifdef CONFIG_PAGEFLAGS_EXTENDED
PG_head,       /* A head page */
PG_tail,       /* A tail page */
#else
PG_compound,       /* A compound page */
#endif
PG_swapcache,    /* Swap page: swp_entry_t in private */
PG_mappedtodisk, /* Has blocks allocated on-disk */
PG_reclaim,    /* To be reclaimed asap */
PG_swapbacked,    /* Page is backed by RAM/swap */
PG_unevictable,    /* Page is "unevictable"  */
#ifdef CONFIG_MMU
PG_mlocked,    /* Page is vma mlocked */
#endif
#ifdef CONFIG_ARCH_USES_PG_UNCACHED
PG_uncached,       /* Page has been mapped as uncached */
#endif
#ifdef CONFIG_MEMORY_FAILURE
PG_hwpoison,       /* hardware poisoned page. Don't touch */
#endif
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
PG_compound_lock,
#endif
__NR_PAGEFLAGS,

/* Filesystems */
PG_checked = PG_owner_priv_1,

/* Two page bits are conscripted by FS-Cache to maintain local caching
   * state.  These bits are set on pages belonging to the netfs's inodes
   * when those inodes are being locally cached.
   */
PG_fscache = PG_private_2,  /* page backed by cache */

/* XEN */
PG_pinned = PG_owner_priv_1,
PG_savepinned = PG_dirty,

/* SLOB */
PG_slob_free = PG_private,

/* SLUB */
PG_slub_frozen = PG_active,
};

（2）
/* PG_readahead is only used for file reads; PG_reclaim is only for writes */
PAGEFLAG(Reclaim, reclaim) TESTCLEARFLAG(Reclaim, reclaim)
PAGEFLAG(Readahead, reclaim)       /* Reminder to do async read-ahead */

#define PAGEFLAG(uname, lname) TESTPAGEFLAG(uname, lname)    \
SETPAGEFLAG(uname, lname) CLEARPAGEFLAG(uname, lname)

#define TESTPAGEFLAG(uname, lname)                \
static inline int Page##uname(struct page *page)          \
         { return test_bit(PG_##lname, &page->flags); }

#define SETPAGEFLAG(uname, lname)                \
static inline void SetPage##uname(struct page *page)          \
         { set_bit(PG_##lname, &page->flags); }

#define CLEARPAGEFLAG(uname, lname)                \
static inline void ClearPage##uname(struct page *page)       \
         { clear_bit(PG_##lname, &page->flags); }

作者: 黎明748 时间: 2014-07-24 14:03

内核里面找不到PG_readahead。。。

作者: chenyu105 时间: 2014-07-24 14:24
1.这个标志是宏生成的
2. 这个标志和reclaim标志复用了在reclaim流程自然不会去预读，所以是互斥

作者: 王非文 时间: 2014-07-24 15:28
1.这个标志是宏生成的
能说具体一点吗？即使是宏生成的，但是在枚举变量pageflags中没有PG_readahead的声明
2. 这个标志和reclaim标志复用了在reclaim流程自然不会去预读，所以是互斥
这句话就更加不明白了，SetPageReadahead() 展开之后其实是置位PG_reclaim，看到是PG_readahead 和 PG_reclaim是等价的。

作者: embeddedlwp 时间: 2014-07-24 15:34
page flags bit是非常紧张的，能复用就复用吧，

作者: embeddedlwp 时间: 2014-07-24 15:35
http://lwn.net/Articles/335768/

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