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page-flags.md

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page-flags

We have seen page flags many times. Each page has different page flags.

Page flags are defined in include/linux/page-flags.h file. Basically every flags are defined as enum values in enum pageflags, for instance PG_reserved and PG_private. And you can find description for each flag.

But flag handling functions, for instance __SetPageReferenced, are not there. You cannot find them with grep tool and any tagging tools. That is because they are defined with macro in page-flags.h

For example, following is definition of PG_locked.

TESTPAGEFLAG(Locked, locked)

What is TESTPAGEFLAG macro?

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

uname is Locked and lname is locked. Let's translate the macro into real code.

static inline int PageLocked(const struct page *page)
			{ return test_bit(PG_locked, &page->flags); }

Finally we can understand that TESTPAGEFLAG macro defines a bit checking function PageLocked. PG_ prefix is omiited to define macro.

Let's check another macro using lru flag.

PAGEFLAG(LRU, lru) __CLEARPAGEFLAG(LRU, lru)

This also omits PG_ prefix of flag name PG_lru. Let's check what PAGEFLAG and CLEARPAGEFLAG are.

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

#define TESTPAGEFLAG(uname, lname)    				\
static inline int Page##uname(const 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); }

#define __CLEARPAGEFLAG(uname, lname)					\
static inline void __ClearPage##uname(struct page *page)		\
			{ __clear_bit(PG_##lname, &page->flags); }

Finally PAGEFLAG and __CLEARPAGEFLAG macros generates PageLru, SetPageLru, ClearPageLru, __ClearPageLru macro functions.

We can find out a nameing policy here.

  • Page: check flag
  • SetPage: set flag
  • ClearPage: clear flag
  • `__ClearPage: clear flag without lock

Now we can understand what each macro sets/clears which flag.