mm: remove flush_kernel_dcache_page

  ``void flush_dcache_page(struct page *page)``

	Any time the kernel writes to a page cache page, _OR_

	the kernel is about to read from a page cache page and

	user space shared/writable mappings of this page potentially

	exist, this routine is called.

        This routines must be called when:

	  a) the kernel did write to a page that is in the page cache page

	     and / or in high memory

	  b) the kernel is about to read from a page cache page and user space

	     shared/writable mappings of this page potentially exist.  Note

	     that {get,pin}_user_pages{_fast} already call flush_dcache_page

	     on any page found in the user address space and thus driver

	     code rarely needs to take this into account.

	.. note::

	      handling vfs symlinks in the page cache need not call

	      this interface at all.

	The phrase "kernel writes to a page cache page" means,

	specifically, that the kernel executes store instructions

	that dirty data in that page at the page->virtual mapping

	of that page.  It is important to flush here to handle

	D-cache aliasing, to make sure these kernel stores are

	visible to user space mappings of that page.

	The corollary case is just as important, if there are users

	which have shared+writable mappings of this file, we must make

	sure that kernel reads of these pages will see the most recent

	stores done by the user.

	If D-cache aliasing is not an issue, this routine may

	simply be defined as a nop on that architecture.

        There is a bit set aside in page->flags (PG_arch_1) as

	"architecture private".  The kernel guarantees that,

	for pagecache pages, it will clear this bit when such

	a page first enters the pagecache.

	This allows these interfaces to be implemented much more

	efficiently.  It allows one to "defer" (perhaps indefinitely)

	the actual flush if there are currently no user processes

	mapping this page.  See sparc64's flush_dcache_page and

	update_mmu_cache implementations for an example of how to go

	about doing this.

	The idea is, first at flush_dcache_page() time, if

	page->mapping->i_mmap is an empty tree, just mark the architecture

	private page flag bit.  Later, in update_mmu_cache(), a check is

	made of this flag bit, and if set the flush is done and the flag

	bit is cleared.

	The phrase "kernel writes to a page cache page" means, specifically,

	that the kernel executes store instructions that dirty data in that

	page at the page->virtual mapping of that page.  It is important to

	flush here to handle D-cache aliasing, to make sure these kernel stores

	are visible to user space mappings of that page.

	The corollary case is just as important, if there are users which have

	shared+writable mappings of this file, we must make sure that kernel

	reads of these pages will see the most recent stores done by the user.

	If D-cache aliasing is not an issue, this routine may simply be defined

	as a nop on that architecture.

        There is a bit set aside in page->flags (PG_arch_1) as "architecture

	private".  The kernel guarantees that, for pagecache pages, it will

	clear this bit when such a page first enters the pagecache.

	This allows these interfaces to be implemented much more efficiently.

	It allows one to "defer" (perhaps indefinitely) the actual flush if

	there are currently no user processes mapping this page.  See sparc64's

	flush_dcache_page and update_mmu_cache implementations for an example

	of how to go about doing this.

	The idea is, first at flush_dcache_page() time, if page_file_mapping()

	returns a mapping, and mapping_mapped on that mapping returns %false,

	just mark the architecture private page flag bit.  Later, in

	update_mmu_cache(), a check is made of this flag bit, and if set the

	flush is done and the flag bit is cleared.

	.. important::

	architectures).  For incoherent architectures, it should flush

	the cache of the page at vmaddr.

  ``void flush_kernel_dcache_page(struct page *page)``

	When the kernel needs to modify a user page is has obtained

	with kmap, it calls this function after all modifications are

	complete (but before kunmapping it) to bring the underlying

	page up to date.  It is assumed here that the user has no

	incoherent cached copies (i.e. the original page was obtained

	from a mechanism like get_user_pages()).  The default

	implementation is a nop and should remain so on all coherent

	architectures.  On incoherent architectures, this should flush

	the kernel cache for page (using page_address(page)).

  ``void flush_icache_range(unsigned long start, unsigned long end)``

  	When the kernel stores into addresses that it will execute

	用。默认的实现是nop（对于所有相干的架构应该保持这样）。对于不一致性

	的架构，它应该刷新vmaddr处的页面缓存。

  ``void flush_kernel_dcache_page(struct page *page)``

	当内核需要修改一个用kmap获得的用户页时，它会在所有修改完成后（但在

	kunmapping之前）调用这个函数，以使底层页面达到最新状态。这里假定用

	户没有不一致性的缓存副本（即原始页面是从类似get_user_pages()的机制

	中获得的）。默认的实现是一个nop，在所有相干的架构上都应该如此。在不

	一致性的架构上，这应该刷新内核缓存中的页面（使用page_address(page)）。

  ``void flush_icache_range(unsigned long start, unsigned long end)``

	当内核存储到它将执行的地址中时（例如在加载模块时），这个函数被调用。

#define ARCH_IMPLEMENTS_FLUSH_DCACHE_PAGE 1

extern void flush_dcache_page(struct page *);

#define ARCH_IMPLEMENTS_FLUSH_KERNEL_VMAP_RANGE 1

static inline void flush_kernel_vmap_range(void *addr, int size)

{

	if ((cache_is_vivt() || cache_is_vipt_aliasing()))

		__flush_anon_page(vma, page, vmaddr);

}

#define ARCH_HAS_FLUSH_KERNEL_DCACHE_PAGE

extern void flush_kernel_dcache_page(struct page *);

#define flush_dcache_mmap_lock(mapping)		xa_lock_irq(&mapping->i_pages)

#define flush_dcache_mmap_unlock(mapping)	xa_unlock_irq(&mapping->i_pages)

}

EXPORT_SYMBOL(flush_dcache_page);

/*

 * Ensure cache coherency for the kernel mapping of this page. We can

 * assume that the page is pinned via kmap.

 *

 * If the page only exists in the page cache and there are no user

 * space mappings, this is a no-op since the page was already marked

 * dirty at creation.  Otherwise, we need to flush the dirty kernel

 * cache lines directly.

 */

void flush_kernel_dcache_page(struct page *page)

{

	if (cache_is_vivt() || cache_is_vipt_aliasing()) {

		struct address_space *mapping;

		mapping = page_mapping_file(page);

		if (!mapping || mapping_mapped(mapping)) {

			void *addr;

			addr = page_address(page);

			/*

			 * kmap_atomic() doesn't set the page virtual

			 * address for highmem pages, and

			 * kunmap_atomic() takes care of cache

			 * flushing already.

			 */

			if (!IS_ENABLED(CONFIG_HIGHMEM) || addr)

				__cpuc_flush_dcache_area(addr, PAGE_SIZE);

		}

	}

}

EXPORT_SYMBOL(flush_kernel_dcache_page);

/*

 * Flush an anonymous page so that users of get_user_pages()

 * can safely access the data.  The expected sequence is:

}

EXPORT_SYMBOL(flush_dcache_page);

void flush_kernel_dcache_page(struct page *page)

{

	__cpuc_flush_dcache_area(page_address(page), PAGE_SIZE);

}

EXPORT_SYMBOL(flush_kernel_dcache_page);

void copy_to_user_page(struct vm_area_struct *vma, struct page *page,

		       unsigned long uaddr, void *dst, const void *src,

		       unsigned long len)