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mm: clean up get_user_pages_fast() documentation

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Move more documentation for get_user_pages_fast into the new kerneldoc comment.
Add some comments for get_user_pages as well.

Also, move get_user_pages_fast declaration up to get_user_pages. It wasn't
there initially because it was once a static inline function.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: Andy Grover <andy.grover@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nick Piggin
2009-06-16 15:31:39 -07:00
committed by Linus Torvalds
parent 7ffc59b4d0
commit d2bf6be8ab
3 changed files with 67 additions and 19 deletions
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50
mm/memory.c
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@ -1360,6 +1360,56 @@ int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
return i;
}
/**
* get_user_pages() - pin user pages in memory
* @tsk: task_struct of target task
* @mm: mm_struct of target mm
* @start: starting user address
* @len: number of pages from start to pin
* @write: whether pages will be written to by the caller
* @force: whether to force write access even if user mapping is
* readonly. This will result in the page being COWed even
* in MAP_SHARED mappings. You do not want this.
* @pages: array that receives pointers to the pages pinned.
* Should be at least nr_pages long. Or NULL, if caller
* only intends to ensure the pages are faulted in.
* @vmas: array of pointers to vmas corresponding to each page.
* Or NULL if the caller does not require them.
*
* Returns number of pages pinned. This may be fewer than the number
* requested. If len is 0 or negative, returns 0. If no pages
* were pinned, returns -errno. Each page returned must be released
* with a put_page() call when it is finished with. vmas will only
* remain valid while mmap_sem is held.
*
* Must be called with mmap_sem held for read or write.
*
* get_user_pages walks a process's page tables and takes a reference to
* each struct page that each user address corresponds to at a given
* instant. That is, it takes the page that would be accessed if a user
* thread accesses the given user virtual address at that instant.
*
* This does not guarantee that the page exists in the user mappings when
* get_user_pages returns, and there may even be a completely different
* page there in some cases (eg. if mmapped pagecache has been invalidated
* and subsequently re faulted). However it does guarantee that the page
* won't be freed completely. And mostly callers simply care that the page
* contains data that was valid *at some point in time*. Typically, an IO
* or similar operation cannot guarantee anything stronger anyway because
* locks can't be held over the syscall boundary.
*
* If write=0, the page must not be written to. If the page is written to,
* set_page_dirty (or set_page_dirty_lock, as appropriate) must be called
* after the page is finished with, and before put_page is called.
*
* get_user_pages is typically used for fewer-copy IO operations, to get a
* handle on the memory by some means other than accesses via the user virtual
* addresses. The pages may be submitted for DMA to devices or accessed via
* their kernel linear mapping (via the kmap APIs). Care should be taken to
* use the correct cache flushing APIs.
*
* See also get_user_pages_fast, for performance critical applications.
*/
int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
unsigned long start, int len, int write, int force,
struct page **pages, struct vm_area_struct **vmas)