__GET_USER_PAGES(9)					    Memory Management in Linux					       __GET_USER_PAGES(9)

NAME

       __get_user_pages - pin user pages in memory

SYNOPSIS

       long __get_user_pages(struct task_struct * tsk, struct mm_struct * mm, unsigned long start, unsigned long nr_pages, unsigned int gup_flags,
			     struct page ** pages, struct vm_area_struct ** vmas, int * nonblocking);

ARGUMENTS

       tsk
	   task_struct of target task

       mm
	   mm_struct of target mm

       start
	   starting user address

       nr_pages
	   number of pages from start to pin

       gup_flags
	   flags modifying pin behaviour

       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.

       nonblocking
	   whether waiting for disk IO or mmap_sem contention

DESCRIPTION

       Returns number of pages pinned. This may be fewer than the number requested. If nr_pages 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 gup_flags & FOLL_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.

       If nonblocking != NULL, __get_user_pages will not wait for disk IO or mmap_sem contention, and if waiting is needed to pin all pages,
       *nonblocking will be set to 0.

       In most cases, get_user_pages or get_user_pages_fast should be used instead of __get_user_pages. __get_user_pages should be used only if
       you need some special gup_flags.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014						       __GET_USER_PAGES(9)