Man page 9 : __get_user_pages

__get_user_pages - pin user pages in memory

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 );

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

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 processAqs 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 wonAqt 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 canAqt 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.