__GET_USER_PAGES - Online Linux Manual PageSection : 9
Updated : June 2018
Source : Kernel Hackers Manual 2.6.
Note : Memory Management in Linux
NAME__get_user_pages − pin user pages in memory
SYNOPSISint __get_user_pages(struct task_struct * tsk, struct mm_struct * mm, unsigned long start, int nr_pages, unsigned int gup_flags, struct page ** pages, struct vm_area_struct ** vmas);
ARGUMENTStsk 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.
DESCRIPTIONReturns 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. 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.
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