Re: [PATCH 1/5] mm: gup: add get_user_pages_locked and get_user_pages_unlocked

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Jan 13, 2015 at 05:37:50PM +0100, Andrea Arcangeli wrote:
> We can leverage the VM_FAULT_RETRY functionality in the page fault
> paths better by using either get_user_pages_locked or
> get_user_pages_unlocked.
> 
> The former allow conversion of get_user_pages invocations that will
> have to pass a "&locked" parameter to know if the mmap_sem was dropped
> during the call. Example from:
> 
>     down_read(&mm->mmap_sem);
>     do_something()
>     get_user_pages(tsk, mm, ..., pages, NULL);
>     up_read(&mm->mmap_sem);
> 
> to:
> 
>     int locked = 1;
>     down_read(&mm->mmap_sem);
>     do_something()
>     get_user_pages_locked(tsk, mm, ..., pages, &locked);
>     if (locked)
>         up_read(&mm->mmap_sem);
> 
> The latter is suitable only as a drop in replacement of the form:
> 
>     down_read(&mm->mmap_sem);
>     get_user_pages(tsk, mm, ..., pages, NULL);
>     up_read(&mm->mmap_sem);
> 
> into:
> 
>     get_user_pages_unlocked(tsk, mm, ..., pages);
> 
> Where tsk, mm, the intermediate "..." paramters and "pages" can be any
> value as before. Just the last parameter of get_user_pages (vmas) must
> be NULL for get_user_pages_locked|unlocked to be usable (the latter
> original form wouldn't have been safe anyway if vmas wasn't null, for
> the former we just make it explicit by dropping the parameter).
> 
> If vmas is not NULL these two methods cannot be used.
> 
> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
> Reviewed-by: Andres Lagar-Cavilla <andreslc@google.com>
> Reviewed-by: Peter Feiner <pfeiner@google.com>

Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

-- 
 Kirill A. Shutemov

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