Re: [PATCH 11/13] mm: extend prefault helpers to fault in more than PAGE_SIZE

Chris Wilson
2011-11-06T22:24:13Z

On Sun, 6 Nov 2011 20:13:58 +0100, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> drm/i915 wants to read/write more than one page in its fastpath
> and hence needs to prefault more than PAGE_SIZE bytes.
>
> I've checked the callsites and they all already clamp size when
> calling fault_in_pages_* to the same as for the subsequent
> __copy_to|from_user and hence don't rely on the implicit clamping
> to PAGE_SIZE.
>
> Also kill a copy&pasted spurious space in both functions while at it.
>
> Cc: linux-mm@kvack.org
> Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> ---
> include/linux/pagemap.h | 28 ++++++++++++++++++----------
> 1 files changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index cfaaa69..689527d 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -408,6 +408,7 @@ extern void add_page_wait_queue(struct page *page, wait_queue_t *waiter);
> static inline int fault_in_pages_writeable(char __user *uaddr, int size)
> {
> int ret;
> + char __user *end = uaddr + size - 1;
>
> if (unlikely(size == 0))
> return 0;
> @@ -416,17 +417,20 @@ static inline int fault_in_pages_writeable(char __user *uaddr, int size)
> * Writing zeroes into userspace here is OK, because we know that if
> * the zero gets there, we'll be overwriting it.
> */
> - ret = __put_user(0, uaddr);
> + while (uaddr <= end) {
> + ret = __put_user(0, uaddr);
> + if (ret != 0)
> + return ret;
> + uaddr += PAGE_SIZE;
> + }
> if (ret == 0) {
> - char __user *end = uaddr + size - 1;
> -
> /*
> * If the page was already mapped, this will get a cache miss
> * for sure, so try to avoid doing it.
> */
> - if (((unsigned long)uaddr & PAGE_MASK) !=
> + if (((unsigned long)uaddr & PAGE_MASK) ==
> ((unsigned long)end & PAGE_MASK))
> - ret = __put_user(0, end);
> + ret = __put_user(0, end);
> }
> return ret;

You leave these functions in a worse mess by introducing a false
compiler warning about an uninitialized ret by the now redundant test
against zero, a do{}while loop would be clearer that the original
behaviour is merely extended upon. And please replace the open-coded
offset_in_page().
-Chris

--
Chris Wilson, Intel Open Source Technology Centre

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