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SubjectRe: [PATCH] mm/gup.c: fix the wrong comments
On Thu, Apr 04, 2019 at 03:23:47PM +0800, Huang Shijie wrote:
> When CONFIG_HAVE_GENERIC_GUP is defined, the kernel will use its own
> get_user_pages_fast().
>
> In the following scenario, we will may meet the bug in the DMA case:
> .....................
> get_user_pages_fast(start,,, pages);
> ......
> sg_alloc_table_from_pages(, pages, ...);
> .....................
>
> The root cause is that sg_alloc_table_from_pages() requires the
> page order to keep the same as it used in the user space, but
> get_user_pages_fast() will mess it up.

I wonder if there is something we can do to change sg_alloc_table_from_pages()
to work? Reading the comment for it there is no indication of this limitation.
So should we update that comment as well?

>
> So change the comments, and make it more clear for the driver
> users.
>
> Signed-off-by: Huang Shijie <sjhuang@iluvatar.ai>
> ---
> mm/gup.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 22acdd0f79ff..b810d15d4db9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1129,10 +1129,6 @@ EXPORT_SYMBOL(get_user_pages_locked);
> * with:
> *
> * get_user_pages_unlocked(tsk, mm, ..., pages);
> - *
> - * It is functionally equivalent to get_user_pages_fast so
> - * get_user_pages_fast should be used instead if specific gup_flags
> - * (e.g. FOLL_FORCE) are not required.
> */
> long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
> struct page **pages, unsigned int gup_flags)
> @@ -2147,6 +2143,10 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
> * If not successful, it will fall back to taking the lock and
> * calling get_user_pages().
> *
> + * This function is different from the get_user_pages_unlocked():
> + * The @pages may has different page order with the result
> + * got by get_user_pages_unlocked().
> + *

I think I would word this a bit more generally. Say:

<quote>
NOTE: Because get_user_pages_fast() walks the page tables to find the pages,
the order of pages returned may be different from those returned by other
get_user_pages_*() calls.
</quote>

Ira

> * 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.
> --
> 2.17.1
>

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