Re: [linux-next RFC v2] mm/gup.c: Convert to use get_user_{page|pages}_fast_only()

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On 2020-05-23 21:27, Souptick Joarder wrote:
API __get_user_pages_fast() renamed to get_user_pages_fast_only()
to align with pin_user_pages_fast_only().

As part of this we will get rid of write parameter. Instead caller
will pass FOLL_WRITE to get_user_pages_fast_only(). This will not
change any existing functionality of the API.

All the callers are changed to pass FOLL_WRITE.

This looks good. A few nits below, but with those fixed, feel free to
add:

    Reviewed-by: John Hubbard <jhubbard@xxxxxxxxxx>


There are few places where 1 is passed to 2nd parameter of
__get_user_pages_fast() and return value is checked for 1
like [1]. Those are replaced with new inline
get_user_page_fast_only().

[1] if (__get_user_pages_fast(hva, 1, 1, &page) == 1)


We try to avoid talking *too* much about the previous version of
the code. Just enough. So, instead of the above two paragraphs,
I'd compress it down to:

Also: introduce get_user_page_fast_only(), and use it in a few
places that hard-code nr_pages to 1.

...
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 93d93bd..8d4597f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1817,10 +1817,16 @@ extern int mprotect_fixup(struct vm_area_struct *vma,
  /*
   * doesn't attempt to fault and will return short.
   */
-int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
-			  struct page **pages);
+int get_user_pages_fast_only(unsigned long start, int nr_pages,
+			unsigned int gup_flags, struct page **pages);

Silly nit:

Can you please leave the original indentation in place? I don't normally
comment about this, but I like the original indentation better, and it matches
the pin_user_pages_fast() below, too.

...
@@ -2786,8 +2792,8 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
   * If the architecture does not support this function, simply return with no
   * pages pinned.
   */
-int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
-			  struct page **pages)
+int get_user_pages_fast_only(unsigned long start, int nr_pages,
+			unsigned int gup_flags, struct page **pages)


Same thing here: you've changed the original indentation, which was (arguably, but
to my mind anyway) more readable, and for no reason. It still would have fit within
80 cols.

I'm sure it's a perfect 50/50 mix of people who prefer either indentation style, and
so for brand new code, I'll remain silent, as long as it is consistent with either
itself and/or the surrounding code. But changing it back and forth is a bit aggravating, and best avoided. :)


thanks,
--
John Hubbard
NVIDIA



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