Re: [PATCH 1/3] mm/gup: track FOLL_PIN pages

On Fri 24-01-20 18:11:13, John Hubbard wrote:

+static __maybe_unused struct page *try_grab_compound_head(struct page *page,

• 					  int refs,
  

• 					  unsigned int flags)
  

+{

• if (flags & FOLL_GET)

• return try_get_compound_head(page, refs);
  

• else if (flags & FOLL_PIN) {

• int orig_refs = refs;
  

• /*
  

•  * When pinning a compound page of order > 1 (which is what
  

•  * hpage_pincount_available() checks for), use an exact count to
  

•  * track it, via hpage_pincount_inc/_dec().
  

•  *
  

•  * However, be sure to *also* increment the normal page refcount
  

•  * field at least once, so that the page really is pinned.
  

•  */
  

• if (!hpage_pincount_available(page))
  

• 	refs *= GUP_PIN_COUNTING_BIAS;
  

• page = try_get_compound_head(page, refs);
  

• if (!page)
  

• 	return NULL;
  

• if (hpage_pincount_available(page))
  

• 	hpage_pincount_inc(page);
  

Umm, adding just 1 to pincount looks dangerous to me as try_grab_compound_head() would not compose - you could not release references acquired by try_grab_compound_head() with refs==2 by two calls to put_compound_head() with refs==1. So I'd rather have here: hpage_pincount_add(page, refs) and similarly in put_compound_head(). Otherwise the patch looks good to me from a quick look.

Honza