On Mon, 23 Oct 2023 at 15:27, Yunsheng Lin [off-list ref] wrote:

On 2023/10/23 19:43, Ilias Apalodimas wrote:

quoted

Hi Yunsheng,

[...]

quoted

+     * 1. 'n == 1': no need to actually overwrite it.
+     * 2. 'n != 1': overwrite it with one, which is the rare case
+     *              for pp_frag_count draining.
      *
-     * The main advantage to doing this is that an atomic_read is
-     * generally a much cheaper operation than an atomic update,
-     * especially when dealing with a page that may be partitioned
-     * into only 2 or 3 pieces.
+     * The main advantage to doing this is that not only we avoid a atomic
+     * update, as an atomic_read is generally a much cheaper operation than
+     * an atomic update, especially when dealing with a page that may be
+     * partitioned into only 2 or 3 pieces; but also unify the pp_frag_count
+     * handling by ensuring all pages have partitioned into only 1 piece
+     * initially, and only overwrite it when the page is partitioned into
+     * more than one piece.
      */
-    if (atomic_long_read(&page->pp_frag_count) == nr)
+    if (atomic_long_read(&page->pp_frag_count) == nr) {
+            /* As we have ensured nr is always one for constant case using
+             * the BUILD_BUG_ON(), only need to handle the non-constant case
+             * here for pp_frag_count draining, which is a rare case.
+             */
+            BUILD_BUG_ON(__builtin_constant_p(nr) && nr != 1);
+            if (!__builtin_constant_p(nr))
+                    atomic_long_set(&page->pp_frag_count, 1);

Aren't we changing the behaviour of the current code here? IIRC is
atomic_long_read(&page->pp_frag_count) == nr we never updated the atomic
pp_frag_count and the reasoning was that the next caller can set it
properly.

If the next caller is calling the page_pool_alloc_frag(), then yes,
because page_pool_fragment_page() will be used to reset the
page->pp_frag_count, so it does not really matter what is the value
of page->pp_frag_count when we are recycling a page.

If the next caller is calling page_pool_alloc_pages() directly without
fragmenting a page, the above code is used to ensure that pp_frag_count
is always one when page_pool_alloc_pages() fetches a page from pool->alloc
or pool->ring, because page_pool_fragment_page() is not used to reset the
page->pp_frag_count for page_pool_alloc_pages() and we have removed the
per page_pool PP_FLAG_PAGE_FRAG in page_pool_is_last_frag().

As we don't know if the caller is page_pool_alloc_frag() or
page_pool_alloc_pages(), so the above code ensure the page in pool->alloc
or pool->ring always have the pp_frag_count being one.

Fair enough, Jakub pulled the series before I managed to ack them, but
that's okay. It's been long overdue apologies. FWIW I went through the
patches and didn't find anything wrong coding-wise

Thanks
/Ilias

quoted

quoted

+
             return 0;
+    }

     ret = atomic_long_sub_return(nr, &page->pp_frag_count);
     WARN_ON(ret < 0);
+
+    /* We are the last user here too, reset pp_frag_count back to 1 to
+     * ensure all pages have been partitioned into 1 piece initially,
+     * this should be the rare case when the last two fragment users call
+     * page_pool_defrag_page() currently.
+     */
+    if (unlikely(!ret))
+            atomic_long_set(&page->pp_frag_count, 1);
+
     return ret;
 }

 [....]

 Thanks
 /Ilias

.