Re: [PATCH net-next 1/2] page_pool: support non-split page with PP_FLAG_PAGE_FRAG

On 2021/8/30 23:05, Alexander Duyck wrote:

On Sun, Aug 29, 2021 at 6:19 PM Yunsheng Lin [off-list ref] wrote:

quoted

Currently when PP_FLAG_PAGE_FRAG is set, the caller is not
expected to call page_pool_alloc_pages() directly because of
the PP_FLAG_PAGE_FRAG checking in __page_pool_put_page().

The patch removes the above checking to enable non-split page
support when PP_FLAG_PAGE_FRAG is set.

Signed-off-by: Yunsheng Lin <redacted>
---
 include/net/page_pool.h |  6 ++++++
 net/core/page_pool.c    | 12 +++++++-----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/include/net/page_pool.h b/include/net/page_pool.h
index a408240..2ad0706 100644
--- a/include/net/page_pool.h
+++ b/include/net/page_pool.h

@@ -238,6 +238,9 @@ static inline void page_pool_set_dma_addr(struct page *page, dma_addr_t addr)

 static inline void page_pool_set_frag_count(struct page *page, long nr)
 {
+       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
+               return;
+
        atomic_long_set(&page->pp_frag_count, nr);
 }

@@ -246,6 +249,9 @@ static inline long page_pool_atomic_sub_frag_count_return(struct page *page,
 {
        long ret;

+       if (PAGE_POOL_DMA_USE_PP_FRAG_COUNT)
+               return 0;
+
        /* As suggested by Alexander, atomic_long_read() may cover up the
         * reference count errors, so avoid calling atomic_long_read() in
         * the cases of freeing or draining the page_frags, where we would

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index 1a69784..ba9f14d 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c

@@ -313,11 +313,14 @@ struct page *page_pool_alloc_pages(struct page_pool *pool, gfp_t gfp)

        /* Fast-path: Get a page from cache */
        page = __page_pool_get_cached(pool);
-       if (page)
-               return page;

        /* Slow-path: cache empty, do real allocation */
-       page = __page_pool_alloc_pages_slow(pool, gfp);
+       if (!page)
+               page = __page_pool_alloc_pages_slow(pool, gfp);
+
+       if (likely(page))
+               page_pool_set_frag_count(page, 1);
+
        return page;
 }
 EXPORT_SYMBOL(page_pool_alloc_pages);

@@ -426,8 +429,7 @@ __page_pool_put_page(struct page_pool *pool, struct page *page,
                     unsigned int dma_sync_size, bool allow_direct)
 {
        /* It is not the last user for the page frag case */
-       if (pool->p.flags & PP_FLAG_PAGE_FRAG &&
-           page_pool_atomic_sub_frag_count_return(page, 1))
+       if (page_pool_atomic_sub_frag_count_return(page, 1))
                return NULL;

Isn't this going to have a negative performance impact on page pool
pages in general? Essentially you are adding an extra atomic operation
for all the non-frag pages.

It would work better if this was doing a check against 1 to determine
if it is okay for this page to be freed here and only if the check
fails then you perform the atomic sub_return.

The page_pool_atomic_sub_frag_count_return() has added the optimization
to not do the atomic sub_return when the caller is the last user of the
page, see page_pool_atomic_sub_frag_count_return():

	/* As suggested by Alexander, atomic_long_read() may cover up the
	 * reference count errors, so avoid calling atomic_long_read() in
	 * the cases of freeing or draining the page_frags, where we would
	 * not expect it to match or that are slowpath anyway.
	 */
        if (__builtin_constant_p(nr) &&
            atomic_long_read(&page->pp_frag_count) == nr)
                return 0;

So the check against 1 is not needed here?

.