On 2021/8/30 23:14, Alexander Duyck wrote:
On Sun, Aug 29, 2021 at 6:19 PM Yunsheng Lin linyunsheng@huawei.com wrote:
As the skb->pp_recycle and page->pp_magic may not be enough to track if a frag page is from page pool after the calling of __skb_frag_ref(), mostly because of a data race, see: commit 2cc3aeb5eccc ("skbuff: Fix a potential race while recycling page_pool packets").
There may be clone and expand head case that might lose the track if a frag page is from page pool or not.
So increment the frag count when __skb_frag_ref() is called, and only use page->pp_magic to indicate if a frag page is from page pool, to avoid the above data race.
For 32 bit systems with 64 bit dma, we preserve the orginial behavior as frag count is used to trace how many time does a frag page is called with __skb_frag_ref().
Signed-off-by: Yunsheng Lin linyunsheng@huawei.com
Is this really a common enough case to justify adding this extra overhead?
I am not sure I understand what does extra overhead mean here. But it seems this patch does not add any explicit overhead? As the added page_pool_is_pp_page() checking in __skb_frag_ref() is neutralized by avoiding the recycle checking in __skb_frag_unref(), and the atomic operation is with either pp_frag_count or _refcount?
include/linux/skbuff.h | 13 ++++++++++++- include/net/page_pool.h | 17 +++++++++++++++++ net/core/page_pool.c | 12 ++---------- 3 files changed, 31 insertions(+), 11 deletions(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h index 6bdb0db..8311482 100644 --- a/include/linux/skbuff.h +++ b/include/linux/skbuff.h @@ -3073,6 +3073,16 @@ static inline struct page *skb_frag_page(const skb_frag_t *frag) */ static inline void __skb_frag_ref(skb_frag_t *frag) {
struct page *page = skb_frag_page(frag);+#ifdef CONFIG_PAGE_POOL
if (!PAGE_POOL_DMA_USE_PP_FRAG_COUNT &&page_pool_is_pp_page(page)) {page_pool_atomic_inc_frag_count(page);return;}+#endif
get_page(skb_frag_page(frag));}
This just seems like a bad idea in general. We are likely increasing the potential for issues with this patch instead of avoiding them. I
Yes, I am agreed that calling the __skb_frag_ref() without calling the __skb_frag_unref() for the same page might be more likely to cause problem for this patch. But we are already depending on the calling of __skb_frag_unref() to free the pp page, making it more likely just enable us to catch the bug more quickly?
Or is there other situation that I am not awared of, which might cause issues?
really feel it would be better for us to just give up on the page and kick it out of the page pool if we are cloning frames and multiple references are being taken on the pages.
For Rx, it seems fine for normal case. For Tx, it seems the cloning and multiple references happens when tso_fragment() is called in tcp_write_xmit(), and the driver need to reliable way to tell if a page is from the page pool, so that the dma mapping can be avoided for Tx too.
@@ -3101,7 +3111,8 @@ static inline void __skb_frag_unref(skb_frag_t *frag, bool recycle) struct page *page = skb_frag_page(frag);
#ifdef CONFIG_PAGE_POOL
if (recycle && page_pool_return_skb_page(page))
if ((!PAGE_POOL_DMA_USE_PP_FRAG_COUNT || recycle) &&page_pool_return_skb_page(page)) return;#endif put_page(page); diff --git a/include/net/page_pool.h b/include/net/page_pool.h index 2ad0706..8b43e3d9 100644 --- a/include/net/page_pool.h +++ b/include/net/page_pool.h @@ -244,6 +244,23 @@ static inline void page_pool_set_frag_count(struct page *page, long nr) atomic_long_set(&page->pp_frag_count, nr); }
+static inline void page_pool_atomic_inc_frag_count(struct page *page) +{
atomic_long_inc(&page->pp_frag_count);+}
+static inline bool page_pool_is_pp_page(struct page *page) +{
/* page->pp_magic is OR'ed with PP_SIGNATURE after the allocation* in order to preserve any existing bits, such as bit 0 for the* head page of compound page and bit 1 for pfmemalloc page, so* mask those bits for freeing side when doing below checking,* and page_is_pfmemalloc() is checked in __page_pool_put_page()* to avoid recycling the pfmemalloc page.*/return (page->pp_magic & ~0x3UL) == PP_SIGNATURE;+}
static inline long page_pool_atomic_sub_frag_count_return(struct page *page, long nr) { diff --git a/net/core/page_pool.c b/net/core/page_pool.c index ba9f14d..442d37b 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -24,7 +24,7 @@ #define DEFER_TIME (msecs_to_jiffies(1000)) #define DEFER_WARN_INTERVAL (60 * HZ)
-#define BIAS_MAX LONG_MAX +#define BIAS_MAX (LONG_MAX / 2)
This piece needs some explaining in the patch. Why are you changing the BIAS_MAX?
When __skb_frag_ref() is called for the pp page that is not drained yet, the pp_frag_count could be overflowed if the BIAS is too big.
static int page_pool_init(struct page_pool *pool, const struct page_pool_params *params) @@ -741,15 +741,7 @@ bool page_pool_return_skb_page(struct page *page) struct page_pool *pp;
page = compound_head(page);
/* page->pp_magic is OR'ed with PP_SIGNATURE after the allocation* in order to preserve any existing bits, such as bit 0 for the* head page of compound page and bit 1 for pfmemalloc page, so* mask those bits for freeing side when doing below checking,* and page_is_pfmemalloc() is checked in __page_pool_put_page()* to avoid recycling the pfmemalloc page.*/if (unlikely((page->pp_magic & ~0x3UL) != PP_SIGNATURE))
if (!page_pool_is_pp_page(page)) return false; pp = page->pp;-- 2.7.4
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