Re: [RFC PATCH] net: Fix one page_pool page leak from skb_frag_unref

[Mina Almasry]

On Thu, Apr 25, 2024 at 1:17 AM Dragos Tatulea <dtatulea@xxxxxxxxxx> wrote:
>
> On Wed, 2024-04-24 at 15:08 -0700, Mina Almasry wrote:
> >  If that doesn't work, I think I prefer
> > reverting a580ea994fd3 ("net: mirror skb frag ref/unref helpers")
> > rather than merging this fix to make sure we removed the underlying
> > cause of the issue.
> This is the safest bet.
>
> So, to recap, I see 2 possibilities:
>
> 1) Revert a580ea994fd3 ("net: mirror skb frag ref/unref helpers"): safe, but it
> will probably have to come back in one way or another.
> 2) Drop the recycle checks from skb_frag_ref/unref: this enforces the rule of
> always referencing/dereferencing pages based on their type (page_pool or
> normal).
>

If this works, I would be very happy. I personally think ref/unref
should be done based on the page type. For me the net stack using the
regular {get|put}_page on a pp page isn't great. It requires special
handling to make sure the ref + unref are in sync. Also if the last pp
ref is dropped while there are pending regular refs,
__page_pool_page_can_be_recycled() check will fail and the page will
not be recycled.

On the other hand, since 0a149ab78ee2 ("page_pool: transition to
reference count management after page draining") I'm not sure there is
any reason to continue to use get/put_page on pp-pages, we can use the
new pp-ref instead.

I don't see any regressions with this diff (needs cleanup), but your
test setup seems much much better than mine (I think this is the
second reffing issue you manage to repro):

diff --git a/include/linux/skbuff_ref.h b/include/linux/skbuff_ref.h
index 4dcdbe9fbc5f..4c72227dce1b 100644
--- a/include/linux/skbuff_ref.h
+++ b/include/linux/skbuff_ref.h
@@ -31,7 +31,7 @@ static inline bool napi_pp_get_page(struct page *page)
 static inline void skb_page_ref(struct page *page, bool recycle)
 {
 #ifdef CONFIG_PAGE_POOL
-       if (recycle && napi_pp_get_page(page))
+       if (napi_pp_get_page(page))
                return;
 #endif
        get_page(page);
@@ -69,7 +69,7 @@ static inline void
 skb_page_unref(struct page *page, bool recycle)
 {
 #ifdef CONFIG_PAGE_POOL
-       if (recycle && napi_pp_put_page(page))
+       if (napi_pp_put_page(page))
                return;
 #endif
        put_page(page);


-- 
Thanks,
Mina