From: Alexander Duyck <alexander.duyck@gmail.com> To: Yunsheng Lin <linyunsheng@huawei.com> Cc: David Miller <davem@davemloft.net>, Jakub Kicinski <kuba@kernel.org>, Russell King - ARM Linux <linux@armlinux.org.uk>, Marcin Wojtas <mw@semihalf.com>, linuxarm@openeuler.org, yisen.zhuang@huawei.com, Salil Mehta <salil.mehta@huawei.com>, thomas.petazzoni@bootlin.com, hawk@kernel.org, Ilias Apalodimas <ilias.apalodimas@linaro.org>, Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, John Fastabend <john.fastabend@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Peter Zijlstra <peterz@infradead.org>, Will Deacon <will@kernel.org>, Matthew Wilcox <willy@infradead.org>, Vlastimil Babka <vbabka@suse.cz>, fenghua.yu@intel.com, guro@fb.com, Peter Xu <peterx@redhat.com>, Feng Tang <feng.tang@intel.com>, Jason Gunthorpe <jgg@ziepe.ca>, Matteo Croce <mcroce@microsoft.com>, Hugh Dickins <hughd@google.com>, Jonathan Lemon <jonathan.lemon@gmail.com>, Alexander Lobakin <alobakin@pm.me>, Willem de Bruijn <willemb@google.com>, wenxu@ucloud.cn, Cong Wang <cong.wang@bytedance.com>, Kevin Hao <haokexin@gmail.com>, nogikh@google.com, Marco Elver <elver@google.com>, Yonghong Song <yhs@fb.com>, kpsingh@kernel.org, andrii@kernel.org, Martin KaFai Lau <kafai@fb.com>, songliubraving@fb.com, Netdev <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, bpf <bpf@vger.kernel.org> Subject: Re: [PATCH rfc v6 2/4] page_pool: add interface to manipulate frag count in page pool Date: Wed, 21 Jul 2021 07:06:26 -0700 [thread overview] Message-ID: <CAKgT0UfwiBowGN+ctqoFZ6qaQAUp-0uGJeukk4OHOEOOfbrEWw@mail.gmail.com> (raw) In-Reply-To: <92e68f4e-49a4-568c-a281-2865b54a146e@huawei.com> On Wed, Jul 21, 2021 at 1:15 AM Yunsheng Lin <linyunsheng@huawei.com> wrote: > > On 2021/7/20 23:43, Alexander Duyck wrote: > > On Mon, Jul 19, 2021 at 8:36 PM Yunsheng Lin <linyunsheng@huawei.com> wrote: > >> > >> For 32 bit systems with 64 bit dma, dma_addr[1] is used to > >> store the upper 32 bit dma addr, those system should be rare > >> those days. > >> > >> For normal system, the dma_addr[1] in 'struct page' is not > >> used, so we can reuse dma_addr[1] for storing frag count, > >> which means how many frags this page might be splited to. > >> > >> In order to simplify the page frag support in the page pool, > >> the PAGE_POOL_DMA_USE_PP_FRAG_COUNT macro is added to indicate > >> the 32 bit systems with 64 bit dma, and the page frag support > >> in page pool is disabled for such system. > >> > >> The newly added page_pool_set_frag_count() is called to reserve > >> the maximum frag count before any page frag is passed to the > >> user. The page_pool_atomic_sub_frag_count_return() is called > >> when user is done with the page frag. > >> > >> Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com> > >> --- > >> include/linux/mm_types.h | 18 +++++++++++++----- > >> include/net/page_pool.h | 41 ++++++++++++++++++++++++++++++++++------- > >> net/core/page_pool.c | 4 ++++ > >> 3 files changed, 51 insertions(+), 12 deletions(-) > >> > > > > <snip> > > > >> +static inline long page_pool_atomic_sub_frag_count_return(struct page *page, > >> + long nr) > >> +{ > >> + long frag_count = atomic_long_read(&page->pp_frag_count); > >> + long ret; > >> + > >> + if (frag_count == nr) > >> + return 0; > >> + > >> + ret = atomic_long_sub_return(nr, &page->pp_frag_count); > >> + WARN_ON(ret < 0); > >> + return ret; > >> } > >> > > > > So this should just be an atomic_long_sub_return call. You should get > > rid of the atomic_long_read portion of this as it can cover up > > reference count errors. > > atomic_long_sub_return() is used to avoid one possible cache bouncing and > barrrier caused by the last user. I assume you mean "atomic_long_read()" here. > You are right that that may cover up the reference count errors. How about > something like below: > > static inline long page_pool_atomic_sub_frag_count_return(struct page *page, > long nr) > { > #ifdef CONFIG_DEBUG_PAGE_REF > long ret = atomic_long_sub_return(nr, &page->pp_frag_count); > > WARN_ON(ret < 0); > > return ret; > #else > if (atomic_long_read(&page->pp_frag_count) == nr) > return 0; > > return atomic_long_sub_return(nr, &page->pp_frag_count); > #end > } > > Or any better suggestion? So the one thing I might change would be to make it so that you only do the atomic_long_read if nr is a constant via __builtin_constant_p. That way you would be performing the comparison in __page_pool_put_page and in the cases of freeing or draining the page_frags you would be using the atomic_long_sub_return which should be paths where you would not expect it to match or that are slowpath anyway. Also I would keep the WARN_ON in both paths just to be on the safe side.
next prev parent reply other threads:[~2021-07-21 14:06 UTC|newest] Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-07-20 3:35 [PATCH rfc v6 0/4] add frag page support " Yunsheng Lin 2021-07-20 3:35 ` [PATCH rfc v6 1/4] page_pool: keep pp info as long as page pool owns the page Yunsheng Lin 2021-07-20 3:35 ` [PATCH rfc v6 2/4] page_pool: add interface to manipulate frag count in page pool Yunsheng Lin 2021-07-20 15:43 ` Alexander Duyck 2021-07-21 8:15 ` Yunsheng Lin 2021-07-21 14:06 ` Alexander Duyck [this message] 2021-07-22 8:07 ` Yunsheng Lin 2021-07-22 15:18 ` Alexander Duyck 2021-07-23 11:12 ` Yunsheng Lin 2021-07-23 16:08 ` Alexander Duyck 2021-07-24 13:07 ` Yunsheng Lin 2021-07-25 16:49 ` Alexander Duyck 2021-07-27 7:54 ` Yunsheng Lin 2021-07-27 18:38 ` Alexander Duyck 2021-08-02 9:17 ` Yunsheng Lin 2021-07-20 3:35 ` [PATCH rfc v6 3/4] page_pool: add frag page recycling support " Yunsheng Lin 2021-07-20 3:35 ` [PATCH rfc v6 4/4] net: hns3: support skb's frag page recycling based on " Yunsheng Lin
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