From: Yunsheng Lin <linyunsheng@huawei.com> To: Alexander Duyck <alexander.duyck@gmail.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: Thu, 22 Jul 2021 16:07:48 +0800 [thread overview] Message-ID: <fffae41f-b0a3-3c43-491f-096d31ba94ca@huawei.com> (raw) In-Reply-To: <CAKgT0UfwiBowGN+ctqoFZ6qaQAUp-0uGJeukk4OHOEOOfbrEWw@mail.gmail.com> On 2021/7/21 22:06, Alexander Duyck wrote: > 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. Yes, sorry for the confusion. > >> 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. If I understand it correctly, we should change it as below, right? static inline long page_pool_atomic_sub_frag_count_return(struct page *page, long nr) { long ret; /* 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; ret = atomic_long_sub_return(nr, &page->pp_frag_count); WARN_ON(ret < 0); return ret; } > . >
next prev parent reply other threads:[~2021-07-22 8:08 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 2021-07-22 8:07 ` Yunsheng Lin [this message] 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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