linux-kernel - Re: [PATCH net-next v3 11/13] net: replace page_frag with page_frag

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Date: Fri, 10 May 2024 10:29:28 -0700 (PDT)
From: Mat Martineau <martineau@...nel.org>
To: Yunsheng Lin <linyunsheng@...wei.com>
cc: davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com, 
    netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
    Alexander Duyck <alexander.duyck@...il.com>, 
    Ayush Sawal <ayush.sawal@...lsio.com>, Eric Dumazet <edumazet@...gle.com>, 
    Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
    Jason Wang <jasowang@...hat.com>, Ingo Molnar <mingo@...hat.com>, 
    Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
    Vincent Guittot <vincent.guittot@...aro.org>, 
    Dietmar Eggemann <dietmar.eggemann@....com>, 
    Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, 
    Mel Gorman <mgorman@...e.de>, 
    Daniel Bristot de Oliveira <bristot@...hat.com>, 
    Valentin Schneider <vschneid@...hat.com>, 
    John Fastabend <john.fastabend@...il.com>, 
    Jakub Sitnicki <jakub@...udflare.com>, David Ahern <dsahern@...nel.org>, 
    Matthieu Baerts <matttbe@...nel.org>, Geliang Tang <geliang@...nel.org>, 
    Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>, 
    Jiri Pirko <jiri@...nulli.us>, Boris Pismenny <borisp@...dia.com>, 
    bpf@...r.kernel.org, mptcp@...ts.linux.dev
Subject: Re: [PATCH net-next v3 11/13] net: replace page_frag with
 page_frag_cache

On Fri, 10 May 2024, Yunsheng Lin wrote:

> On 2024/5/10 0:22, Mat Martineau wrote:
>> On Wed, 8 May 2024, Yunsheng Lin wrote:
>>
>>> Use the newly introduced prepare/probe/commit API to
>>> replace page_frag with page_frag_cache for sk_page_frag().
>>>
>>> CC: Alexander Duyck <alexander.duyck@...il.com>
>>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
>>> ---
>>> .../chelsio/inline_crypto/chtls/chtls.h       |   3 -
>>> .../chelsio/inline_crypto/chtls/chtls_io.c    | 100 ++++---------
>>> .../chelsio/inline_crypto/chtls/chtls_main.c  |   3 -
>>> drivers/net/tun.c                             |  28 ++--
>>> include/linux/sched.h                         |   4 +-
>>> include/net/sock.h                            |  14 +-
>>> kernel/exit.c                                 |   3 +-
>>> kernel/fork.c                                 |   3 +-
>>> net/core/skbuff.c                             |  32 ++--
>>> net/core/skmsg.c                              |  22 +--
>>> net/core/sock.c                               |  46 ++++--
>>> net/ipv4/ip_output.c                          |  33 +++--
>>> net/ipv4/tcp.c                                |  35 ++---
>>> net/ipv4/tcp_output.c                         |  28 ++--
>>> net/ipv6/ip6_output.c                         |  33 +++--
>>> net/kcm/kcmsock.c                             |  30 ++--
>>> net/mptcp/protocol.c                          |  70 +++++----
>>> net/sched/em_meta.c                           |   2 +-
>>> net/tls/tls_device.c                          | 139 ++++++++++--------
>>> 19 files changed, 331 insertions(+), 297 deletions(-)
>>>
>>
>> <snip>
>>
>>> diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
>>> index bb8f96f2b86f..ab844011d442 100644
>>> --- a/net/mptcp/protocol.c
>>> +++ b/net/mptcp/protocol.c
>>> @@ -960,17 +960,18 @@ static bool mptcp_skb_can_collapse_to(u64 write_seq,
>>> }
>>>
>>> /* we can append data to the given data frag if:
>>> - * - there is space available in the backing page_frag
>>> - * - the data frag tail matches the current page_frag free offset
>>> + * - there is space available for the current page
>>> + * - the data frag tail matches the current page and offset
>>>  * - the data frag end sequence number matches the current write seq
>>>  */
>>> static bool mptcp_frag_can_collapse_to(const struct mptcp_sock *msk,
>>> -                       const struct page_frag *pfrag,
>>> +                       const struct page *page,
>>> +                       const unsigned int offset,
>>> +                       const unsigned int size,
>>
>> Hi Yunsheng -
>>
>> Why add the 'size' parameter here? It's checked to be a nonzero value, 
>> but it can only be 0 if page is also NULL. In this case "page == 
>> df->page" will be false, so the function will return false even without 
>> checking 'size'.
>
> Is it possible that the pfrag->page is also NULL, which may cause
> mptcp_frag_can_collapse_to() to return true?

Not sure. But I do know that df->page will never be NULL, so "page == 
df->page" will always be false when page == NULL.

>
> I just found out that the 'size' is not set to zero when return
> NULL for the implementation of probe API for the current version.
> Perhaps it makes more sense to expect the API caller to make sure
> the the returned 'page' not being NULL before using the 'offset',
> 'size' and 'va', like below:
>
> df && page && page == df->page
>

Given that df->page is never NULL, I don't think the extra "&& page" is 
needed.

- Mat