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From:   Menglong Dong <menglong8.dong@gmail.com>
Date:   Tue, 1 Jun 2021 22:18:29 +0800
Message-ID: <CADxym3baupJJ7Q9otxtoQ-DH5e-J2isg-LZj2CsOqRPo70AL4A@mail.gmail.com>
Subject: The value of FB_MTU eats two pages
To:     jmaloy@redhat.com
Cc:     ying.xue@windriver.com, David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, etdev@vger.kernel.org,
        tipc-discussion@lists.sourceforge.net,
        LKML <linux-kernel@vger.kernel.org>
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Hello!

I have a question about the value of FB_MTU in tipc, how does the '3744' form?
I notice that it is used in 'tipc_msg_build()' when memory allocation
fails, and it
tries to fall back to a smaller MTU to avoid unnecessary sending failures.

However, the size of the data allocated will be more than 4096 when FB_MTU
is 3744. I did a rough calculation, the size of data will more than 4200:

(FB_MTU + TIPCHDR + BUF_HEADROOM + sizeof(struct skb_shared_info))

Therefore, 8192 will be allocated from slab, and about 4000 of it will
not be used.

FB_MTU is used for low memory, and I think eating two pages will make it worse.
Do I miss something?

Thanks!
Menglong Dong