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From: Xie He <>
To: Willem de Bruijn <>
Cc: "David S. Miller" <>,
	Jakub Kicinski <>,
	Network Development <>,
	linux-kernel <>,
	Linux X25 <>,
	Martin Schiller <>
Subject: Re: [PATCH net] drivers/net/wan/lapbether: Added needed_tailroom
Date: Sat, 15 Aug 2020 19:28:14 -0700	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

I took some time to look at the history of needed_tailroom. I found it
was added in this commit:
f5184d267c1a (net: Allow netdevices to specify needed head/tailroom)

The author tried to make use of needed_tailroom at various places in
the kernel by replacing the macro LL_RESERVED_SPACE with his new macro

However, the macro LL_ALLOCATED_SPACE was later found to have
problems. So it was removed 3 years later and was replaced by explicit
handling of needed_tailroom. See:

So maybe only those places considered by these two authors have taken
needed_tailroom into account.

Other places might not have taken needed_tailroom into account because
of the rarity of the usage of needed_tailroom.

The second author also said in the commit message of his Patch 5/6
(which changes af_packet.c), that:
    While auditing LL_ALLOCATED_SPACE I noticed that packet_sendmsg_spkt
    did not include needed_tailroom when allocating an skb.  This isn't
    a fatal error as we should always tolerate inadequate tail room but
    it isn't optimal.

This shows not taking needed_tailroom into account is not a bug but
it'd be better to take it into account.

  reply	other threads:[~2020-08-16  2:30 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-08 17:52 Xie He
2020-08-09  8:48 ` Willem de Bruijn
2020-08-09 17:11   ` Xie He
2020-08-10  7:31     ` Willem de Bruijn
2020-08-10 18:13       ` Xie He
2020-08-16  2:28         ` Xie He [this message]
2020-08-19  0:17 ` Xie He
2020-08-19 22:11 ` Xie He

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