netdev - Re: [PATCH net] net: Don't copy pfmemalloc flag in __copy_skb

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Message-Id: <20180712.151532.651394681141690693.davem@davemloft.net>
Date:   Thu, 12 Jul 2018 15:15:32 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     sbrivio@...hat.com
Cc:     mgorman@...e.de, eric.dumazet@...il.com, fw@...len.de,
        ptalbert@...hat.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] net: Don't copy pfmemalloc flag in
 __copy_skb_header()

From: Stefano Brivio <sbrivio@...hat.com>
Date: Wed, 11 Jul 2018 14:39:42 +0200

> The pfmemalloc flag indicates that the skb was allocated from
> the PFMEMALLOC reserves, and the flag is currently copied on skb
> copy and clone.
> 
> However, an skb copied from an skb flagged with pfmemalloc
> wasn't necessarily allocated from PFMEMALLOC reserves, and on
> the other hand an skb allocated that way might be copied from an
> skb that wasn't.
> 
> So we should not copy the flag on skb copy, and rather decide
> whether to allow an skb to be associated with sockets unrelated
> to page reclaim depending only on how it was allocated.
> 
> Move the pfmemalloc flag before headers_start[0] using an
> existing 1-bit hole, so that __copy_skb_header() doesn't copy
> it.
> 
> When cloning, we'll now take care of this flag explicitly,
> contravening to the warning comment of __skb_clone().
> 
> While at it, restore the newline usage introduced by commit
> b19372273164 ("net: reorganize sk_buff for faster
> __copy_skb_header()") to visually separate bytes used in
> bitfields after headers_start[0], that was gone after commit
> a9e419dc7be6 ("netfilter: merge ctinfo into nfct pointer storage
> area"), and describe the pfmemalloc flag in the kernel-doc
> structure comment.
> 
> This doesn't change the size of sk_buff or cacheline boundaries,
> but consolidates the 15 bits hole before tc_index into a 2 bytes
> hole before csum, that could now be filled more easily.
> 
> Reported-by: Patrick Talbert <ptalbert@...hat.com>
> Fixes: c93bdd0e03e8 ("netvm: allow skb allocation to use PFMEMALLOC reserves")
> Signed-off-by: Stefano Brivio <sbrivio@...hat.com>

Applied and queued up for -stable, thank you.