netdev - Re: [PATCH net-next 1/5] bpf: use __GFP

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Message-ID: <2fd601e3-5679-08f4-1610-b3c22de80935@oracle.com>
Date:   Wed, 12 Sep 2018 09:21:53 -0700
From:   Tushar Dave <tushar.n.dave@...cle.com>
To:     ast@...nel.org, daniel@...earbox.net, davem@...emloft.net,
        santosh.shilimkar@...cle.com, jakub.kicinski@...ronome.com,
        quentin.monnet@...ronome.com, jiong.wang@...ronome.com,
        sandipan@...ux.vnet.ibm.com, john.fastabend@...il.com,
        kafai@...com, rdna@...com, yhs@...com, netdev@...r.kernel.org,
        rds-devel@....oracle.com, sowmini.varadhan@...cle.com
Subject: Re: [PATCH net-next 1/5] bpf: use __GFP_COMP while allocating page



On 09/11/2018 12:38 PM, Tushar Dave wrote:
> Helper bpg_msg_pull_data() can allocate multiple pages while
> linearizing multiple scatterlist elements into one shared page.
> However, if the shared page has size > PAGE_SIZE, using
> copy_page_to_iter() causes below warning.
> 
> e.g.
> [ 6367.019832] WARNING: CPU: 2 PID: 7410 at lib/iov_iter.c:825
> page_copy_sane.part.8+0x0/0x8
> 
> To avoid above warning, use __GFP_COMP while allocating multiple
> contiguous pages.
> 
> Signed-off-by: Tushar Dave <tushar.n.dave@...cle.com>
> ---
>   net/core/filter.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index d301134..0b40f95 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2344,7 +2344,8 @@ struct sock *do_msg_redirect_map(struct sk_msg_buff *msg)
>   	if (unlikely(bytes_sg_total > copy))
>   		return -EINVAL;
>   
> -	page = alloc_pages(__GFP_NOWARN | GFP_ATOMIC, get_order(copy));
> +	page = alloc_pages(__GFP_NOWARN | GFP_ATOMIC | __GFP_COMP,
> +			   get_order(copy));
>   	if (unlikely(!page))
>   		return -ENOMEM;
>   	p = page_address(page);

I should have mentioned that I could re-order this patch anywhere in
patch series (as long as it doesn't break git bisect). I kept it first
because I think it is more like a bug fix. I sent it along with these
patch series considering we have a context of why and for what I need
this patch!

Daniel, John,

Not sure if you guys hit this page_copy_sane warning. I hit it when RDS
copy sg page to userspace using copy_page_to_iter().

example:

RDS packet size 8KB represented in scatterlist:
sg_data[0].length = 1400
sg_data[1].length = 1448
sg_data[2].length = 1448
sg_data[3].length = 1448
sg_data[4].length = 1448
sg_data[5].length = 1000

If start=0 and end=8192, bpf_msg_pull_data() will linearize all
sg_data elements into one shared page. e.g. sg_data[0].length = 8192.
Using this sg_data[0].page in function copy_page_to_iter() causes:
WARNING: CPU: 2 PID: 7410 at lib/iov_iter.c:825
page_copy_sane.part.8+0x0/0x8

(FYI, patch 4 has code that does copy_page_to_iter)


Comments?

Thanks in advance,
-Tushar