netdev - Re: mlx5 XDP redirect leaking memory on kernel 6.3

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Message-ID: <6fef5ce8-c414-f182-f607-8ce199519dff@redhat.com>
Date: Wed, 24 May 2023 17:28:22 +0200
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
To: Dragos Tatulea <dtatulea@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
 "ttoukan.linux@...il.com" <ttoukan.linux@...il.com>,
 "jbrouer@...hat.com" <jbrouer@...hat.com>, Saeed Mahameed
 <saeedm@...dia.com>, "saeed@...nel.org" <saeed@...nel.org>,
 "linyunsheng@...wei.com" <linyunsheng@...wei.com>,
 "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc: brouer@...hat.com, "maxtram95@...il.com" <maxtram95@...il.com>,
 "lorenzo@...nel.org" <lorenzo@...nel.org>,
 "alexander.duyck@...il.com" <alexander.duyck@...il.com>,
 "kheib@...hat.com" <kheib@...hat.com>,
 "ilias.apalodimas@...aro.org" <ilias.apalodimas@...aro.org>,
 "mkabat@...hat.com" <mkabat@...hat.com>, "atzin@...hat.com"
 <atzin@...hat.com>, "fmaurer@...hat.com" <fmaurer@...hat.com>,
 "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
 "jbenc@...hat.com" <jbenc@...hat.com>
Subject: Re: mlx5 XDP redirect leaking memory on kernel 6.3



On 23/05/2023 18.35, Dragos Tatulea wrote:
> 
> On Tue, 2023-05-23 at 17:55 +0200, Jesper Dangaard Brouer wrote:
>>
>> When the mlx5 driver runs an XDP program doing XDP_REDIRECT, then memory
>> is getting leaked. Other XDP actions, like XDP_DROP, XDP_PASS and XDP_TX
>> works correctly. I tested both redirecting back out same mlx5 device and
>> cpumap redirect (with XDP_PASS), which both cause leaking.
>>
>> After removing the XDP prog, which also cause the page_pool to be
>> released by mlx5, then the leaks are visible via the page_pool periodic
>> inflight reports. I have this bpftrace[1] tool that I also use to detect
>> the problem faster (not waiting 60 sec for a report).
>>
>>    [1]
>> https://github.com/xdp-project/xdp-project/blob/master/areas/mem/bpftrace/page_pool_track_shutdown01.bt
>>
>> I've been debugging and reading through the code for a couple of days,
>> but I've not found the root-cause, yet. I would appreciate new ideas
>> where to look and fresh eyes on the issue.
>>
>>
>> To Lin, it looks like mlx5 uses PP_FLAG_PAGE_FRAG, and my current
>> suspicion is that mlx5 driver doesn't fully release the bias count (hint
>> see MLX5E_PAGECNT_BIAS_MAX).
>>
> 
> Thanks for the report Jesper. Incidentally I've just picked up this issue today
> as well.
> 
> On XDP redirect and tx, the page is set to skip the bias counter release with
> the expectation that page_pool_put_defragged_page will be called from [1]. But,
> as I found out now, during XDP redirect only one fragment of the page is
> released in xdp core [2]. This is where the leak is coming from.
> 

Ohh, I guess I see the problem now. (As Lin also says indirectly) the
page_pool_put_defragged_page() call is not allowed or not intended to be
invoked directly.

In [1] the driver actually free a PP page that have been fragmented (via
page_pool_fragment_page), but not "defragged" yet.  Meaning
page->pp_frag_count will still be 64 (MLX5E_PAGECNT_BIAS_MAX).

I though about catching this invalid API usage in page_pool, but due to
an (atomic_read) optimization (in page_pool_defrag_page), we cannot
detect this reliably.

> We'll provide a fix soon.
> 
> [1]
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c#n665
> 
> [2]
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/net/core/xdp.c#n390