Re: [PATCH 1/1] mm: avoid re-using pfmemalloc page in page_frag_alloc()

[Dongli Zhang]

On 11/4/20 4:51 AM, Eric Dumazet wrote:
> 
> 
> On 11/4/20 1:36 PM, Matthew Wilcox wrote:
>> On Wed, Nov 04, 2020 at 09:50:30AM +0100, Eric Dumazet wrote:
>>> On 11/4/20 2:16 AM, Rama Nichanamatlu wrote:
>>>>> Thanks for providing the numbers.  Do you think that dropping (up to)
>>>>> 7 packets is acceptable?
>>>>
>>>> net.ipv4.tcp_syn_retries = 6
>>>>
>>>> tcp clients wouldn't even get that far leading to connect establish issues.
>>>
>>> This does not really matter. If host was under memory pressure,
>>> dropping a few packets is really not an issue.
>>>
>>> Please do not add expensive checks in fast path, just to "not drop a packet"
>>> even if the world is collapsing.
>>
>> Right, that was my first patch -- to only recheck if we're about to
>> reuse the page.  Do you think that's acceptable, or is that still too
>> close to the fast path?
> 
> I think it is totally acceptable.
> 
> The same strategy is used in NIC drivers, before recycling a page.
> 
> If page_is_pfmemalloc() returns true, they simply release the 'problematic'page
> and attempt a new allocation.
> 
> ( git grep -n page_is_pfmemalloc -- drivers/net/ethernet/ )

While the drivers may implement their own page_frag_cache to manage skb->frags ...

... the skb->data is usually allocated via __netdev_alloc_skb() or
napi_alloc_skb(), which end up to the global this_cpu_ptr(&netdev_alloc_cache)
or this_cpu_ptr(&napi_alloc_cache.page).

> 
> 
>>
>>> Also consider that NIC typically have thousands of pre-allocated page/frags
>>> for their RX ring buffers, they might all have pfmemalloc set, so we are speaking
>>> of thousands of packet drops before the RX-ring can be refilled with normal (non pfmemalloc) page/frags.
>>>
>>> If we want to solve this issue more generically, we would have to try
>>> to copy data into a non pfmemalloc frag instead of dropping skb that
>>> had frags allocated minutes ago under memory pressure.
>>
>> I don't think we need to copy anything.  We need to figure out if the
>> system is still under memory pressure, and if not, we can clear the
>> pfmemalloc bit on the frag, as in my second patch.  The 'least change'
>> way of doing that is to try to allocate a page, but the VM could export
>> a symbol that says "we're not under memory pressure any more".
>>
>> Did you want to move checking that into the networking layer, or do you
>> want to keep it in the pagefrag allocator?
> 
> I think your proposal is fine, thanks !

Hi Matthew, are you going to send out the patch to avoid pfmemalloc recycle?

Thank you very much!

Dongli Zhang