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

[Eric Dumazet]

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/ )


> 
>> 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 !