linux-kernel - Re: [PATCH v5] net/mlx5: Reclaim max 50K pages at once

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Date: Wed, 26 Jun 2024 04:19:17 +0800
From: Zhu Yanjun <yanjun.zhu@...ux.dev>
To: Anand Khoje <anand.a.khoje@...cle.com>,
 Jesse Brandeburg <jesse.brandeburg@...el.com>, linux-rdma@...r.kernel.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc: saeedm@...lanox.com, leon@...nel.org, tariqt@...dia.com,
 edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net
Subject: Re: [PATCH v5] net/mlx5: Reclaim max 50K pages at once

在 2024/6/25 13:00, Anand Khoje 写道:
> 
> On 6/25/24 02:11, Jesse Brandeburg wrote:
>> On 6/24/2024 8:33 AM, Anand Khoje wrote:
>>
>>> --- a/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
>>> +++ b/drivers/net/ethernet/mellanox/mlx5/core/pagealloc.c
>>> @@ -608,6 +608,7 @@ enum {
>>>       RELEASE_ALL_PAGES_MASK = 0x4000,
>>>   };
>>> +#define MAX_RECLAIM_NPAGES -50000
>> Can you please explain why this is negative? There doesn't seem to be
>> any reason mentioned in the commit message or code.
>>
>> At the very least it's super confusing to have a MAX be negative, and at
>> worst it's a bug. I don't have any other context on this code besides
>> this patch, so an explanation would be helpful.
>>
>>
>>
> Hi Jesse,
> 
> The way Mellanox ConnectX5 driver handles 'release of allocated pages 
> from HCA' or 'allocation of pages to HCA', is by sending an event to the 
> host. This event will have number of pages in it. If the number is 
> positive, that indicates HCA is requesting that number of pages to be 
> allocated. And if that number is negative, it is the HCA indicating that 
> that number of pages can be reclaimed by the host.
> 
> In this patch we are restricting the maximum number of pages that can be 
> reclaimed to be 50000 (effectively this would be -50000 as it is 
> reclaim). This limit is based on the capability of the firmware as it 
> cannot release more than 50000 back to the host in one go.
> 
> I hope that explains.

To be honest, I am also obvious why this MACRO is defined as a negative 
number. From the above, I can understand why. I think, perhaps many 
people also wonder why it is defined as a negative. IMO, it is better 
that you put the above explanations into the source code as comments.
When users check the source code, from the comments, users will know why 
it is defined as a negative number.

Thanks a lot.
Zhu Yanjun

> 
> Thanks,
> 
> Anand
>