netdev - Re: [PATCH net-next] page_pool: check if nmdesc->pp is !NULL to confirm its usage as pp for net

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Message-ID: <8d833a3f-ae18-4ea6-9092-ddaa48290a63@gmail.com>
Date: Fri, 17 Oct 2025 13:33:43 +0100
From: Pavel Begunkov <asml.silence@...il.com>
To: Byungchul Park <byungchul@...com>, axboe@...nel.dk, kuba@...nel.org,
 pabeni@...hat.com, almasrymina@...gle.com
Cc: davem@...emloft.net, edumazet@...gle.com, horms@...nel.org,
 hawk@...nel.org, ilias.apalodimas@...aro.org, sdf@...ichev.me,
 dw@...idwei.uk, ap420073@...il.com, dtatulea@...dia.com, toke@...hat.com,
 io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org, kernel_team@...ynix.com, max.byungchul.park@...il.com
Subject: Re: [PATCH net-next] page_pool: check if nmdesc->pp is !NULL to
 confirm its usage as pp for net_iov

On 10/16/25 08:21, Byungchul Park wrote:
> On Thu, Oct 16, 2025 at 03:36:57PM +0900, Byungchul Park wrote:
>> ->pp_magic field in struct page is current used to identify if a page
>> belongs to a page pool.  However, ->pp_magic will be removed and page
>> type bit in struct page e.g. PGTY_netpp should be used for that purpose.
>>
>> As a preparation, the check for net_iov, that is not page-backed, should
>> avoid using ->pp_magic since net_iov doens't have to do with page type.
>> Instead, nmdesc->pp can be used if a net_iov or its nmdesc belongs to a
>> page pool, by making sure nmdesc->pp is NULL otherwise.
>>
>> For page-backed netmem, just leave unchanged as is, while for net_iov,
>> make sure nmdesc->pp is initialized to NULL and use nmdesc->pp for the
>> check.
> 
> IIRC,
> 
> Suggested-by: Pavel Begunkov <asml.silence@...il.com>

Pointing out a problem in a patch with a fix doesn't qualify to
me as "suggested-by", you don't need to worry about that.

Did you get the PGTY bits merged? There is some uneasiness about
this patch as it does nothing good by itself, it'd be much better
to have it in a series finalising the page_pool conversion. And
I don't think it simplify merging anyhow, hmm?

...>> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
>> index 723e4266b91f..cf78227c0ca6 100644
>> --- a/io_uring/zcrx.c
>> +++ b/io_uring/zcrx.c
>> @@ -450,6 +450,10 @@ static int io_zcrx_create_area(struct io_zcrx_ifq *ifq,
>>   		area->freelist[i] = i;
>>   		atomic_set(&area->user_refs[i], 0);
>>   		niov->type = NET_IOV_IOURING;
>> +
>> +		/* niov->desc.pp is already initialized to NULL by
>> +		 * kvmalloc_array(__GFP_ZERO).
>> +		 */

Please drop this hunk if you'll be resubmitting, it's not
needed.

-- 
Pavel Begunkov