linux-kernel - 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: <20251018044653.GA66683@system.software.com>
Date: Sat, 18 Oct 2025 13:46:53 +0900
From: Byungchul Park <byungchul@...com>
To: Mina Almasry <almasrymina@...gle.com>
Cc: Pavel Begunkov <asml.silence@...il.com>, axboe@...nel.dk,
	kuba@...nel.org, pabeni@...hat.com, 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 Fri, Oct 17, 2025 at 08:13:14AM -0700, Mina Almasry wrote:
> On Fri, Oct 17, 2025 at 5:32 AM Pavel Begunkov <asml.silence@...il.com> wrote:
> >
> > 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?
> >
> 
> +1 honestly.
> 
> If you want to 'extract the networking bits' into its own patch,  let
> it be a patch series where this is a patch doing pre-work, and the
> next patches in the series are adding the page_flag.

Okay.  Then is it possible that one for mm tree and the other for
net-next in the same patch series?  I've never tried patches that way.

> I don't want added netmem_is_net_iov checks unnecessarily tbh. These
> checks are bad and only used when absolutely necessary, so let the
> patch series that adds them also do something useful (i.e. add the
> page flag), if possible. But I honestly think this patch was almost
> good as-is:

Hm.. but the following patch includes both networking changes and mm
changes.  Jakub thinks it should go to mm and I don't know how Andrew
thinks it should be.  It's not clear even to me.

That's why I splitted it into two, and this is the networking part, and
I will post the mm part to mm folks later.  Any suggestions?

	Byungchul

> https://lore.kernel.org/all/20250729110210.48313-1-byungchul@sk.com/
> 
> You just need to address Jakub's review comments and resubmit? Not
> sure why we want to split, but if you want let it be a patch series
> that does something useful.
> 
> --
> Thanks,
> Mina