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: <CAHS8izPeFvTNPTAqmfkAxR9aKd00HW-G45r77Ex16QQSjfQibg@mail.gmail.com>
Date: Sat, 18 Oct 2025 10:05:51 -0500
From: Mina Almasry <almasrymina@...gle.com>
To: Byungchul Park <byungchul@...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 11:47 PM Byungchul Park <byungchul@...com> wrote:
>
> 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?
>

I think 1 series with all the mm and networking changes targeting mm
and Cc: netdev@ would work (and lets see if Andrew prefers something
different). The networking changes are very small.

-- 
Thanks,
Mina