Subject: Re: [PATCH net-next v3 2/5] mm: add a signature in struct page

From: Jesper Dangaard Brouer <[email-masked]>
Date: Wed, 14 Apr 2021 21:41:32 +0200

On Sat, 10 Apr 2021 21:27:31 +0300
Ilias Apalodimas <[email-masked]> wrote:

> On Sat, Apr 10, 2021 at 10:42:30AM -0700, Shakeel Butt wrote:
>
> > On Sat, Apr 10, 2021 at 9:16 AM Ilias Apalodimas
> > <[email-masked]> wrote:  
> > >
> > > Hi Matthew
> > >
> > > On Sat, Apr 10, 2021 at 04:48:24PM +0100, Matthew Wilcox wrote:  
> > > > On Sat, Apr 10, 2021 at 12:37:58AM +0200, Matteo Croce wrote:  
> > > > > This is needed by the page_pool to avoid recycling a page not allocated
> > > > > via page_pool.  
> > > >
> > > > Is the PageType mechanism more appropriate to your needs?  It wouldn't
> > > > be if you use page->_mapcount (ie mapping it to userspace).  
> > >
> > > Interesting!
> > > Please keep in mind this was written ~2018 and was stale on my branches for
> > > quite some time.  So back then I did try to use PageType, but had not free
> > > bits.  Looking at it again though, it's cleaned up.  So yes I think this can
> > > be much much cleaner.  Should we go and define a new PG_pagepool?
> > >
> > 
> > Can this page_pool be used for TCP RX zerocopy? If yes then PageType
> > can not be used.  
> 
> Yes it can, since it's going to be used as your default allocator for
> payloads, which might end up on an SKB.

I'm not sure we want or should "allow" page_pool be used for TCP RX
zerocopy.
For several reasons.

(1) This implies mapping these pages page to userspace, which AFAIK
means using page->mapping and page->index members (right?).

(2) It feels wrong (security wise) to keep the DMA-mapping (for the
device) and also map this page into userspace.

(3) The page_pool is optimized for refcnt==1 case, and AFAIK TCP-RX
zerocopy will bump the refcnt, which means the page_pool will not
recycle the page when it see the elevated refcnt (it will instead
release its DMA-mapping).

(4) I remember vaguely that this code path for (TCP RX zerocopy) uses
page->private for tricks.  And our patch [3/5] use page->private for
storing xdp_mem_info.

IMHO when the SKB travel into this TCP RX zerocopy code path, we should
call page_pool_release_page() to release its DMA-mapping.

> > [1] https://lore.kernel.org/linux-mm/[email-masked]/  

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer