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

[Ilias Apalodimas]

Hi Shakeel, 

On Sat, Apr 10, 2021 at 10:42:30AM -0700, Shakeel Butt wrote:
> On Sat, Apr 10, 2021 at 9:16 AM Ilias Apalodimas
> <ilias.apalodimas@xxxxxxxxxx> 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.
So we have to keep the extra added field on struct page for our mark.
Matthew had an intersting idea.  He suggested keeping it, but changing the 
magic number, so it can't be a kernel address, but I'll let him follow 
up on the details.

> 
> There is a recent discussion [1] on memcg accounting of TCP RX
> zerocopy and I am wondering if this work can somehow help in that
> regard. I will take a look at the series.
> 

I'll try having a look on this as well. The idea behind the patchset is to
allow lower speed NICs that use the API already, gain recycling 'easily'.  
Using page_pool for the driver comes with a penalty to begin with.
Allocating pages instead of SKBs has a measurable difference. By enabling them
to recycle they'll get better performance, since you skip the
reallocation/remapping and only care for syncing the buffers correctly.

> [1] https://lore.kernel.org/linux-mm/20210316013003.25271-1-arjunroy.kdev@xxxxxxxxx/