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

From: Ilias Apalodimas <[email-masked]>
Date: Sat, 10 Apr 2021 19:16:45 +0300

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?

Thanks!
/Ilias
> 
> > Signed-off-by: Matteo Croce <[email-masked]>
> > ---
> >  include/linux/mm_types.h | 1 +
> >  include/net/page_pool.h  | 2 ++
> >  net/core/page_pool.c     | 4 ++++
> >  3 files changed, 7 insertions(+)
> > 
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 6613b26a8894..ef2d0d5f62e4 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -101,6 +101,7 @@ struct page {
> >  			 * 32-bit architectures.
> >  			 */
> >  			dma_addr_t dma_addr;
> > +			unsigned long signature;
> >  		};
> >  		struct {	/* slab, slob and slub */
> >  			union {
> > diff --git a/include/net/page_pool.h b/include/net/page_pool.h
> > index b5b195305346..b30405e84b5e 100644
> > --- a/include/net/page_pool.h
> > +++ b/include/net/page_pool.h
> > @@ -63,6 +63,8 @@
> >   */
> >  #define PP_ALLOC_CACHE_SIZE	128
> >  #define PP_ALLOC_CACHE_REFILL	64
> > +#define PP_SIGNATURE		0x20210303
> > +
> >  struct pp_alloc_cache {
> >  	u32 count;
> >  	void *cache[PP_ALLOC_CACHE_SIZE];
> > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> > index ad8b0707af04..2ae9b554ef98 100644
> > --- a/net/core/page_pool.c
> > +++ b/net/core/page_pool.c
> > @@ -232,6 +232,8 @@ static struct page *__page_pool_alloc_pages_slow(struct page_pool *pool,
> >  		page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
> >  
> >  skip_dma_map:
> > +	page->signature = PP_SIGNATURE;
> > +
> >  	/* Track how many pages are held 'in-flight' */
> >  	pool->pages_state_hold_cnt++;
> >  
> > @@ -302,6 +304,8 @@ void page_pool_release_page(struct page_pool *pool, struct page *page)
> >  			     DMA_ATTR_SKIP_CPU_SYNC);
> >  	page->dma_addr = 0;
> >  skip_dma_unmap:
> > +	page->signature = 0;
> > +
> >  	/* This may be the last page returned, releasing the pool, so
> >  	 * it is not safe to reference pool afterwards.
> >  	 */
> > -- 
> > 2.30.2
> >