netdev - Re: [PATCH net-next 04/12] mm: Make the page_frag

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Date: Sat, 27 May 2023 08:47:58 -0700
From: Alexander H Duyck <alexander.duyck@...il.com>
To: Yunsheng Lin <linyunsheng@...wei.com>, David Howells
 <dhowells@...hat.com>,  netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>,  Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
 <pabeni@...hat.com>, Willem de Bruijn <willemdebruijn.kernel@...il.com>,
 David Ahern <dsahern@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 Jens Axboe <axboe@...nel.dk>, linux-mm@...ck.org, 
 linux-kernel@...r.kernel.org, Jeroen de Borst <jeroendb@...gle.com>, 
 Catherine Sullivan <csully@...gle.com>, Shailend Chand
 <shailend@...gle.com>, Felix Fietkau <nbd@....name>, John Crispin
 <john@...ozen.org>, Sean Wang <sean.wang@...iatek.com>, Mark Lee
 <Mark-MC.Lee@...iatek.com>, Lorenzo Bianconi <lorenzo@...nel.org>, Matthias
 Brugger <matthias.bgg@...il.com>,  AngeloGioacchino Del Regno
 <angelogioacchino.delregno@...labora.com>, Keith Busch <kbusch@...nel.org>,
 Jens Axboe <axboe@...com>,  Christoph Hellwig <hch@....de>, Sagi Grimberg
 <sagi@...mberg.me>, Chaitanya Kulkarni <kch@...dia.com>, Andrew Morton
 <akpm@...ux-foundation.org>,  linux-arm-kernel@...ts.infradead.org,
 linux-mediatek@...ts.infradead.org,  linux-nvme@...ts.infradead.org
Subject: Re: [PATCH net-next 04/12] mm: Make the page_frag_cache allocator
 use multipage folios

On Fri, 2023-05-26 at 19:56 +0800, Yunsheng Lin wrote:
> On 2023/5/24 23:33, David Howells wrote:
> > Change the page_frag_cache allocator to use multipage folios rather than
> > groups of pages.  This reduces page_frag_free to just a folio_put() or
> > put_page().
> 
> Hi, David
> 
> put_page() is not used in this patch, perhaps remove it to avoid
> the confusion?
> Also, Is there any significant difference between __free_pages()
> and folio_put()? IOW, what does the 'reduces' part means here?
> 
> I followed some disscusion about folio before, but have not really
> understood about real difference between 'multipage folios' and
> 'groups of pages' yet. Is folio mostly used to avoid the confusion
> about whether a page is 'headpage of compound page', 'base page' or
> 'tailpage of compound page'? Or is there any abvious benefit about
> folio that I missed?
> 
> > 
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 306a3d1a0fa6..d7c52a5979cc 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -420,18 +420,13 @@ static inline void *folio_get_private(struct folio *folio)
> >  }
> >  
> >  struct page_frag_cache {
> > -	void * va;
> > -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE)
> > -	__u16 offset;
> > -	__u16 size;
> > -#else
> > -	__u32 offset;
> > -#endif
> > +	struct folio	*folio;
> > +	unsigned int	offset;
> >  	/* we maintain a pagecount bias, so that we dont dirty cache line
> >  	 * containing page->_refcount every time we allocate a fragment.
> >  	 */
> > -	unsigned int		pagecnt_bias;
> > -	bool pfmemalloc;
> > +	unsigned int	pagecnt_bias;
> > +	bool		pfmemalloc;
> >  };
> 
> It seems 'va' and 'size' field is used to avoid touching 'stuct page' to
> avoid possible cache bouncing when there is more frag can be allocated
> from the page while other frags is freed at the same time before this patch?
> It might be worth calling that out in the commit log or split it into another
> patch to make it clearer and easier to review?

Yes, there is a cost for going from page to virtual address. That is
why we only use the page when we finally get to freeing or resetting
the pagecnt_bias.

Also I have some concerns about going from page to folio as it seems
like the folio_alloc setups the transparent hugepage destructor instead
of using the compound page destructor. I would think that would slow
down most users as it looks like there is a spinlock that is taken in
the hugepage destructor that isn't there in the compound page
destructor.