Re: [PATCH net-next 04/12] mm: Make the page_frag_cache allocator use multipage folios

From: Alexander H Duyck <alexander.duyck@gmail.com>
To: Yunsheng Lin <linyunsheng@huawei.com>,
	David Howells <dhowells@redhat.com>,
	netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
	David Ahern <dsahern@kernel.org>,
	Matthew Wilcox <willy@infradead.org>,
	Jens Axboe <axboe@kernel.dk>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Jeroen de Borst <jeroendb@google.com>,
	Catherine Sullivan <csully@google.com>,
	Shailend Chand <shailend@google.com>,
	Felix Fietkau <nbd@nbd.name>, John Crispin <john@phrozen.org>,
	Sean Wang <sean.wang@mediatek.com>,
	Mark Lee <Mark-MC.Lee@mediatek.com>,
	Lorenzo Bianconi <lorenzo@kernel.org>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	AngeloGioacchino Del Regno 
	<angelogioacchino.delregno@collabora.com>,
	Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-mediatek@lists.infradead.org,
	linux-nvme@lists.infradead.org
Subject: Re: [PATCH net-next 04/12] mm: Make the page_frag_cache allocator use multipage folios
Date: Sat, 27 May 2023 08:47:58 -0700	[thread overview]
Message-ID: <f7919c2c9e1cb6218a0b0f55ddaa9a34f7d2b9a7.camel@gmail.com> (raw)
In-Reply-To: <a819dd80-54cc-695f-f142-e3d42ce815a7@huawei.com>

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.