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

From: "Mika Penttilä" <mpenttil@redhat.com>
To: David Howells <dhowells@redhat.com>,
	Yunsheng Lin <linyunsheng@huawei.com>,
	Matthew Wilcox <willy@infradead.org>
Cc: netdev@vger.kernel.org, "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>, 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: Fri, 26 May 2023 17:06:55 +0300	[thread overview]
Message-ID: <5dd62fee-56bf-0b54-2e91-c31068a2b040@redhat.com> (raw)
In-Reply-To: <739166.1685105220@warthog.procyon.org.uk>

Hi,

On 26.5.2023 15.47, David Howells wrote:
> Yunsheng Lin <linyunsheng@huawei.com> 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().
>>
>> put_page() is not used in this patch, perhaps remove it to avoid
>> the confusion?
> 
> Will do if I need to respin the patches.
> 
>> Also, Is there any significant difference between __free_pages()
>> and folio_put()? IOW, what does the 'reduces' part means here?
> 
> I meant that the folio code handles page compounding for us and we don't need
> to work out how big the page is for ourselves.
> 
> If you look at __free_pages(), you can see a PageHead() call.  folio_put()
> doesn't need that.
> 
>> 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?
> 
> There is a benefit: a folio pointer always points to the head page and so we
> never need to do "is this compound? where's the head?" logic to find it.  When
> going from a page pointer, we still have to find the head.
> 

But page_frag_free() uses folio_put(virt_to_folio(addr)) and 
virt_to_folio() depends on the compound infrastructure to get the head 
page and folio.

> Ultimately, the aim is to reduce struct page to a typed pointer to massively
> reduce the amount of space consumed by mem_map[].  A page struct will then
> point at a folio or a slab struct or one of a number of different types.  But
> to get to that point, we have to stop a whole lot of things from using page
> structs, but rather use some other type, such as folio.
> 
> Eventually, there won't be a need for head pages and tail pages per se - just
> memory objects of different sizes.
> 
>>> 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?
> 
> Hmmm... fair point, though va is calculated from the page pointer on most
> arches without the need to dereference struct page (only arc, m68k and sparc
> define WANT_PAGE_VIRTUAL).
> 
> David
> 

--Mika