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

From: David Howells <dhowells@redhat.com>
To: Alexander H Duyck <alexander.duyck@gmail.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>,
	David Howells <dhowells@redhat.com>,
	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>,
	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: Tue, 06 Jun 2023 09:25:13 +0100	[thread overview]
Message-ID: <1841913.1686039913@warthog.procyon.org.uk> (raw)
In-Reply-To: <f7919c2c9e1cb6218a0b0f55ddaa9a34f7d2b9a7.camel@gmail.com>

Alexander H Duyck <alexander.duyck@gmail.com> wrote:

> 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.

Note that this code is going to have to move to folios[*] at some point.
"Old-style" compound pages are going to go away, I believe.  Matthew Wilcox
and the mm folks are on a drive towards simplifying memory management,
formalising chunks larger than a single page - with the ultimate aim of
reducing the page struct to a single, typed pointer.

So, take, for example, a folio: As I understand it, this will no longer
overlay struct page, but rather will become a single, dynamically-allocated
struct that covers a pow-of-2 number of pages.  A contiguous subset of page
structs will point at it.

However, rather than using a folio, we could define a "page fragment" memory
type.  Rather than having all the flags and fields to be found in struct
folio, it could have just the set to be found in page_frag_cache.

David

[*] It will be possible to have some other type than "folio".  See "struct
slab" in mm/slab.h for example.  struct slab corresponds to a set of pages
and, in the future, a number of struct pages will point at it.