Re: [PATCH net-next 03/12] mm: Make the page_frag_cache allocator alignment param a pow-of-2

From: Yunsheng Lin <linyunsheng@huawei.com>
To: Alexander H Duyck <alexander.duyck@gmail.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 03/12] mm: Make the page_frag_cache allocator alignment param a pow-of-2
Date: Thu, 30 Nov 2023 17:00:52 +0800	[thread overview]
Message-ID: <e841d2f4-b221-9099-8312-6b859ccb98de@huawei.com> (raw)
In-Reply-To: <51161740e832334594960ed43430b868a6f892c3.camel@gmail.com>

On 2023/5/27 23:54, Alexander H Duyck wrote:
> On Wed, 2023-05-24 at 16:33 +0100, David Howells wrote:
>> Make the page_frag_cache allocator's alignment parameter a power of 2
>> rather than a mask and give a warning if it isn't.
>>
>> This means that it's consistent with {napi,netdec}_alloc_frag_align() and
>> allows __{napi,netdev}_alloc_frag_align() to be removed.

I am trying to rmove the page frag implemetation in
vhost_net_page_frag_refill() by using page_frag_alloc_align(), and
I ended up having a simiar patch as this one.

>>
> 
> This goes against the original intention of these functions. One of the
> reasons why this is being used is because when somebody enables
> something like 2K jumbo frames they don't necessarily want to have to
> allocate 4K SLABs. Instead they can just add a bit of overhead and get
> almost twice the utilization out of an order 3 page.
> 
> The requirement should only be cache alignment, not power of 2
> alignment. This isn't meant to be a slab allocator. We are just
> sectioning up pages to handle mixed workloads. In the case of
> networking we can end up getting everything from 60B packets, to 1514B
> in the standard cases. That was why we started sectioning up pages in
> the first place so putting a power of 2 requirement on it doens't fit
> our use case at all and is what we were trying to get away from with
> the SLAB allocators.

It seems that is_power_of_2() checking in this patch does not excluding
the non-align case if we are passing 'align' being 1, which means we still
can support the 'everything from 60B packets, to 1514B' case.

> 
> .
> 