linux-kernel - Re: [RFC v2 1/2] dmapool: Move pool metadata into non-DMA memory

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Message-ID: <Zz6_CFj-MiMk73Tm@infradead.org>
Date: Wed, 20 Nov 2024 21:03:04 -0800
From: Christoph Hellwig <hch@...radead.org>
To: Brian Johannesmeyer <bjohannesmeyer@...il.com>
Cc: Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
	Raphael Isemann <teemperor@...il.com>,
	Cristiano Giuffrida <giuffrida@...vu.nl>,
	Herbert Bos <h.j.bos@...nl>, Greg KH <gregkh@...uxfoundation.org>,
	Keith Busch <kbusch@...nel.org>
Subject: Re: [RFC v2 1/2] dmapool: Move pool metadata into non-DMA memory

On Wed, Nov 20, 2024 at 04:46:40PM -0700, Brian Johannesmeyer wrote:
> Thank you for the suggestion. I hacked together a bitmap-based
> approach as you proposed, and while it does improve memory efficiency
> by reducing the per-block metadata overhead, it unfortunately appears
> to significantly impact the runtime performance.
>
> My guess as to why: The current linked list implementation allows us
> to find the next free block in constant time (`O(1)`) by directly
> dereferencing `pool->next_block`, and then following the `next_block`
> pointers for subsequent free blocks. In contrast, the bitmap approach
> requires iterating over all pages in `page->page_list` and, for each
> page, iterating through its bitmap to find the first zero bit. This
> results in a worst-case complexity of `O(n * b)`, where `n` is the
> number of pages and `b` is the number of bits in each page's bitmap.

Indeed.  You'd probably need to split the linkage of the pages into
a list of those that have free blocks and those that don't as a minimum.

Can you share your current version?