linux-kernel - Re: [RFC v2 0/2] dmapool: Mitigate device-controllable mem. corruption

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Message-ID: <CAOZ5it0LKViPsOJrfLXmB-zk1OGWhKT6XMrj03etV-MA8-HOkQ@mail.gmail.com>
Date: Thu, 21 Nov 2024 12:07:36 -0700
From: Brian Johannesmeyer <bjohannesmeyer@...il.com>
To: Keith Busch <kbusch@...nel.org>
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>
Subject: Re: [RFC v2 0/2] dmapool: Mitigate device-controllable mem. corruption

On Thu, Nov 21, 2024 at 11:06 AM Keith Busch <kbusch@...nel.org> wrote:
> If you have the time, could you compare with using xarray instead?

Sure. Good idea.

**With the submitted patches applied AND using an xarray for
vaddr-to-block translations:**
```
dmapool test: size:16   align:16   blocks:8192 time:37954
dmapool test: size:64   align:64   blocks:8192 time:40036
dmapool test: size:256  align:256  blocks:8192 time:41942
dmapool test: size:1024 align:1024 blocks:2048 time:10964
dmapool test: size:4096 align:4096 blocks:1024 time:6101
dmapool test: size:68   align:32   blocks:8192 time:41307
```

The xarray approach shows a slight improvement in performance compared
to the maple tree approach.

FWIW, I implemented the two with slightly different semantics:
- In the maple tree implementation, I saved the `block`'s entire
`vaddr` range, allowing any `vaddr` within the `block` to be passed to
`dma_pool_free()`.
- In the xarray implementation, I saved only the `block's` base
`vaddr`, requiring `dma_pool_free()` to be called with the exact
`vaddr` returned by `dma_pool_alloc()`. This aligns with the DMA pool
API documentation, which specifies that the `vaddr` returned by
`dma_pool_alloc()` should be passed to `dma_pool_free()`.

Let me know if you'd like further adjustments.

Thanks,

Brian Johannesmeyer