linux-kernel - Re: [PATCH] dma-debug: dynamic allocation of hash table

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Message-ID: <a305bd1f-8142-6557-4228-aae10c5114e1@arm.com>
Date:   Fri, 31 Jan 2020 11:33:48 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, Christoph Hellwig <hch@....de>,
        Joerg Roedel <jroedel@...e.de>,
        Linux IOMMU <iommu@...ts.linux-foundation.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] dma-debug: dynamic allocation of hash table

On 2020-01-31 9:06 am, Geert Uytterhoeven wrote:
> Hi Robin,
> 
> On Fri, Jan 31, 2020 at 12:46 AM Robin Murphy <robin.murphy@....com> wrote:
>> On 2020-01-30 7:10 pm, Eric Dumazet via iommu wrote:
>>> Increasing the size of dma_entry_hash size by 327680 bytes
>>> has reached some bootloaders limitations.
>>
>> [ That might warrant some further explanation - I don't quite follow how
>> this would relate to a bootloader specifically :/ ]
> 
> Increasing the size of a static array increases kernel size.
> Some (all? ;-) bootloaders have limitations on the maximum size of a
> kernel image they can boot (usually something critical gets overwritten
> when handling a too large image).  While boot loaders can be fixed and
> upgraded, this is usually much more cumbersome than updating the
> kernel.

Ah, OK - I'm all too familiar with bootloaders having image size limits, 
but I'm also used to implicitly-initialised statics being collected into 
a runtime-initialised .bss section, so I hadn't realised that there 
might still be platforms where that space is actually allocated in the 
image at link-time.

> Besides, a static array always consumes valuable unswapable memory,
> even when the feature would not be used (e.g. disabled by a command
> line option).

Indeed, and that alone might have been a reasonable rationale for the 
patch - I was merely querying the wording of the commit message, not its 
intent :)

Thanks,
Robin.