linux-kernel - Re: [PATCH 2/7] mmc: Don't use PF

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Date:	Wed, 18 Nov 2009 20:15:10 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-mmc@...r.kernel.org
Subject: Re: [PATCH 2/7] mmc: Don't use PF_MEMALLOC

On Wed, Nov 18, 2009 at 7:54 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2009-11-18 at 19:31 +0900, Minchan Kim wrote:
>> >
>> > Sure some generic blocklevel infrastructure might work, _but_ you cannot
>> > take away the responsibility of determining the amount of memory needed,
>> > nor does any of this have any merit if you do not limit yourself to that
>> > amount.
>>
>> Yes. Some one have to take a responsibility.
>>
>> The intention was we could take away the responsibility from block driver.
>> Instead of driver, VM would take the responsibility.
>>
>> You mean althgouth VM could take the responsiblity, it is hard to
>> expect amout of pages needed by block drivers?
>
> Correct, its near impossible for the VM to accurately guess the amount
> of memory needed for a driver, or limit the usage of the driver.
>
> The driver could be very simple in that it'll just start a DMA on the
> page and get an interrupt when done, not consuming much (if any) memory
> beyond the generic BIO structure, but it could also be some iSCSI
> monstrosity which involves the full network stack and userspace.

Wow, Thanks for good example.
Until now, I don't know iSCSI is such memory hog driver.

> That is why I generally prefer the user of PF_MEMALLOC to take
> responsibility, because it knows its own consumption and can limit its
> own consumption.

Okay. I understand your point by good explanation.

> Now, I don't think (but I could be wring here) that you need to bother
> with PF_MEMALLOC unless you're swapping. File based pages should always
> be able to free some memory due to the dirty-limit, whcih basically
> guarantees that there are some clean file pages for every dirty file
> page.
>
> My swap-over-nfs series used to have a block-layer hook to expose the
> swap-over-block behaviour:
>
> http://programming.kicks-ass.net/kernel-patches/vm_deadlock/v12.99/blk_queue_swapdev.patch
>
> That gives block devices the power to refuse being swapped over, which
> could be an alternative to using PF_MEMALLOC.
>

Thanks for noticing me.
I will look at your patches.
Thanks again, Peter.





-- 
Kind regards,
Minchan Kim
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