Subject: Re: [PATCH 2/7] mmc: Don't use PF_MEMALLOC
From: Peter Zijlstra <peterz@infradead.org>
To: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
       Andrew Morton <akpm@linux-foundation.org>, linux-mmc@vger.kernel.org
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Date: Wed, 18 Nov 2009 11:54:56 +0100
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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.

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.

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.

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