linux-kernel - Re: mmotm boot panic bootmem-avoid-dma32-zone-by-default.patch

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Date:	Sat, 6 Mar 2010 03:24:15 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Greg Thelen <gthelen@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, linux-mm@...ck.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: mmotm boot panic bootmem-avoid-dma32-zone-by-default.patch

On Fri, Mar 05, 2010 at 05:50:39PM -0800, Yinghai Lu wrote:
> On 03/05/2010 03:58 PM, Johannes Weiner wrote:
> > Hello Yinghai,
> > 
> > On Fri, Mar 05, 2010 at 10:41:56AM -0800, Yinghai Lu wrote:
> >> On 03/04/2010 09:17 PM, Greg Thelen wrote:
> >>> On Thu, Mar 4, 2010 at 7:21 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> >>>> On Thu, Mar 04, 2010 at 01:21:41PM -0800, Greg Thelen wrote:
> >>>>> On several systems I am seeing a boot panic if I use mmotm
> >>>>> (stamp-2010-03-02-18-38).  If I remove
> >>>>> bootmem-avoid-dma32-zone-by-default.patch then no panic is seen.  I
> >>>>> find that:
> >>>>> * 2.6.33 boots fine.
> >>>>> * 2.6.33 + mmotm w/o bootmem-avoid-dma32-zone-by-default.patch: boots fine.
> >>>>> * 2.6.33 + mmotm (including
> >>>>> bootmem-avoid-dma32-zone-by-default.patch): panics.
> >> ...
> >>>
> >>> Note: mmotm has been recently updated to stamp-2010-03-04-18-05.  I
> >>> re-tested with 'make defconfig' to confirm the panic with this later
> >>> mmotm.
> >>
> >> please check
> >>
> >> [PATCH] early_res: double check with updated goal in alloc_memory_core_early
> >>
> >> Johannes Weiner pointed out that new early_res replacement for alloc_bootmem_node
> >> change the behavoir about goal.
> >> original bootmem one will try go further regardless of goal.
> >>
> >> and it will break his patch about default goal from MAX_DMA to MAX_DMA32...
> >> also broke uncommon machines with <=16M of memory.
> >> (really? our x86 kernel still can run on 16M system?)
> >>
> >> so try again with update goal.
> > 
> > Thanks for the patch, it seems to be correct.
> > 
> > However, I have a more generic question about it, regarding the future of the
> > early_res allocator.
> > 
> > Did you plan on keeping the bootmem API for longer?  Because my impression was,
> > emulating it is a temporary measure until all users are gone and bootmem can
> > be finally dropped.
> 
> that depends on every arch maintainer.
> 
> user can compare them on x86 to check if...

Humm, now that is a bit disappointing.  Because it means we will never get rid
of bootmem as long as it works for the other architectures.  And your changeset
just added ~900 lines of code, some of it being a rather ugly compatibility
layer in bootmem that I hoped could go away again sooner than later.

I do not know what the upsides for x86 are from no longer using bootmem but it
would suck from a code maintainance point of view to get stuck half way through
this transition and have now TWO implementations of the bootmem interface we
would like to get rid of.

> next step will be make fw_mem_map to generiaized and combine them with lmb.
> 
> > 
> > But then this would require some sort of handling of 'user does not need DMA[32]
> > memory, so avoid it' and 'user can only use DMA[32] memory' in the early_res
> > allocator as well.
> > 
> > I ask this specifically because you move this fix into the bootmem compatibility
> > code while there is not yet a way to tell early_res the same thing, so switching
> > a user that _needs_ to specify this requirement from bootmem to early_res is not
> > yet possible, is it?
> 
> just let caller set the goal.

That means that every caller must be aware of where the DMA zone ends and if
it is non-empty and open-code the fallback to the DMA zone if the non-DMA zone
is exhausted?
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