Re: Top kernel oopses/warnings for the week of May 16th 2008

From: Robert Hancock
Date: Sun May 18 2008 - 22:25:28 EST

Andrea Arcangeli wrote:
On Sat, May 17, 2008 at 07:57:26AM -0700, Arjan van de Ven wrote:
Andrea Arcangeli wrote:
The reason I touched that code, is that a change introduced during
2.6.25-rc initialized the isa dma pool even if not necessary and that
broke the reserved-ram patch that requires no __GFP_DMA
allocations. There was no crash in 2.6.24 based kernels, the
regression started in 2.6.25-rc.
I'd not really call "breaks external patch" a regression ;)

The external patch only allowed me to notice the regression when
nobody else noticed it. For mainline the regression was to put ram
into the bounce buffer pool even if no dma could ever require the
bounce buffering. There's no point to initialize the pool when total
ram < highest dma address. That is the regression. My patch turned the
regression from a waste of ram, to a kernel crash at boot. That's the
only relation between the reserved-ram patch this bug.

I assume Robert has a similar issue with some debugging code checking
for GFP_KERNEL allocations inside atomic context or similar, I assume
his driver has a bug and calls that function in the wrong context. But
if this didn't happen in 2.4.24, it means such bug has nothing to do
with the bug in blk-settings.c. It's just that such a bug or the
reserved-ram patches are required to notice the regression in

Well, it's not really documented what the locking semantics are supposed to be for blk_queue_bounce_limit. Based on the implementation, though, it's OK to call it under spin_lock_irqsave (it only sets some variables) unless you hit the case where dma is set to 1 and we do init_emergency_isa_pool. That's the problem, that case should not be hit with a DMA mask of 32-bit, but with Yang Shi's change to blk-settings.c, now we are.

The code in that function seems rather hackish, actually. It seems like a lot of those assumptions it's making should be in architecture-specific code..

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