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Date: Thu, 19 Sep 2013 09:50:35 -0700
From: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Does iommu_init_table need to use GFP_ATOMIC allocations?
Message-ID: <20130919165035.GA20144@linux.vnet.ibm.com>
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Cc: Paul Mackerras <paulus@samba.org>, linuxppc-dev@lists.ozlabs.org,
 Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>,
 Anton Blanchard <anton@samba.org>
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Under heavy (DLPAR?) stress, we tripped this panic() in
arch/powerpc/kernel/iommu.c::iommu_init_table():

        page = alloc_pages_node(nid, GFP_ATOMIC, get_order(sz));
        if (!page)
                panic("iommu_init_table: Can't allocate %ld bytes\n",
sz);

Before the panic() we got a page allocation failure for an order-2
allocation. There appears to be memory free, but perhaps not in the
ATOMIC context. I looked through all the call-sites of
iommu_init_table() and didn't see any obvious reason to need an ATOMIC
allocation. Most call-sites in fact have an explicit GFP_KERNEL
allocation shortly before the call to iommu_init_table(), indicating we
are not in an atomic context. There is some indirection for some paths,
but I didn't see any locks indicating that GFP_KERNEL is inappropriate.
Does anyone know if/why ATOMIC allocations are necessary here?

Thanks,
Nish