Subject: Re: [PATCH] powerpc/iommu: use GFP_KERNEL instead of GFP_ATOMIC in iommu_init_table()

From: Thadeu Lima de Souza Cascardo <[email-masked]>
Date: Wed, 2 Oct 2013 09:32:37 -0300

On Tue, Oct 01, 2013 at 02:04:53PM -0700, Nishanth Aravamudan wrote:
> 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.
> 
> With this change under the same conditions, we have not been able to
> reproduce the panic.
>     
> Signed-off-by: Nishanth Aravamudan <[email-masked]>
> 
> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
> index 0adab06..572bb5b 100644
> --- a/arch/powerpc/kernel/iommu.c
> +++ b/arch/powerpc/kernel/iommu.c
> @@ -661,7 +661,7 @@ struct iommu_table *iommu_init_table(struct iommu_table *tbl, int nid)
>  	/* number of bytes needed for the bitmap */
>  	sz = BITS_TO_LONGS(tbl->it_size) * sizeof(unsigned long);
> 
> -	page = alloc_pages_node(nid, GFP_ATOMIC, get_order(sz));
> +	page = alloc_pages_node(nid, GFP_KERNEL, get_order(sz));
>  	if (!page)
>  		panic("iommu_init_table: Can't allocate %ld bytes\n", sz);
>  	tbl->it_map = page_address(page);

I didn't respond to the previous message, but also checked if there were
any history on the logs, and found this was as it is from the start. I
also found no other reasons why it needs to be atomic. Therefore,

Acked-by: Thadeu Lima de Souza Cascardo <[email-masked]>