[RFC 1/1] bootmem: move big allocations behing 4G

[Jiri Slaby <jslaby@suse.cz>]

Hi, I'm fighting a bug where Grub loads the kernel just fine, whereas
isolinux doesn't. I found out, it's due to different addresses of
loaded initrd. On a machine with 128G of memory, grub loads the
initrd at 895M in our case and flat mem_map (2G long) is allocated
above 4G due to 2-4G BIOS reservation.

On the other hand, with isolinux, the 0-2G is free and mem_map is
placed there leaving no space for others, hence kernel panics for
swiotlb which needs to be below 4G.

I use the patch below, but it seems, from the code, like it won't
work out for section allocations.

Any ideas?

--

If there is a big amount of memory (128G) in a machine and 2G of
low 4 gigs are reserved by BIOS, the rest of the "low" memory is
consumed by mem_map with flat mapping enabled.

Consequent allocations with limit being 4G (e.g. swiotlb) fails to
allocate and kernel panics.

Try to avoid that situation on 64-bit by allocating space bigger
than 128M above 4G if possible. With that, mem_map is allocated above
4G and there is enough space for others (swiotlb) in low 4G.
---
 mm/bootmem.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/mm/bootmem.c b/mm/bootmem.c
index 7d14868..365a0d1 100644
--- a/mm/bootmem.c
+++ b/mm/bootmem.c
@@ -486,6 +486,11 @@ static void * __init alloc_bootmem_core(struct bootmem_data *bdata,
 
 	step = max(align >> PAGE_SHIFT, 1UL);
 
+	/* on 64-bit: allocate 128M+ at 4G if satisfies limit */
+	if (BITS_PER_LONG == 64 && size >= (128UL << 20) &&
+			(4UL << 30) + size < (max << PAGE_SHIFT))
+		goal = 4UL << (30 - PAGE_SHIFT);
+
 	if (goal && min < goal && goal < max)
 		start = ALIGN(goal, step);
 	else
-- 
1.6.5.7

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