sparc64: disable fast-GUP due to unexplained oopses
HAVE_FAST_GUP enables the lockless quick page table walker for simple cases, and is a nice optimization for some random loads that can then use get_user_pages_fast() rather than the more careful page walker. However, for some unexplained reason, it seems to be subtly broken on sparc64. The breakage is only with some compiler versions and some hardware, and nobody seems to have figured out what triggers it, although there's a simple reprodicer for the problem when it does trigger. The problem was introduced with the conversion to the generic GUP code in commit 7b9afb86 ("sparc64: use the generic get_user_pages_fast code"), but nothing looks obviously wrong in that conversion. It may be a compiler bug that just hits us with the code reorganization. Or it may be something very specific to sparc64. This disables HAVE_FAST_GUP entirely. That makes things like futexes a bit slower, but at least they work. If we can figure out the trigger, that would be lovely, but it's been three months already.. Link: https://lore.kernel.org/lkml/20190717215956.GA30369@altlinux.org/ Fixes: 7b9afb86 ("sparc64: use the generic get_user_pages_fast code") Reported-by: NDmitry V Levin <ldv@altlinux.org> Reported-by: NAnatoly Pugachev <matorola@gmail.com> Requested-by: NMeelis Roos <mroos@linux.ee> Suggested-by: NChristoph Hellwig <hch@infradead.org> Cc: David Miller <davem@davemloft.net> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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