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 7b9afb86b632 ("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: 7b9afb86b632 ("sparc64: use the generic get_user_pages_fast code")
Reported-by: Dmitry V Levin <ldv@altlinux.org>
Reported-by: Anatoly Pugachev <matorola@gmail.com>
Requested-by: Meelis Roos <mroos@linux.ee>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index fbc1aecf0f94ce085bc099c82c2b4a555e7a8b6c..eb24cb1afc11fbcc044583c3277b55aca9234f71 100644 (file)
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -29,7 +29,6 @@ config SPARC
        select RTC_DRV_M48T59
        select RTC_SYSTOHC
        select HAVE_ARCH_JUMP_LABEL if SPARC64
-       select HAVE_FAST_GUP if SPARC64
        select GENERIC_IRQ_SHOW
        select ARCH_WANT_IPC_PARSE_VERSION
        select GENERIC_PCI_IOMAP