From: Linus Torvalds torvalds@linux-foundation.org
[ Upstream commit 8e0d0ad206f08506c893326ca7c9c3d9cc042cef ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- arch/sparc/Kconfig | 1 - 1 file changed, 1 deletion(-)
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig index 7926a2e11bdc2..6a31f240840d4 100644 --- a/arch/sparc/Kconfig +++ b/arch/sparc/Kconfig @@ -28,7 +28,6 @@ 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