On Mon, Oct 23, 2017 at 06:07:51PM +0000, McFarland, Brian wrote: > Using the TI's 6AM.1.3 release for DRA7x I have an oom-killer problem > that only occurs with LPAE enabled. > > That is to say - I have a baseline system runs without oom-killer on a > non-LPAE kernel, but recently enabled LPAE to allow access to 4GB of > RAM (my baseline HW 2G). When doing so, I see oom-killer issues early > and often. > > I believe I've narrowed down the problem to an issue with the > SGX/PowerVR driver, pvrsrvkm (open source-ish but built out-of-tree). > When LPAE is disabled, the driver allocates using GFP_HIGHMEM because > all memory is "32-bit safe", making it accessible to the SGX core. > > When LPAE is enabled, instead the driver uses GFP_DMA32, which on > arch's where the zone does not exists - falls back to ZONE_NORMAL or > ZONE_DMA, which are constricted to the first approx 768MB. [...] > This all leads me to believe I need another zone. A zone that spans > from arm_lowmem_limit to the end of 32-bit space, which in the case of > dra7x is nominally about 1.3GB. > > This appears to be the general idea behind ZONE_DMA32 but the zone has > comes in & out of multiple arch's and I cannot find any clear > information on the exact intention of said zone. It used to be on > arm64 but was since removed. As far as I can tell, no one ever used it > on arm32+LPAE. The way the Linux page allocator behaves won't help you even with a ZONE_DMA32. Basically it expects that zones do not overlap and also expects a fallback allocation: e.g. when ZONE_NORMAL allocation fails, it can try the lower index zone which is currently ZONE_DMA for arch/arm. Since ZONE_NORMAL must end at 32-bit virtual address range (where ZONE_HIGHMEM begins), you can't have it fall back to a lower index ZONE_DMA32 which would go beyond the 4G virtual address space (see include/linux/gfp.h for the fallback order). The quickest solution for you is to rebuild the kernel with a 2:2 memory split so that ZONE_DMA is bigger (around 1.7GB). An alternative might be to invent a new zone, something like ZONE_HIGHMEM32, which is highmem but still within 32-bit physical address space. However, it will take some effort to justify and upstream it. -- Catalin -- To unsubscribe from this list: send the line "unsubscribe linux-omap" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
