On Sat, Jan 20, 2024 at 12:04:27PM +0000, Ryan Roberts wrote: > However, after this patch, each allocation is in its own VMA, and there is a 2M > gap between each VMA. This causes 2 problems: 1) mmap becomes MUCH slower > because there are so many VMAs to check to find a new 1G gap. 2) It fails once > it hits the VMA limit (/proc/sys/vm/max_map_count). Hitting this limit then > causes a subsequent calloc() to fail, which causes the test to fail. > > Looking at the code, I think the problem is that arm64 selects > ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT. But __thp_get_unmapped_area() allocates > len+2M then always aligns to the bottom of the discovered gap. That causes the > 2M hole. As far as I can see, x86 allocates bottom up, so you don't get a hole. As a quick hack, perhaps #ifdef ARCH_WANT_DEFAULT_TOPDOWN_MMAP_LAYOUT take-the-top-half #else current-take-bottom-half-code #endif ?
