On Mon, 11 Nov 2019 18:06:42 -0800, Andrii Nakryiko wrote: > So let's say if sizeof(struct bpf_array) is 300, then I'd have to either: > > - somehow make sure that I allocate 4k (for data) + 300 (for struct > bpf_array) in such a way that those 4k of data are 4k-aligned. Is > there any way to do that? > - assuming there isn't, then another way would be to allocate entire > 4k page for struct bpf_array itself, but put it at the end of that > page, so that 4k of data is 4k-aligned. While wasteful, the bigger > problem is that pointer to bpf_array is not a pointer to allocated > memory anymore, so we'd need to remember that and adjust address > before calling vfree(). > > Were you suggesting #2 as a solution? Or am I missing some other way to do this? I am suggesting #2, that's the way to do it in the kernel. You could make the assumption that if you're allocating memory aligned to PAGE_SIZE, the address for vfree() is: addr = map; if (map->flags & MMAPABLE) addr = round_down(addr, PAGE_SIZE); vfree(addr); Just make a note of the fact that we depend on vmalloc()s alignment in bpf_map_area_alloc().
