On Wed, Jul 08, 2020 at 10:45:16AM -0700, Mike Kravetz wrote: > On 7/7/20 12:56 PM, Andrew Morton wrote: > > On Tue, 7 Jul 2020 16:02:04 +1200 Barry Song <song.bao.hua@xxxxxxxxxxxxx> wrote: > > > >> hugetlb_cma[0] can be NULL due to various reasons, for example, node0 has > >> no memory. so NULL hugetlb_cma[0] doesn't necessarily mean cma is not > >> enabled. gigantic pages might have been reserved on other nodes. > > > > I'm trying to figure out whether this should be backported into 5.7.1, > > but the changelog doesn't describe any known user-visible effects of > > the bug. Are there any? > > Barry must have missed this email. He reported the issue so I was hoping > he would reply. > > Based on the code changes, I believe the following could happen: > - Someone uses 'hugetlb_cma=' kernel command line parameter to reserve > CMA for gigantic pages. > - The system topology is such that no memory is on node 0. Therefore, > no CMA can be reserved for gigantic pages on node 0. CMA is reserved > on other nodes. > - The user also specifies a number of gigantic pages to pre-allocate on > the command line with hugepagesz=<gigantic_page_size> hugepages=<N> > - The routine which allocates gigantic pages from the bootmem allocator > will not detect CMA has been reserved as there is no memory on node 0. > Therefore, pages will be pre-allocated from bootmem allocator as well > as reserved in CMA. > > This double allocation (bootmem and CMA) is the worst case scenario. Not > sure if this is what Barry saw, and I suspect this would rarely happen. > > After writing this, I started to think that perhaps command line parsing > should be changed. If hugetlb_cma= is specified, it makes no sense to > pre-allocate gigantic pages. Therefore, the hugepages=<N> paramemter > should be ignored and flagged with a warning if hugetlb_cma= is specified. > This could be checked at parsing time and there would be no need for such > a check in the allocation code (except for sanity cheching). > > Thoughts? I just cleaned up the parsing code and could make such a change > quite easily. I agree. Basically, if hugetlb_cma_size > 0, we should not pre-allocate gigantic pages. It would be much simpler and more reliable than the existing code. Thank you!
