On Fri, 7 Dec 2018, Vlastimil Babka wrote: > > This reverts commit 89c83fb539f95491be80cdd5158e6f0ce329e317. > > > > There are a couple of issues with 89c83fb539f9 independent of its partial > > revert in 2f0799a0ffc0 ("mm, thp: restore node-local hugepage > > allocations"): > > > > Firstly, the interaction between alloc_hugepage_direct_gfpmask() and > > alloc_pages_vma() is racy wrt __GFP_THISNODE and MPOL_BIND. > > alloc_hugepage_direct_gfpmask() makes sure not to set __GFP_THISNODE for > > an MPOL_BIND policy but the policy used in alloc_pages_vma() may not be > > the same for shared vma policies, triggering the WARN_ON_ONCE() in > > policy_node(). > > AFAICS 2f0799a0ffc0 removed the policy check in > alloc_hugepage_direct_gfpmask() comlpetely, so it's not racy and the > warning will always trigger for a MPOL_BIND policy right now? >
Yup, looks like you hit it on the head. This revert should have been done alongside 2f0799a0ffc0 ("mm, thp: restore node-local hugepage allocations"), I didn't appreciate how invasive the consolidation patch was. I noticed the race in 89c83fb539f9 ("mm, thp: consolidate THP gfp handling into alloc_hugepage_direct_gfpmask") that is fixed by the revert, but as you noted it didn't cleanup the second part which is the balancing act for gfp flags between alloc_hugepage_direct_gfpmask() and alloc_pages_vma(). Syzbot found this to trigger the WARN_ON_ONCE() you mention. So we certainly need this patch for 4.20 as a follow-up to 2f0799a0ffc0. It's likely better to regroup and discuss NUMA aspects of all thp allocations separately with a stable 4.20.