On Wed, Jul 15, 2020 at 03:35:13PM +0300, Kirill A. Shutemov wrote: > mremap(2) does not allow source and destination regions to overlap, but > shift_arg_pages() calls move_page_tables() directly and in this case the > source and destination overlap often. It confuses move_normal_pmd(): > > WARNING: CPU: 3 PID: 27091 at mm/mremap.c:211 move_page_tables+0x6ef/0x720 > > move_normal_pmd() expects the destination PMD to be empty, but when > ranges overlap nobody removes PTE page tables on source side. > move_ptes() only removes PTE entries, leaving tables behind. > When the source PMD becomes destination and alignment/size is right we > step onto the warning. > > The warning is harmless: kernel correctly fallbacks to handle entries on > per-entry basis. A link to the debugging effort could be added to the change log: https://lore.kernel.org/lkml/20200713025354.GB3644504@xxxxxxxxxx/ > The fix is to avoid move_normal_pmd() if we see that source and > destination ranges overlap. Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> And one thing that bothers me: > mm/mremap.c | 14 +++++++++++++- > 1 file changed, 13 insertions(+), 1 deletion(-) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 5dd572d57ca9..e33fcee541fe 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -245,6 +245,18 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > unsigned long extent, next, old_end; > struct mmu_notifier_range range; > pmd_t *old_pmd, *new_pmd; > + bool overlaps; > + > + /* > + * shift_arg_pages() can call move_page_tables() on overlapping ranges. > + * In this case we cannot use move_normal_pmd() because destination pmd > + * might be established page table: move_ptes() doesn't free page > + * table. > + */ > + if (old_addr > new_addr) > + overlaps = old_addr - new_addr < len; > + else > + overlaps = new_addr - old_addr < len; Does the code really work properly if old_addr < new_addr and overlaps == true ? If not, then we should add a warning here in the else IMHO: if (old_addr >= new_addr) { overlaps = old_addr - new_addr < len; } else { overlaps = new_addr - old_addr < len; WARN_ON(overlaps); } (More so, since you have added code that detects overlaps for such a case). thanks, - Joel > > old_end = old_addr + len; > flush_cache_range(vma, old_addr, old_end); > @@ -282,7 +294,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > split_huge_pmd(vma, old_pmd, old_addr); > if (pmd_trans_unstable(old_pmd)) > continue; > - } else if (extent == PMD_SIZE) { > + } else if (!overlaps && extent == PMD_SIZE) { > #ifdef CONFIG_HAVE_MOVE_PMD > /* > * If the extent is PMD-sized, try to speed the move by > -- > 2.26.2 > >
