On Wed, Jul 15, 2020 at 09:14:51AM -0400, Joel Fernandes wrote: > 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> Thanks. > 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). Right, I'll update. But we should warn for old_addr == new_addr too. Nobody should do this. > > 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 > > > > > -- Kirill A. Shutemov
