On Tue, Aug 22, 2023 at 01:54:54AM +0000, Joel Fernandes (Google) wrote: > Recently, we see reports [1] of a warning that triggers due to > move_page_tables() doing a downward and overlapping move on a > mutually-aligned offset within a PMD. By mutual alignment, I > mean the source and destination addresses of the mremap are at > the same offset within a PMD. > > This mutual alignment along with the fact that the move is downward is > sufficient to cause a warning related to having an allocated PMD that > does not have PTEs in it. > > This warning will only trigger when there is mutual alignment in the > move operation. A solution, as suggested by Linus Torvalds [2], is to > initiate the copy process at the PMD level whenever such alignment is > present. Implementing this approach will not only prevent the warning > from being triggered, but it will also optimize the operation as this > method should enhance the speed of the copy process whenever there's a > possibility to start copying at the PMD level. > > Some more points: > a. The optimization can be done only when both the source and > destination of the mremap do not have anything mapped below it up to a > PMD boundary. I add support to detect that. > > b. #1 is not a problem for the call to move_page_tables() from exec.c as > nothing is expected to be mapped below the source. However, for > non-overlapping mutually aligned moves as triggered by mremap(2), I > added support for checking such cases. > > c. I currently only optimize for PMD moves, in the future I/we can build > on this work and do PUD moves as well if there is a need for this. But I > want to take it one step at a time. > > d. We need to be careful about mremap of ranges within the VMA itself. > For this purpose, I added checks to determine if the address after > alignment falls within its VMA itself. > > [1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@xxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/all/CAHk-=whd7msp8reJPfeGNyt0LiySMT0egExx3TVZSX3Ok6X=9g@xxxxxxxxxxxxxx/ > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> > --- > mm/mremap.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 62 insertions(+) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 11e06e4ab33b..035fbf542a8f 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -489,6 +489,53 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma, > return moved; > } > > +/* > + * A helper to check if a previous mapping exists. Required for > + * move_page_tables() and realign_addr() to determine if a previous mapping > + * exists before we can do realignment optimizations. > + */ > +static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_align, > + unsigned long mask) > +{ > + unsigned long addr_masked = addr_to_align & mask; > + > + /* > + * If @addr_to_align of either source or destination is not the beginning > + * of the corresponding VMA, we can't align down or we will destroy part > + * of the current mapping. > + */ > + if (vma->vm_start != addr_to_align) > + return false; > + > + /* > + * Make sure the realignment doesn't cause the address to fall on an > + * existing mapping. > + */ > + return find_vma_intersection(vma->vm_mm, addr_masked, addr_to_align) == NULL; > +} > + > +/* Opportunistically realign to specified boundary for faster copy. */ > +static void try_realign_addr(unsigned long *old_addr, struct vm_area_struct *old_vma, > + unsigned long *new_addr, struct vm_area_struct *new_vma, > + unsigned long mask) > +{ > + /* Skip if the addresses are already aligned. */ > + if ((*old_addr & ~mask) == 0) > + return; > + > + /* Only realign if the new and old addresses are mutually aligned. */ > + if ((*old_addr & ~mask) != (*new_addr & ~mask)) > + return; > + > + /* Ensure realignment doesn't cause overlap with existing mappings. */ > + if (!can_align_down(old_vma, *old_addr, mask) || > + !can_align_down(new_vma, *new_addr, mask)) > + return; > + > + *old_addr = *old_addr & mask; > + *new_addr = *new_addr & mask; > +} > + > unsigned long move_page_tables(struct vm_area_struct *vma, > unsigned long old_addr, struct vm_area_struct *new_vma, > unsigned long new_addr, unsigned long len, > @@ -508,6 +555,14 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > return move_hugetlb_page_tables(vma, new_vma, old_addr, > new_addr, len); > > + /* > + * If possible, realign addresses to PMD boundary for faster copy. > + * Only realign if the mremap copying hits a PMD boundary. > + */ > + if ((vma != new_vma) > + && (len >= PMD_SIZE - (old_addr & ~PMD_MASK))) > + try_realign_addr(&old_addr, vma, &new_addr, new_vma, PMD_MASK); > + > flush_cache_range(vma, old_addr, old_end); > mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma->vm_mm, > old_addr, old_end); > @@ -577,6 +632,13 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > > mmu_notifier_invalidate_range_end(&range); > > + /* > + * Prevent negative return values when {old,new}_addr was realigned > + * but we broke out of the above loop for the first PMD itself. > + */ > + if (len + old_addr < old_end) > + return 0; > + > return len + old_addr - old_end; /* how much done */ > } > > -- > 2.42.0.rc1.204.g551eb34607-goog > Looks good to me! Thanks for the changes :) Reviewed-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
