On 10/12/2018 03:37 AM, Joel Fernandes (Google) wrote: > Android needs to mremap large regions of memory during memory management > related operations. The mremap system call can be really slow if THP is > not enabled. The bottleneck is move_page_tables, which is copying each > pte at a time, and can be really slow across a large map. Turning on THP > may not be a viable option, and is not for us. This patch speeds up the > performance for non-THP system by copying at the PMD level when possible. > > The speed up is three orders of magnitude. On a 1GB mremap, the mremap > completion times drops from 160-250 millesconds to 380-400 microseconds. > > Before: > Total mremap time for 1GB data: 242321014 nanoseconds. > Total mremap time for 1GB data: 196842467 nanoseconds. > Total mremap time for 1GB data: 167051162 nanoseconds. > > After: > Total mremap time for 1GB data: 385781 nanoseconds. > Total mremap time for 1GB data: 388959 nanoseconds. > Total mremap time for 1GB data: 402813 nanoseconds. > > Incase THP is enabled, the optimization is skipped. I also flush the > tlb every time we do this optimization since I couldn't find a way to > determine if the low-level PTEs are dirty. It is seen that the cost of > doing so is not much compared the improvement, on both x86-64 and arm64. > > Cc: minchan@xxxxxxxxxx > Cc: pantin@xxxxxxxxxx > Cc: hughd@xxxxxxxxxx > Cc: lokeshgidra@xxxxxxxxxx > Cc: dancol@xxxxxxxxxx > Cc: mhocko@xxxxxxxxxx > Cc: kirill@xxxxxxxxxxxxx > Cc: akpm@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 9e68a02a52b1..d82c485822ef 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -191,6 +191,54 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd, > drop_rmap_locks(vma); > } > > +static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr, > + unsigned long new_addr, unsigned long old_end, > + pmd_t *old_pmd, pmd_t *new_pmd, bool *need_flush) > +{ > + spinlock_t *old_ptl, *new_ptl; > + struct mm_struct *mm = vma->vm_mm; > + > + if ((old_addr & ~PMD_MASK) || (new_addr & ~PMD_MASK) > + || old_end - old_addr < PMD_SIZE) > + return false; > + > + /* > + * The destination pmd shouldn't be established, free_pgtables() > + * should have release it. > + */ > + if (WARN_ON(!pmd_none(*new_pmd))) > + return false; > + > + /* > + * We don't have to worry about the ordering of src and dst > + * ptlocks because exclusive mmap_sem prevents deadlock. > + */ > + old_ptl = pmd_lock(vma->vm_mm, old_pmd); > + if (old_ptl) { > + pmd_t pmd; > + > + new_ptl = pmd_lockptr(mm, new_pmd); > + if (new_ptl != old_ptl) > + spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > + > + /* Clear the pmd */ > + pmd = *old_pmd; > + pmd_clear(old_pmd); Adding Martin Schwidefsky. Is this mapping maybe still in use on other CPUs? If yes, I think for s390 we need to flush here as well (in other word we might need to introduce pmd_clear_flush). On s390 you have to use instructions like CRDTE,IPTE or IDTE to modify page table entries that are still in use. Otherwise you can get a delayed access exception which is - in contrast to page faults - not recoverable. > + > + VM_BUG_ON(!pmd_none(*new_pmd)); > + > + /* Set the new pmd */ > + set_pmd_at(mm, new_addr, new_pmd, pmd); > + if (new_ptl != old_ptl) > + spin_unlock(new_ptl); > + spin_unlock(old_ptl); > + > + *need_flush = true; > + return true; > + } > + return false; > +} > + > 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, > @@ -239,7 +287,21 @@ 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) { > + bool moved; > + > + /* See comment in move_ptes() */ > + if (need_rmap_locks) > + take_rmap_locks(vma); > + moved = move_normal_pmd(vma, old_addr, new_addr, > + old_end, old_pmd, new_pmd, > + &need_flush); > + if (need_rmap_locks) > + drop_rmap_locks(vma); > + if (moved) > + continue; > } > + > if (pte_alloc(new_vma->vm_mm, new_pmd)) > break; > next = (new_addr + PMD_SIZE) & PMD_MASK; >