Re: [PATCH] mm: Speed up mremap on large regions

[Jann Horn]

On Fri, Oct 12, 2018 at 7:29 AM Juergen Gross <jgross@xxxxxxxx> wrote:
> On 12/10/2018 05:21, Jann Horn wrote:
> > +cc xen maintainers and kvm folks
> >
> > On Fri, Oct 12, 2018 at 4:40 AM Joel Fernandes (Google)
> > <joel@xxxxxxxxxxxxxxxxx> 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.
> > [...]
> >> +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)
> >> +{
> > [...]
> >> +       /*
> >> +        * 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);
> >> +
> >> +               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);
> >
> > How does this interact with Xen PV? From a quick look at the Xen PV
> > integration code in xen_alloc_ptpage(), it looks to me as if, in a
> > config that doesn't use split ptlocks, this is going to temporarily
> > drop Xen's type count for the page to zero, causing Xen to de-validate
> > and then re-validate the L1 pagetable; if you first set the new pmd
> > before clearing the old one, that wouldn't happen. I don't know how
> > this interacts with shadow paging implementations.
>
> No, this isn't an issue. As the L1 pagetable isn't being released it
> will stay pinned, so there will be no need to revalidate it.

Where exactly is the L1 pagetable pinned? xen_alloc_ptpage() does:

        if (static_branch_likely(&xen_struct_pages_ready))
            SetPagePinned(page);

        if (!PageHighMem(page)) {
            xen_mc_batch();

            __set_pfn_prot(pfn, PAGE_KERNEL_RO);

            if (level == PT_PTE && USE_SPLIT_PTE_PTLOCKS)
                __pin_pagetable_pfn(MMUEXT_PIN_L1_TABLE, pfn);

            xen_mc_issue(PARAVIRT_LAZY_MMU);
        } else {
            /* make sure there are no stray mappings of
               this page */
            kmap_flush_unused();
        }

which means that if USE_SPLIT_PTE_PTLOCKS is false, the table doesn't
get pinned and only stays typed as long as it is referenced by an L2
table, right?