Re: [PATCH v2 3/6] mm: Speedup mremap on 1GB or larger regions

[Kalesh Singh <kaleshsingh@google.com>]

Hi Kirill, thank you for the feedback.

On Fri, Oct 2, 2020 at 12:51 PM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> On Fri, Oct 02, 2020 at 04:20:48PM +0000, Kalesh Singh wrote:
> > Android needs to move large memory regions for garbage collection.
> > The GC requires moving physical pages of multi-gigabyte heap
> > using mremap. During this move, the application threads have to
> > be paused for correctness. It is critical to keep this pause as
> > short as possible to avoid jitters during user interaction.
> >
> > Optimize mremap for >= 1GB-sized regions by moving at the PUD/PGD
> > level if the source and destination addresses are PUD-aligned.
> > For CONFIG_PGTABLE_LEVELS == 3, moving at the PUD level in effect moves
> > PGD entries, since the PUD entry is “folded back” onto the PGD entry.
> > Add HAVE_MOVE_PUD so that architectures where moving at the PUD level
> > isn't supported/tested can turn this off by not selecting the config.
> >
> > Fix build test error from v1 of this series reported by
> > kernel test robot in [1].
> >
> > [1] https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org/thread/CKPGL4FH4NG7TGH2CVYX2UX76L25BTA3/
> >
> > Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
> > Reported-by: kernel test robot <lkp@intel.com>
> > ---
> > Changes in v2:
> >   - Update commit message with description of Android GC's use case.
> >   - Move set_pud_at() to a separate patch.
> >   - Use switch() instead of ifs in move_pgt_entry()
> >   - Fix build test error reported by kernel test robot on x86_64 in [1].
> >     Guard move_huge_pmd() with IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE),
> >     since this section doesn't get optimized out in the kernel test
> >     robot's build test when HAVE_MOVE_PUD is enabled.
> >   - Keep WARN_ON_ONCE(1) instead of BUILD_BUG() for the aforementioned
> >     reason.
>
> Okay, but IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) on the caller side would
> do the trick, I believe.
I tried moving this to the caller side in move_page_tables(),
-                       if (extent == HPAGE_PMD_SIZE &&
+                       if (extent == HPAGE_PMD_SIZE &&
IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
but it produces the same error as reported by kernel test robot:
ld.lld: error: undefined symbol: move_huge_pmd
I'm not sure why these are different but the kernel test robot
compiler complains.
>
> >
> >  arch/Kconfig |   7 ++
> >  mm/mremap.c  | 220 ++++++++++++++++++++++++++++++++++++++++++++-------
> >  2 files changed, 197 insertions(+), 30 deletions(-)
> >
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index af14a567b493..5eabaa00bf9b 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -602,6 +602,13 @@ config HAVE_IRQ_TIME_ACCOUNTING
> >         Archs need to ensure they use a high enough resolution clock to
> >         support irq time accounting and then call enable_sched_clock_irqtime().
> >
> > +config HAVE_MOVE_PUD
> > +     bool
> > +     help
> > +       Architectures that select this are able to move page tables at the
> > +       PUD level. If there are only 3 page table levels, the move effectively
> > +       happens at the PGD level.
> > +
> >  config HAVE_MOVE_PMD
> >       bool
> >       help
> > diff --git a/mm/mremap.c b/mm/mremap.c
> > index 138abbae4f75..c1d6ab667d70 100644
> > --- a/mm/mremap.c
> > +++ b/mm/mremap.c
> > @@ -249,14 +249,176 @@ static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
> >
> >       return true;
> >  }
> > +#else
> > +static inline bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
> > +               unsigned long new_addr, pmd_t *old_pmd, pmd_t *new_pmd)
> > +{
> > +     return false;
> > +}
> >  #endif
> >
> > +#ifdef CONFIG_HAVE_MOVE_PUD
> > +static pud_t *get_old_pud(struct mm_struct *mm, unsigned long addr)
> > +{
> > +     pgd_t *pgd;
> > +     p4d_t *p4d;
> > +     pud_t *pud;
> > +
> > +     pgd = pgd_offset(mm, addr);
> > +     if (pgd_none_or_clear_bad(pgd))
> > +             return NULL;
> > +
> > +     p4d = p4d_offset(pgd, addr);
> > +     if (p4d_none_or_clear_bad(p4d))
> > +             return NULL;
> > +
> > +     pud = pud_offset(p4d, addr);
> > +     if (pud_none_or_clear_bad(pud))
> > +             return NULL;
> > +
> > +     return pud;
> > +}
> > +
> > +static pud_t *alloc_new_pud(struct mm_struct *mm, struct vm_area_struct *vma,
> > +                         unsigned long addr)
> > +{
> > +     pgd_t *pgd;
> > +     p4d_t *p4d;
> > +     pud_t *pud;
> > +
> > +     pgd = pgd_offset(mm, addr);
> > +     p4d = p4d_alloc(mm, pgd, addr);
> > +     if (!p4d)
> > +             return NULL;
> > +     pud = pud_alloc(mm, p4d, addr);
> > +     if (!pud)
> > +             return NULL;
> > +
> > +     return pud;
> > +}
>
> Looks like a code duplication.
>
> Could you move these two helpers out of #ifdef CONFIG_HAVE_MOVE_PUD and
> make get_old_pmd() and alloc_new_pmd() use them?
Yes, that will be cleaner. I'll update it in the next version.
>
> > +
> > +static bool move_normal_pud(struct vm_area_struct *vma, unsigned long old_addr,
> > +               unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
> > +{
> > +     spinlock_t *old_ptl, *new_ptl;
> > +     struct mm_struct *mm = vma->vm_mm;
> > +     pud_t pud;
> > +
> > +     /*
> > +      * The destination pud shouldn't be established, free_pgtables()
> > +      * should have released it.
> > +      */
> > +     if (WARN_ON_ONCE(!pud_none(*new_pud)))
> > +             return false;
> > +
> > +     /*
> > +      * We don't have to worry about the ordering of src and dst
> > +      * ptlocks because exclusive mmap_lock prevents deadlock.
> > +      */
> > +     old_ptl = pud_lock(vma->vm_mm, old_pud);
> > +     new_ptl = pud_lockptr(mm, new_pud);
> > +     if (new_ptl != old_ptl)
> > +             spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
> > +
> > +     /* Clear the pud */
> > +     pud = *old_pud;
> > +     pud_clear(old_pud);
> > +
> > +     VM_BUG_ON(!pud_none(*new_pud));
> > +
> > +     /* Set the new pud */
> > +     set_pud_at(mm, new_addr, new_pud, pud);
> > +     flush_tlb_range(vma, old_addr, old_addr + PUD_SIZE);
> > +     if (new_ptl != old_ptl)
> > +             spin_unlock(new_ptl);
> > +     spin_unlock(old_ptl);
> > +
> > +     return true;
> > +}
> > +#else
> > +static inline bool move_normal_pud(struct vm_area_struct *vma, unsigned long old_addr,
> > +               unsigned long new_addr, pud_t *old_pud, pud_t *new_pud)
> > +{
> > +     return false;
> > +}
> > +#endif
> > +
> > +enum pgt_entry {
> > +     NORMAL_PMD,
> > +     HPAGE_PMD,
> > +     NORMAL_PUD,
> > +};
> > +
> > +/*
> > + * Returns an extent of the corresponding size for the pgt_entry specified if valid.
> > + * Else returns a smaller extent bounded by the end of the source and destination
> > + * pgt_entry. Returns 0 if an invalid pgt_entry is specified.
> > + */
> > +static unsigned long get_extent(enum pgt_entry entry, unsigned long old_addr,
> > +                     unsigned long old_end, unsigned long new_addr)
> > +{
> > +     unsigned long next, extent, mask, size;
> > +
> > +     if (entry == NORMAL_PMD || entry == HPAGE_PMD) {
> > +             mask = PMD_MASK;
> > +             size = PMD_SIZE;
> > +     } else if (entry == NORMAL_PUD) {
> > +             mask = PUD_MASK;
> > +             size = PUD_SIZE;
> > +     } else
> > +             return 0;
>
> Em. Who would ever specify invalid pgt_entry? It's bug.
> Again, switch()?
Sounds good. I'll use BUG() and switch() instead.
>
> > +
> > +     next = (old_addr + size) & mask;
> > +     /* even if next overflowed, extent below will be ok */
> > +     extent = (next > old_end) ? old_end - old_addr : next - old_addr;
> > +     next = (new_addr + size) & mask;
> > +     if (extent > next - new_addr)
> > +             extent = next - new_addr;
> > +     return extent;
> > +}
> > +
> > +/*
> > + * Attempts to speedup the move by moving entry at the level corresponding to
> > + * pgt_entry. Returns true if the move was successful, else false.
> > + */
> > +static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma,
> > +                     unsigned long old_addr, unsigned long new_addr, void *old_entry,
> > +                     void *new_entry, bool need_rmap_locks)
> > +{
> > +     bool moved = false;
> > +
> > +     /* See comment in move_ptes() */
> > +     if (need_rmap_locks)
> > +             take_rmap_locks(vma);
> > +
> > +     switch (entry) {
> > +     case NORMAL_PMD:
> > +             moved =  move_normal_pmd(vma, old_addr, new_addr, old_entry, new_entry);
>
> Nit: here and below, double space after '='. Why?
Sorry, editing mistake on my end. I'll clean this up before resending.
>
> > +             break;
> > +     case NORMAL_PUD:
> > +             moved =  move_normal_pud(vma, old_addr, new_addr, old_entry, new_entry);
> > +             break;
> > +     case HPAGE_PMD:
> > +             moved =  IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
> > +                     move_huge_pmd(vma, old_addr, new_addr, old_entry, new_entry);
> > +             break;
> > +     default:
> > +             WARN_ON_ONCE(1);
> > +             break;
> > +     }
> > +
> > +     if (need_rmap_locks)
> > +             drop_rmap_locks(vma);
> > +
> > +     return moved;
> > +}
> > +
> >  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,
> >               bool need_rmap_locks)
> >  {
> > -     unsigned long extent, next, old_end;
> > +     unsigned long extent, old_end;
> >       struct mmu_notifier_range range;
> >       pmd_t *old_pmd, *new_pmd;
> >
> > @@ -269,14 +431,27 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> >
> >       for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
> >               cond_resched();
> > -             next = (old_addr + PMD_SIZE) & PMD_MASK;
> > -             /* even if next overflowed, extent below will be ok */
> > -             extent = next - old_addr;
> > -             if (extent > old_end - old_addr)
> > -                     extent = old_end - old_addr;
> > -             next = (new_addr + PMD_SIZE) & PMD_MASK;
> > -             if (extent > next - new_addr)
> > -                     extent = next - new_addr;
> > +#ifdef CONFIG_HAVE_MOVE_PUD
>
> Any chance  if (IS_ENABLED(CONFIG_HAVE_MOVE_PUD)) would work here?
Once we move get_old_put() and alloc_new_pud() out of the #ifdefs as
you suggested
above, it should work. It would also now be possible to replace the
#ifdef CONFIG_HAVE_MOVE_PMD in move_page_tables() with
IS_ENABLED(CONFIG_HAVE_MOVE_PMD).

Thanks,
Kalesh
>
> > +             /*
> > +              * If extent is PUD-sized try to speed up the move by moving at the
> > +              * PUD level if possible.
> > +              */
> > +             extent = get_extent(NORMAL_PUD, old_addr, old_end, new_addr);
> > +             if (extent == PUD_SIZE) {
> > +                     pud_t *old_pud, *new_pud;
> > +
> > +                     old_pud = get_old_pud(vma->vm_mm, old_addr);
> > +                     if (!old_pud)
> > +                             continue;
> > +                     new_pud = alloc_new_pud(vma->vm_mm, vma, new_addr);
> > +                     if (!new_pud)
> > +                             break;
> > +                     if (move_pgt_entry(NORMAL_PUD, vma, old_addr, new_addr,
> > +                                        old_pud, new_pud, need_rmap_locks))
> > +                             continue;
> > +             }
> > +#endif
> > +             extent = get_extent(NORMAL_PMD, old_addr, old_end, new_addr);
> >               old_pmd = get_old_pmd(vma->vm_mm, old_addr);
> >               if (!old_pmd)
> >                       continue;
> > @@ -284,18 +459,10 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> >               if (!new_pmd)
> >                       break;
> >               if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd) || pmd_devmap(*old_pmd)) {
> > -                     if (extent == HPAGE_PMD_SIZE) {
> > -                             bool moved;
> > -                             /* See comment in move_ptes() */
> > -                             if (need_rmap_locks)
> > -                                     take_rmap_locks(vma);
> > -                             moved = move_huge_pmd(vma, old_addr, new_addr,
> > -                                                   old_pmd, new_pmd);
> > -                             if (need_rmap_locks)
> > -                                     drop_rmap_locks(vma);
> > -                             if (moved)
> > -                                     continue;
> > -                     }
> > +                     if (extent == HPAGE_PMD_SIZE &&
> > +                         move_pgt_entry(HPAGE_PMD, vma, old_addr, new_addr, old_pmd,
> > +                                        new_pmd, need_rmap_locks))
> > +                             continue;
> >                       split_huge_pmd(vma, old_pmd, old_addr);
> >                       if (pmd_trans_unstable(old_pmd))
> >                               continue;
> > @@ -305,15 +472,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
> >                        * If the extent is PMD-sized, try to speed the move by
> >                        * moving at the PMD level if possible.
> >                        */
> > -                     bool moved;
> > -
> > -                     if (need_rmap_locks)
> > -                             take_rmap_locks(vma);
> > -                     moved = move_normal_pmd(vma, old_addr, new_addr,
> > -                                             old_pmd, new_pmd);
> > -                     if (need_rmap_locks)
> > -                             drop_rmap_locks(vma);
> > -                     if (moved)
> > +                     if (move_pgt_entry(NORMAL_PMD, vma, old_addr, new_addr, old_pmd,
> > +                                        new_pmd, need_rmap_locks))
> >                               continue;
> >  #endif
> >               }
> > --
> > 2.28.0.806.g8561365e88-goog
> >
>
> --
>  Kirill A. Shutemov
>
> --
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