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-...@lists.01.org/thread/CKPGL4FH4NG7TGH2CVYX2UX76L25BTA3/
>
> Signed-off-by: Kalesh Singh <kaleshsi...@google.com>
> Reported-by: kernel test robot <l...@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.
>
> 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?
> +
> +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()?
> +
> + 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?
> + 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?
> + /*
> + * 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
