Re: [mm-unstable v7 09/18] mm/madvise: introduce MADV_COLLAPSE sync hugepage collapse

[Yang Shi <shy828301@gmail.com>]

On Wed, Jul 6, 2022 at 5:06 PM Zach O'Keefe <zokeefe@google.com> wrote:
>
> This idea was introduced by David Rientjes[1].
>
> Introduce a new madvise mode, MADV_COLLAPSE, that allows users to request a
> synchronous collapse of memory at their own expense.
>
> The benefits of this approach are:
>
> * CPU is charged to the process that wants to spend the cycles for the
>   THP
> * Avoid unpredictable timing of khugepaged collapse
>
> Semantics
>
> This call is independent of the system-wide THP sysfs settings, but will
> fail for memory marked VM_NOHUGEPAGE.  If the ranges provided span
> multiple VMAs, the semantics of the collapse over each VMA is
> independent from the others.  This implies a hugepage cannot cross a VMA
> boundary.  If collapse of a given hugepage-aligned/sized region fails,
> the operation may continue to attempt collapsing the remainder of memory
> specified.
>
> The memory ranges provided must be page-aligned, but are not required to
> be hugepage-aligned.  If the memory ranges are not hugepage-aligned, the
> start/end of the range will be clamped to the first/last
> hugepage-aligned address covered by said range.  The memory ranges must
> span at least one hugepage-sized region.
>
> All non-resident pages covered by the range will first be
> swapped/faulted-in, before being internally copied onto a freshly
> allocated hugepage.  Unmapped pages will have their data directly
> initialized to 0 in the new hugepage.  However, for every eligible hugepage
> aligned/sized region to-be collapsed, at least one page must currently be
> backed by memory (a PMD covering the address range must already exist).
>
> Allocation for the new hugepage may enter direct reclaim and/or
> compaction, regardless of VMA flags.  When the system has multiple NUMA
> nodes, the hugepage will be allocated from the node providing the most
> native pages.  This operation operates on the current state of the
> specified process and makes no persistent changes or guarantees on how
> pages will be mapped, constructed, or faulted in the future
>
> Return Value
>
> If all hugepage-sized/aligned regions covered by the provided range were
> either successfully collapsed, or were already PMD-mapped THPs, this
> operation will be deemed successful.  On success, process_madvise(2)
> returns the number of bytes advised, and madvise(2) returns 0.  Else, -1
> is returned and errno is set to indicate the error for the most-recently
> attempted hugepage collapse.  Note that many failures might have
> occurred, since the operation may continue to collapse in the event a
> single hugepage-sized/aligned region fails.
>
>         ENOMEM  Memory allocation failed or VMA not found
>         EBUSY   Memcg charging failed
>         EAGAIN  Required resource temporarily unavailable.  Try again
>                 might succeed.
>         EINVAL  Other error: No PMD found, subpage doesn't have Present
>                 bit set, "Special" page no backed by struct page, VMA
>                 incorrectly sized, address not page-aligned, ...
>
> Most notable here is ENOMEM and EBUSY (new to madvise) which are
> intended to provide the caller with actionable feedback so they may take
> an appropriate fallback measure.

Don't forget to update man-pages. And cc'ed linux-api.

>
> Use Cases
>
> An immediate user of this new functionality are malloc() implementations
> that manage memory in hugepage-sized chunks, but sometimes subrelease
> memory back to the system in native-sized chunks via MADV_DONTNEED;
> zapping the pmd.  Later, when the memory is hot, the implementation
> could madvise(MADV_COLLAPSE) to re-back the memory by THPs to regain
> hugepage coverage and dTLB performance.  TCMalloc is such an
> implementation that could benefit from this[2].
>
> Only privately-mapped anon memory is supported for now, but additional
> support for file, shmem, and HugeTLB high-granularity mappings[2] is
> expected.  File and tmpfs/shmem support would permit:
>
> * Backing executable text by THPs.  Current support provided by
>   CONFIG_READ_ONLY_THP_FOR_FS may take a long time on a large system which
>   might impair services from serving at their full rated load after
>   (re)starting.  Tricks like mremap(2)'ing text onto anonymous memory to
>   immediately realize iTLB performance prevents page sharing and demand
>   paging, both of which increase steady state memory footprint.  With
>   MADV_COLLAPSE, we get the best of both worlds: Peak upfront performance
>   and lower RAM footprints.
> * Backing guest memory by hugapages after the memory contents have been
>   migrated in native-page-sized chunks to a new host, in a
>   userfaultfd-based live-migration stack.
>
> [1] https://lore.kernel.org/linux-mm/d098c392-273a-36a4-1a29-59731cdf5d3d@google.com/
> [2] https://github.com/google/tcmalloc/tree/master/tcmalloc
>
> Suggested-by: David Rientjes <rientjes@google.com>
> Signed-off-by: Zach O'Keefe <zokeefe@google.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  arch/alpha/include/uapi/asm/mman.h           |   2 +
>  arch/mips/include/uapi/asm/mman.h            |   2 +
>  arch/parisc/include/uapi/asm/mman.h          |   2 +
>  arch/xtensa/include/uapi/asm/mman.h          |   2 +
>  include/linux/huge_mm.h                      |  14 ++-
>  include/uapi/asm-generic/mman-common.h       |   2 +
>  mm/khugepaged.c                              | 118 ++++++++++++++++++-
>  mm/madvise.c                                 |   5 +
>  tools/include/uapi/asm-generic/mman-common.h |   2 +
>  9 files changed, 146 insertions(+), 3 deletions(-)
>
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 4aa996423b0d..763929e814e9 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -76,6 +76,8 @@
>
>  #define MADV_DONTNEED_LOCKED   24      /* like DONTNEED, but drop locked pages too */
>
> +#define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
> index 1be428663c10..c6e1fc77c996 100644
> --- a/arch/mips/include/uapi/asm/mman.h
> +++ b/arch/mips/include/uapi/asm/mman.h
> @@ -103,6 +103,8 @@
>
>  #define MADV_DONTNEED_LOCKED   24      /* like DONTNEED, but drop locked pages too */
>
> +#define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
> index a7ea3204a5fa..22133a6a506e 100644
> --- a/arch/parisc/include/uapi/asm/mman.h
> +++ b/arch/parisc/include/uapi/asm/mman.h
> @@ -70,6 +70,8 @@
>  #define MADV_WIPEONFORK 71             /* Zero memory on fork, child only */
>  #define MADV_KEEPONFORK 72             /* Undo MADV_WIPEONFORK */
>
> +#define MADV_COLLAPSE  73              /* Synchronous hugepage collapse */
> +
>  #define MADV_HWPOISON     100          /* poison a page for testing */
>  #define MADV_SOFT_OFFLINE 101          /* soft offline page for testing */
>
> diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
> index 7966a58af472..1ff0c858544f 100644
> --- a/arch/xtensa/include/uapi/asm/mman.h
> +++ b/arch/xtensa/include/uapi/asm/mman.h
> @@ -111,6 +111,8 @@
>
>  #define MADV_DONTNEED_LOCKED   24      /* like DONTNEED, but drop locked pages too */
>
> +#define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index 00312fc251c1..39193623442e 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -218,6 +218,9 @@ void __split_huge_pud(struct vm_area_struct *vma, pud_t *pud,
>
>  int hugepage_madvise(struct vm_area_struct *vma, unsigned long *vm_flags,
>                      int advice);
> +int madvise_collapse(struct vm_area_struct *vma,
> +                    struct vm_area_struct **prev,
> +                    unsigned long start, unsigned long end);
>  void vma_adjust_trans_huge(struct vm_area_struct *vma, unsigned long start,
>                            unsigned long end, long adjust_next);
>  spinlock_t *__pmd_trans_huge_lock(pmd_t *pmd, struct vm_area_struct *vma);
> @@ -361,9 +364,16 @@ static inline void split_huge_pmd_address(struct vm_area_struct *vma,
>  static inline int hugepage_madvise(struct vm_area_struct *vma,
>                                    unsigned long *vm_flags, int advice)
>  {
> -       BUG();
> -       return 0;
> +       return -EINVAL;
>  }
> +
> +static inline int madvise_collapse(struct vm_area_struct *vma,
> +                                  struct vm_area_struct **prev,
> +                                  unsigned long start, unsigned long end)
> +{
> +       return -EINVAL;
> +}
> +
>  static inline void vma_adjust_trans_huge(struct vm_area_struct *vma,
>                                          unsigned long start,
>                                          unsigned long end,
> diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> index 6c1aa92a92e4..6ce1f1ceb432 100644
> --- a/include/uapi/asm-generic/mman-common.h
> +++ b/include/uapi/asm-generic/mman-common.h
> @@ -77,6 +77,8 @@
>
>  #define MADV_DONTNEED_LOCKED   24      /* like DONTNEED, but drop locked pages too */
>
> +#define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index c7a09cc9a0e8..2b2d832e44f2 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -976,7 +976,8 @@ static int alloc_charge_hpage(struct page **hpage, struct mm_struct *mm,
>                               struct collapse_control *cc)
>  {
>         /* Only allocate from the target node */
> -       gfp_t gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
> +       gfp_t gfp = (cc->is_khugepaged ? alloc_hugepage_khugepaged_gfpmask() :
> +                    GFP_TRANSHUGE) | __GFP_THISNODE;
>         int node = khugepaged_find_target_node(cc);
>
>         if (!khugepaged_alloc_page(hpage, gfp, node))
> @@ -2356,3 +2357,118 @@ void khugepaged_min_free_kbytes_update(void)
>                 set_recommended_min_free_kbytes();
>         mutex_unlock(&khugepaged_mutex);
>  }
> +
> +static int madvise_collapse_errno(enum scan_result r)
> +{
> +       /*
> +        * MADV_COLLAPSE breaks from existing madvise(2) conventions to provide
> +        * actionable feedback to caller, so they may take an appropriate
> +        * fallback measure depending on the nature of the failure.
> +        */
> +       switch (r) {
> +       case SCAN_ALLOC_HUGE_PAGE_FAIL:
> +               return -ENOMEM;
> +       case SCAN_CGROUP_CHARGE_FAIL:
> +               return -EBUSY;
> +       /* Resource temporary unavailable - trying again might succeed */
> +       case SCAN_PAGE_LOCK:
> +       case SCAN_PAGE_LRU:
> +               return -EAGAIN;
> +       /*
> +        * Other: Trying again likely not to succeed / error intrinsic to
> +        * specified memory range. khugepaged likely won't be able to collapse
> +        * either.
> +        */
> +       default:
> +               return -EINVAL;
> +       }
> +}
> +
> +int madvise_collapse(struct vm_area_struct *vma, struct vm_area_struct **prev,
> +                    unsigned long start, unsigned long end)
> +{
> +       struct collapse_control *cc;
> +       struct mm_struct *mm = vma->vm_mm;
> +       unsigned long hstart, hend, addr;
> +       int thps = 0, last_fail = SCAN_FAIL;
> +       bool mmap_locked = true;
> +
> +       BUG_ON(vma->vm_start > start);
> +       BUG_ON(vma->vm_end < end);
> +
> +       cc = kmalloc(sizeof(*cc), GFP_KERNEL);
> +       if (!cc)
> +               return -ENOMEM;
> +       cc->is_khugepaged = false;
> +       cc->last_target_node = NUMA_NO_NODE;
> +
> +       *prev = vma;
> +
> +       /* TODO: Support file/shmem */
> +       if (!vma->anon_vma || !vma_is_anonymous(vma))
> +               return -EINVAL;
> +
> +       hstart = (start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> +       hend = end & HPAGE_PMD_MASK;
> +
> +       if (!hugepage_vma_check(vma, vma->vm_flags, false, false, false))
> +               return -EINVAL;
> +
> +       mmgrab(mm);
> +       lru_add_drain_all();
> +
> +       for (addr = hstart; addr < hend; addr += HPAGE_PMD_SIZE) {
> +               int result = SCAN_FAIL;
> +
> +               if (!mmap_locked) {
> +                       cond_resched();
> +                       mmap_read_lock(mm);
> +                       mmap_locked = true;
> +                       result = hugepage_vma_revalidate(mm, addr, &vma, cc);
> +                       if (result  != SCAN_SUCCEED) {
> +                               last_fail = result;
> +                               goto out_nolock;
> +                       }
> +               }
> +               mmap_assert_locked(mm);
> +               memset(cc->node_load, 0, sizeof(cc->node_load));
> +               result = khugepaged_scan_pmd(mm, vma, addr, &mmap_locked, cc);
> +               if (!mmap_locked)
> +                       *prev = NULL;  /* Tell caller we dropped mmap_lock */
> +
> +               switch (result) {
> +               case SCAN_SUCCEED:
> +               case SCAN_PMD_MAPPED:
> +                       ++thps;
> +                       break;
> +               /* Whitelisted set of results where continuing OK */
> +               case SCAN_PMD_NULL:
> +               case SCAN_PTE_NON_PRESENT:
> +               case SCAN_PTE_UFFD_WP:
> +               case SCAN_PAGE_RO:
> +               case SCAN_LACK_REFERENCED_PAGE:
> +               case SCAN_PAGE_NULL:
> +               case SCAN_PAGE_COUNT:
> +               case SCAN_PAGE_LOCK:
> +               case SCAN_PAGE_COMPOUND:
> +               case SCAN_PAGE_LRU:
> +                       last_fail = result;
> +                       break;
> +               default:
> +                       last_fail = result;
> +                       /* Other error, exit */
> +                       goto out_maybelock;
> +               }
> +       }
> +
> +out_maybelock:
> +       /* Caller expects us to hold mmap_lock on return */
> +       if (!mmap_locked)
> +               mmap_read_lock(mm);
> +out_nolock:
> +       mmap_assert_locked(mm);
> +       mmdrop(mm);
> +
> +       return thps == ((hend - hstart) >> HPAGE_PMD_SHIFT) ? 0
> +                       : madvise_collapse_errno(last_fail);
> +}
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 851fa4e134bc..9f08e958ea86 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -59,6 +59,7 @@ static int madvise_need_mmap_write(int behavior)
>         case MADV_FREE:
>         case MADV_POPULATE_READ:
>         case MADV_POPULATE_WRITE:
> +       case MADV_COLLAPSE:
>                 return 0;
>         default:
>                 /* be safe, default to 1. list exceptions explicitly */
> @@ -1057,6 +1058,8 @@ static int madvise_vma_behavior(struct vm_area_struct *vma,
>                 if (error)
>                         goto out;
>                 break;
> +       case MADV_COLLAPSE:
> +               return madvise_collapse(vma, prev, start, end);
>         }
>
>         anon_name = anon_vma_name(vma);
> @@ -1150,6 +1153,7 @@ madvise_behavior_valid(int behavior)
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>         case MADV_HUGEPAGE:
>         case MADV_NOHUGEPAGE:
> +       case MADV_COLLAPSE:
>  #endif
>         case MADV_DONTDUMP:
>         case MADV_DODUMP:
> @@ -1339,6 +1343,7 @@ int madvise_set_anon_name(struct mm_struct *mm, unsigned long start,
>   *  MADV_NOHUGEPAGE - mark the given range as not worth being backed by
>   *             transparent huge pages so the existing pages will not be
>   *             coalesced into THP and new pages will not be allocated as THP.
> + *  MADV_COLLAPSE - synchronously coalesce pages into new THP.
>   *  MADV_DONTDUMP - the application wants to prevent pages in the given range
>   *             from being included in its core dump.
>   *  MADV_DODUMP - cancel MADV_DONTDUMP: no longer exclude from core dump.
> diff --git a/tools/include/uapi/asm-generic/mman-common.h b/tools/include/uapi/asm-generic/mman-common.h
> index 6c1aa92a92e4..6ce1f1ceb432 100644
> --- a/tools/include/uapi/asm-generic/mman-common.h
> +++ b/tools/include/uapi/asm-generic/mman-common.h
> @@ -77,6 +77,8 @@
>
>  #define MADV_DONTNEED_LOCKED   24      /* like DONTNEED, but drop locked pages too */
>
> +#define MADV_COLLAPSE  25              /* Synchronous hugepage collapse */
> +
>  /* compatibility flags */
>  #define MAP_FILE       0
>
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>