On Wed, Jul 6, 2022 at 5:06 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> 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@xxxxxxxxxx/ > [2] https://github.com/google/tcmalloc/tree/master/tcmalloc > > Suggested-by: David Rientjes <rientjes@xxxxxxxxxx> > Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx> > --- > 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 >
