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Wed, 20 May 2026 22:11:45 -0700 (PDT) Date: Thu, 21 May 2026 13:11:18 +0800 From: Vernon Yang To: Wei Yang Cc: Nico Pache , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, david@kernel.org, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, liam@infradead.org, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com Subject: Re: [PATCH mm-unstable v17 11/14] mm/khugepaged: Introduce mTHP collapse support Message-ID: <91015820-f39a-4b06-89de-b49e5ca465fd@gmail.com> References: <20260511185817.686831-1-npache@redhat.com> <20260511185817.686831-12-npache@redhat.com> <8f9834db-8981-4eb1-ae46-94908943da3d@gmail.com> <20260521024654.2a7teoe665porz76@master> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260521024654.2a7teoe665porz76@master> On Thu, May 21, 2026 at 02:46:54AM +0000, Wei Yang wrote: > On Thu, May 21, 2026 at 10:36:15AM +0800, Vernon Yang wrote: > >On Mon, May 11, 2026 at 12:58:11PM -0600, Nico Pache wrote: > >> Enable khugepaged to collapse to mTHP orders. This patch implements the > >> main scanning logic using a bitmap to track occupied pages and a stack > >> structure that allows us to find optimal collapse sizes. > >> > >> Previous to this patch, PMD collapse had 3 main phases, a light weight > >> scanning phase (mmap_read_lock) that determines a potential PMD > >> collapse, an alloc phase (mmap unlocked), then finally heavier collapse > >> phase (mmap_write_lock). > >> > >> To enabled mTHP collapse we make the following changes: > >> > >> During PMD scan phase, track occupied pages in a bitmap. When mTHP > >> orders are enabled, we remove the restriction of max_ptes_none during the > >> scan phase to avoid missing potential mTHP collapse candidates. Once we > >> have scanned the full PMD range and updated the bitmap to track occupied > >> pages, we use the bitmap to find the optimal mTHP size. > >> > >> Implement collapse_scan_bitmap() to perform binary recursion on the bitmap > >> and determine the best eligible order for the collapse. A stack structure > >> is used instead of traditional recursion to manage the search. This also > >> prevents a traditional recursive approach when the kernel stack struct is > >> limited. The algorithm recursively splits the bitmap into smaller chunks to > >> find the highest order mTHPs that satisfy the collapse criteria. We start > >> by attempting the PMD order, then moved on the consecutively lower orders > >> (mTHP collapse). The stack maintains a pair of variables (offset, order), > >> indicating the number of PTEs from the start of the PMD, and the order of > >> the potential collapse candidate. > >> > >> The algorithm for consuming the bitmap works as such: > >> 1) push (0, HPAGE_PMD_ORDER) onto the stack > >> 2) pop the stack > >> 3) check if the number of set bits in that (offset,order) pair > >> statisfy the max_ptes_none threshold for that order > >> 4) if yes, attempt collapse > >> 5) if no (or collapse fails), push two new stack items representing > >> the left and right halves of the current bitmap range, at the > >> next lower order > >> 6) repeat at step (2) until stack is empty. > >> > >> Below is a diagram representing the algorithm and stack items: > >> > >> offset mid_offset > >> | | > >> | | > >> v v > >> ____________________________________ > >> | PTE Page Table | > >> -------------------------------------- > >> <-------><-------> > >> order-1 order-1 > >> > >> mTHP collapses reject regions containing swapped out or shared pages. > >> This is because adding new entries can lead to new none pages, and these > >> may lead to constant promotion into a higher order mTHP. A similar > >> issue can occur with "max_ptes_none > HPAGE_PMD_NR/2" due to a collapse > >> introducing at least 2x the number of pages, and on a future scan will > >> satisfy the promotion condition once again. This issue is prevented via > >> the collapse_max_ptes_none() function which imposes the max_ptes_none > >> restrictions above. > >> > >> We currently only support mTHP collapse for max_ptes_none values of 0 > >> and HPAGE_PMD_NR - 1. resulting in the following behavior: > >> > >> - max_ptes_none=0: Never introduce new empty pages during collapse > >> - max_ptes_none=HPAGE_PMD_NR-1: Always try collapse to the highest > >> available mTHP order > >> > >> Any other max_ptes_none value will emit a warning and skip mTHP collapse > >> attempts. There should be no behavior change for PMD collapse. > >> > >> Once we determine what mTHP sizes fits best in that PMD range a collapse > >> is attempted. A minimum collapse order of 2 is used as this is the lowest > >> order supported by anon memory as defined by THP_ORDERS_ALL_ANON. > >> > >> Currently madv_collapse is not supported and will only attempt PMD > >> collapse. > >> > >> We can also remove the check for is_khugepaged inside the PMD scan as > >> the collapse_max_ptes_none() function handles this logic now. > >> > >> Signed-off-by: Nico Pache > >> --- > >> mm/khugepaged.c | 182 +++++++++++++++++++++++++++++++++++++++++++++--- > >> 1 file changed, 174 insertions(+), 8 deletions(-) > >> > >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >> index 3492b135d667..39bf7ea8a6e8 100644 > >> --- a/mm/khugepaged.c > >> +++ b/mm/khugepaged.c > >> @@ -100,6 +100,30 @@ static DEFINE_READ_MOSTLY_HASHTABLE(mm_slots_hash, MM_SLOTS_HASH_BITS); > >> > >> static struct kmem_cache *mm_slot_cache __ro_after_init; > >> > >> +#define KHUGEPAGED_MIN_MTHP_ORDER 2 > >> +/* > >> + * mthp_collapse() does an iterative DFS over a binary tree, from > >> + * HPAGE_PMD_ORDER down to KHUGEPAGED_MIN_MTHP_ORDER. The max stack > >> + * size needed for a DFS on a binary tree is height + 1, where > >> + * height = HPAGE_PMD_ORDER - KHUGEPAGED_MIN_MTHP_ORDER. > >> + * > >> + * ilog2 is used in place of HPAGE_PMD_ORDER because some architectures > >> + * (e.g. ppc64le) do not define HPAGE_PMD_ORDER until after build time. > >> + */ > >> +#define MTHP_STACK_SIZE (ilog2(MAX_PTRS_PER_PTE) - KHUGEPAGED_MIN_MTHP_ORDER + 1) > >> + > >> +/* > >> + * Defines a range of PTE entries in a PTE page table which are being > >> + * considered for mTHP collapse. > >> + * > >> + * @offset: the offset of the first PTE entry in a PMD range. > >> + * @order: the order of the PTE entries being considered for collapse. > >> + */ > >> +struct mthp_range { > >> + u16 offset; > >> + u8 order; > >> +}; > >> + > >> struct collapse_control { > >> bool is_khugepaged; > >> > >> @@ -111,6 +135,12 @@ struct collapse_control { > >> > >> /* nodemask for allocation fallback */ > >> nodemask_t alloc_nmask; > >> + > >> + /* Each bit represents a single occupied (!none/zero) page. */ > >> + DECLARE_BITMAP(mthp_bitmap, MAX_PTRS_PER_PTE); > >> + /* A mask of the current range being considered for mTHP collapse. */ > >> + DECLARE_BITMAP(mthp_bitmap_mask, MAX_PTRS_PER_PTE); > >> + struct mthp_range mthp_bitmap_stack[MTHP_STACK_SIZE]; > >> }; > >> > >> /** > >> @@ -1404,20 +1434,140 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s > >> return result; > >> } > >> > >> +static void collapse_mthp_stack_push(struct collapse_control *cc, int *stack_size, > >> + u16 offset, u8 order) > >> +{ > >> + const int size = *stack_size; > >> + struct mthp_range *stack = &cc->mthp_bitmap_stack[size]; > >> + > >> + VM_WARN_ON_ONCE(size >= MTHP_STACK_SIZE); > >> + stack->order = order; > >> + stack->offset = offset; > >> + (*stack_size)++; > >> +} > >> + > >> +static struct mthp_range collapse_mthp_stack_pop(struct collapse_control *cc, > >> + int *stack_size) > >> +{ > >> + const int size = *stack_size; > >> + > >> + VM_WARN_ON_ONCE(size <= 0); > >> + (*stack_size)--; > >> + return cc->mthp_bitmap_stack[size - 1]; > >> +} > >> + > >> +static unsigned int collapse_mthp_count_present(struct collapse_control *cc, > >> + u16 offset, unsigned int nr_ptes) > >> +{ > >> + bitmap_zero(cc->mthp_bitmap_mask, MAX_PTRS_PER_PTE); > >> + bitmap_set(cc->mthp_bitmap_mask, offset, nr_ptes); > >> + return bitmap_weight_and(cc->mthp_bitmap, cc->mthp_bitmap_mask, MAX_PTRS_PER_PTE); > >> +} > >> + > >> +/* > >> + * mthp_collapse() consumes the bitmap that is generated during > >> + * collapse_scan_pmd() to determine what regions and mTHP orders fit best. > >> + * > >> + * Each bit in cc->mthp_bitmap represents a single occupied (!none/zero) page. > >> + * A stack structure cc->mthp_bitmap_stack is used to check different regions > >> + * of the bitmap for collapse eligibility. The stack maintains a pair of > >> + * variables (offset, order), indicating the number of PTEs from the start of > >> + * the PMD, and the order of the potential collapse candidate respectively. We > >> + * start at the PMD order and check if it is eligible for collapse; if not, we > >> + * add two entries to the stack at a lower order to represent the left and right > >> + * halves of the PTE page table we are examining. > >> + * > >> + * offset mid_offset > >> + * | | > >> + * | | > >> + * v v > >> + * -------------------------------------- > >> + * | cc->mthp_bitmap | > >> + * -------------------------------------- > >> + * <-------><-------> > >> + * order-1 order-1 > >> + * > >> + * For each of these, we determine how many PTE entries are occupied in the > >> + * range of PTE entries we propose to collapse, then we compare this to a > >> + * threshold number of PTE entries which would need to be occupied for a > >> + * collapse to be permitted at that order (accounting for max_ptes_none). > >> + * > >> + * If a collapse is permitted, we attempt to collapse the PTE range into a > >> + * mTHP. > >> + */ > >> +static int mthp_collapse(struct mm_struct *mm, unsigned long address, > >> + int referenced, int unmapped, struct collapse_control *cc, > >> + unsigned long enabled_orders) > >> +{ > >> + unsigned int nr_occupied_ptes, nr_ptes; > >> + int max_ptes_none, collapsed = 0, stack_size = 0; > >> + unsigned long collapse_address; > >> + struct mthp_range range; > >> + u16 offset; > >> + u8 order; > >> + > >> + collapse_mthp_stack_push(cc, &stack_size, 0, HPAGE_PMD_ORDER); > >> + > >> + while (stack_size) { > >> + range = collapse_mthp_stack_pop(cc, &stack_size); > >> + order = range.order; > >> + offset = range.offset; > >> + nr_ptes = 1UL << order; > >> + > >> + if (!test_bit(order, &enabled_orders)) > >> + goto next_order; > >> + > >> + max_ptes_none = collapse_max_ptes_none(cc, NULL, order); > >> + > >> + if (max_ptes_none < 0) > >> + return collapsed; > >> + > >> + nr_occupied_ptes = collapse_mthp_count_present(cc, offset, > >> + nr_ptes); > >> + > >> + if (nr_occupied_ptes >= nr_ptes - max_ptes_none) { > >> + int ret; > >> + > >> + collapse_address = address + offset * PAGE_SIZE; > >> + ret = collapse_huge_page(mm, collapse_address, referenced, > >> + unmapped, cc, order); > >> + if (ret == SCAN_SUCCEED) { > >> + collapsed += nr_ptes; > >> + continue; > >> + } > >> + } > >> + > >> +next_order: > >> + if (order > KHUGEPAGED_MIN_MTHP_ORDER) { > > > >Hi Nico, thank you very much for your contributions to this series. > > > >I found a minor issue, for MADV_COLLAPSE, if collapse_huge_page() fails > >for some reason (e.g. allocate folio), it goes to next_order and > >continues splitting to the next small order. However, enabled_orders > >only supports HPAGE_PMD_ORDER, so it keeps runing the split operations > >without any effective work until KHUGEPAGED_MIN_MTHP_ORDER is reached > >before exiting. For khugepaged, e.g. setting only 2MB to always, also > >same phenomenon. > > Yes, but it does no actual work since it is checked after pop up. > > > > >This does not affect the overall functionality of mthp collapse, just > >redundant. > > > >The redundant operations can be easily skipped with the following > >modification. If I miss some thing, please let me know. Thanks! > > > >diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >index 1a25af3d6d0f..fa407cce525c 100644 > >--- a/mm/khugepaged.c > >+++ b/mm/khugepaged.c > >@@ -1574,7 +1574,7 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address, > > } > > > > next_order: > >- if (order > KHUGEPAGED_MIN_MTHP_ORDER) { > >+ if ((BIT(order) - 1) & enabled_orders) { > > const u8 next_order = order - 1; > > const u16 mid_offset = offset + (nr_ptes / 2); > > > > This would stop the iteration if there are other lower enabled order, right? ^^^^ ^^^^^^^^^^^^^^^^^^^ NO :) For more details, please refer to the following information. | Scenario | Old Behavior (order > 2) | New Behavior ((BIT(order)-1) & enabled_orders) | |-------------------------------------|--------------------------|------------------------------------------------| | MADV_COLLAPSE | Splits 9,8,7,...,3 | No split | | khugepaged, only 2MB enabled | Splits 9,8,7,...,3 | No split | | khugepaged, only 2MB + 64KB enabled | Splits 9,8,7,...,3 | Splits 9,8,7,...,5 | | khugepaged, only 32KB enabled | Splits 9,8,7,...,3 | Splits 9,8,7,...,4 | | khugepaged, only 16KB enabled | Splits 9,8,7,...,3 | Splits 9,8,7,...,3 | | khugepaged, all mTHP enabled | Splits 9,8,7,...,3 | Splits 9,8,7,...,3 | -- Cheers, Vernon