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b=apqaJtWfBTqTN+KmE10cgxfUIvzUJ4bFVOwiSnmdSo2kmJJI0K3hd5QiXb+aSWZRT 7eB+2YDtliulbsFu8wuQqer2oOUIo9iRRRBBDgm2XUmFaqU2orT8Gerbp8IHQTJn26 oqDwpFAQPuUHswzAIvrEZyw5icryOY9KUixQ6hDQLrhVOTyECMIsFsRCPxO576Riy+ +Z4DrCo3ELWqzPu6taEr8+LAkcQYQVeLAwDuS/SKwwv5M9S0Q4V4+6JNqhaYxW8Lrw KZCk0sf2fm2xzFkZ8t10fR/NLh8p+UWzvD7M7d42hKkcVtmbLHqEQdZS4O+N6lqZde 22AlX1OWxY2YA== Message-ID: <2553caae-9e0e-42a7-8b61-d1216f1e81fa@kernel.org> Date: Mon, 8 Jun 2026 16:56:37 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v19 11/14] mm/khugepaged: Introduce mTHP collapse support To: Lance Yang , npache@redhat.com Cc: 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, 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, 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, richard.weiyang@gmail.com, 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 References: <20260605161422.213817-12-npache@redhat.com> <20260606102800.26940-1-lance.yang@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: D9BD52000E X-Rspam-User: X-Stat-Signature: 4fqnn4wntxg44mwn4pbayjuna9468in8 X-Rspamd-Server: rspam08 X-HE-Tag: 1780930613-633352 X-HE-Meta: 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 RG+0kDH3 04fwt6p4t7REIj5O8g+9jPE5GR106WUwLQkHNVVE7JxnackovHjXn9WXjN3rC8jSjz602F5PdnmwLrtwyGEKuPhgJ6LM0HsVu6dOyGNuG1Y8C8oLszEacQsk+6skV3EJ8AKaGUtL+fVzsr/cM+L/Cp/3FBCXUpHNFs/ZAhKmWx3rH2bU5E0NFV5GPCwwVUd/MxUhR0KrItFCJRA0skzRboQJwwytgIkPY6X4R/k9OfO46SUxjg9PN/GAoOGSlz3rq7kAWAMxfshWB8+LOtbdoEWvMXPFHtC6y17iNRT8yHLsVXx6UnJDeAnc4mFEi6Ov9nkKGA3jyLH2dRdLMYht67klttyvC0Mu7a1rKOLYMH2SXSLg= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/6/26 12:28, Lance Yang wrote: > > On Fri, Jun 05, 2026 at 10:14:18AM -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 the >> algorithm 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 mthp_collapse() to walk forward through the bitmap and >> determine the best eligible order for each naturally-aligned region. The >> algorithm starts at the beginning of the PMD range and, for each offset, >> tries the highest order that fits the alignment. If the number of >> occupied PTEs in that region satisfies the max_ptes_none threshold for >> that order, a collapse is attempted. On failure, the order is >> decremented and the same offset is retried at the next smaller size. Once >> the smallest enabled order is exhausted (or a collapse succeeds), the >> offset advances past the region just processed, and the next attempt >> starts at the highest order permitted by the new offset's natural >> alignment. >> >> The algorithm works as follows: >> 1) set offset=0 and order=HPAGE_PMD_ORDER >> 2) if the order is not enabled, go to step (5) >> 3) count occupied PTEs in the (offset, order) range using >> bitmap_weight_from() >> 4) if the count satisfies the max_ptes_none threshold, attempt >> collapse; on success, advance to step (6) >> 5) if a smaller enabled order exists, decrement order and retry >> from step (2) at the same offset >> 6) advance offset past the current region and compute the next >> order from the new offset's natural alignment via __ffs(offset), >> capped at HPAGE_PMD_ORDER >> 7) repeat from step (2) until the full PMD range is covered >> >> 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 default mTHP >> collapse to max_ptes_none=0. 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 | 146 +++++++++++++++++++++++++++++++++++++++++++++--- >> 1 file changed, 138 insertions(+), 8 deletions(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index ec886a031952..430047316f43 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -99,6 +99,8 @@ 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 >> + >> struct collapse_control { >> bool is_khugepaged; >> >> @@ -110,6 +112,9 @@ struct collapse_control { >> >> /* nodemask for allocation fallback */ >> nodemask_t alloc_nmask; >> + >> + /* Each bit represents a single occupied (!none/zero) page. */ >> + DECLARE_BITMAP(mthp_present_ptes, MAX_PTRS_PER_PTE); >> }; >> >> /** >> @@ -1440,20 +1445,130 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s >> return result; >> } >> >> +/* Return the highest naturally aligned order that fits at @offset within a PMD. */ >> +static unsigned int max_order_from_offset(unsigned int offset) >> +{ >> + if (offset == 0) >> + return HPAGE_PMD_ORDER; >> + >> + return min_t(unsigned int, __ffs(offset), HPAGE_PMD_ORDER); >> +} >> + >> +/* >> + * 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_present_ptes represents a single occupied (!none/zero) >> + * page. We start at the PMD order and check if it is eligible for collapse; >> + * if not, we check the left and right halves of the PTE page table we are >> + * examining at a lower order. >> + * >> + * 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 enum scan_result 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, max_ptes_none; >> + enum scan_result last_result = SCAN_FAIL; >> + int collapsed = 0; >> + bool alloc_failed = false; >> + unsigned long collapse_address; >> + unsigned int offset = 0; >> + unsigned int order = HPAGE_PMD_ORDER; >> + >> + while (offset < HPAGE_PMD_NR) { >> + nr_ptes = 1UL << order; >> + >> + if (!test_bit(order, &enabled_orders)) >> + goto next_order; >> + >> + max_ptes_none = collapse_max_ptes_none(cc, NULL, order); >> + nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset, >> + offset + nr_ptes); >> + >> + if (nr_occupied_ptes >= nr_ptes - max_ptes_none) { > > Looks broken for swap PTEs in PMD collapse ... > > collapse_scan_pmd() allows them up to max_ptes_swap and record them in > unmapped, but they don't get a bit in mthp_present_ptes. And then > mthp_collapse() does the check above: Right. I assumed this is implicitly handled by the optimization in collapse_scan_pmd: if (enabled_orders != BIT(HPAGE_PMD_ORDER)) max_ptes_none = KHUGEPAGED_MAX_PTES_LIMIT; But we perform the check a second time. > > nr_occupied_ptes >= nr_ptes - max_ptes_none > > So max_ptes_none=0 + 511 present PTEs + one allowed swap PTE won't even > call collapse_huge_page() for PMD order. > > Shouldn't we account for them in the PMD-order check? Something like: > > if (is_pmd_order(order)) > nr_occupied_ptes += unmapped; As an alternative, we could either 1) skip the check there for pmd order (as the check was already done); or 2) introduce+maintain a bitmap that tracks non-present PTEs. @@ -1475,7 +1477,9 @@ static enum scan_result mthp_collapse(struct mm_struct *mm, nr_occupied_ptes = bitmap_weight_from(cc->mthp_present_ptes, offset, offset + nr_ptes); - if (nr_occupied_ptes >= nr_ptes - max_ptes_none) { + /* Check was already done in the caller. */ + if (is_pmd_order(order) || + nr_occupied_ptes >= nr_ptes - max_ptes_none) { enum scan_result ret; collapse_address = address + offset * PAGE_SIZE; 2) would probably be cleanest long-term. -- Cheers, David