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Thu, 18 Jun 2026 02:49:05 -0700 (PDT) From: Wang Lian To: sj@kernel.org, akpm@linux-foundation.org Cc: npache@redhat.com, gutierrez.asier@huawei-partners.com, daichaobing@sangfor.com.cn, linux-mm@kvack.org, linux-kernel@vger.kernel.org, lianux.mm@gmail.com, kunwu.chan@gmail.com Subject: [RFC PATCH 5/6] mm/damon/vaddr: implement DAMOS_MTHP_SPLIT handler Date: Thu, 18 Jun 2026 17:48:37 +0800 Message-ID: <20260618094838.32805-6-lianux.mm@gmail.com> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260618094838.32805-1-lianux.mm@gmail.com> References: <20260618094838.32805-1-lianux.mm@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Implement the DAMOS_MTHP_SPLIT action for vaddr-based DAMON operations. Walk the region in PMD-sized aligned chunks, use folio_walk_start() to locate THP folios, and call split_folio_to_order() when the folio order exceeds the target_order. Unlike COLLAPSE which is limited to anonymous memory via collapse_huge_page(), split_folio_to_order() supports both anon and shmem folios. This is critical for tmpfs THP-backed KVM guest memory, where cold and hot pages bundled together in a single PMD THP cause DAMON to overestimate hot regions. The handler holds mmap_read_lock per chunk for VMA lookup and folio_walk_start(), then releases it before the next iteration. split_folio_to_order() does not reacquire mmap locks internally, so this pattern is safe. Co-developed-by: Kunwu Chan Signed-off-by: Kunwu Chan Signed-off-by: Wang Lian --- mm/damon/vaddr.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 68 insertions(+) diff --git a/mm/damon/vaddr.c b/mm/damon/vaddr.c index 2a3757c13bf0..1957e390a277 100644 --- a/mm/damon/vaddr.c +++ b/mm/damon/vaddr.c @@ -934,6 +934,71 @@ static unsigned long damos_va_collapse(struct damon_target *target, return applied; } +static unsigned long damos_va_mthp_split(struct damon_target *target, + struct damon_region *r, struct damos *s, + unsigned long *sz_filter_passed) +{ + unsigned long addr, end, chunk_sz; + unsigned int target_order = s->target_order; + unsigned long applied = 0; + struct mm_struct *mm; + struct vm_area_struct *vma; + struct folio *folio; + struct folio_walk fw; + + mm = damon_get_mm(target); + if (!mm) + return 0; + + chunk_sz = PAGE_SIZE << HPAGE_PMD_ORDER; + addr = ALIGN_DOWN(r->ar.start, chunk_sz); + end = ALIGN(r->ar.end, chunk_sz); + + while (addr < end) { + mmap_read_lock(mm); + vma = find_vma(mm, addr); + /* + * split_folio_to_order() supports both anon and shmem + * folios, so we accept any VMA that has a folio at @addr. + * This covers important use cases like tmpfs THP-backed + * KVM guest memory where cold and hot pages are bundled + * together in a single PMD THP. + */ + if (!vma || addr < vma->vm_start) + goto unlock; + + folio = folio_walk_start(&fw, vma, addr, 0); + if (!folio) + goto unlock; + + if (folio_order(folio) > target_order) { + if (!folio_trylock(folio)) { + folio_walk_end(&fw, vma); + goto unlock; + } + folio_get(folio); + folio_walk_end(&fw, vma); + + if (!split_folio_to_order(folio, target_order)) + applied += chunk_sz; + + folio_unlock(folio); + folio_put(folio); + } else { + folio_walk_end(&fw, vma); + } + +unlock: + *sz_filter_passed += chunk_sz; + addr += chunk_sz; + mmap_read_unlock(mm); + cond_resched(); + } + + mmput(mm); + return applied; +} + static unsigned long damon_va_apply_scheme(struct damon_ctx *ctx, struct damon_target *t, struct damon_region *r, struct damos *scheme, unsigned long *sz_filter_passed) @@ -967,6 +1032,9 @@ static unsigned long damon_va_apply_scheme(struct damon_ctx *ctx, return damos_va_migrate(t, r, scheme, sz_filter_passed); case DAMOS_STAT: return damos_va_stat(t, r, scheme, sz_filter_passed); + case DAMOS_MTHP_SPLIT: + return damos_va_mthp_split(t, r, scheme, + sz_filter_passed); default: /* * DAMOS actions that are not yet supported by 'vaddr'. -- 2.50.1 (Apple Git-155)