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([177.73.136.69]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-29526871693sm30113185ad.20.2025.10.31.10.48.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 31 Oct 2025 10:48:48 -0700 (PDT) From: Pedro Demarchi Gomes To: David Hildenbrand , Andrew Morton Cc: Xu Xin , Chengming Zhou , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Pedro Demarchi Gomes Subject: [PATCH v2 2/3] ksm: perform a range-walk in break_ksm Date: Fri, 31 Oct 2025 14:46:24 -0300 Message-ID: <20251031174625.127417-3-pedrodemargomes@gmail.com> X-Mailer: git-send-email 2.43.0 In-Reply-To: <20251031174625.127417-1-pedrodemargomes@gmail.com> References: <20251031174625.127417-1-pedrodemargomes@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: quoted-printable Content-Type: text/plain; charset="utf-8" Make break_ksm() receive an address range and change break_ksm_pmd_entry() to perform a range-walk and return the address of the first ksm page found. This change allows break_ksm() to skip unmapped regions instead of iterating every page address. When unmerging large sparse VMAs, this significantly reduces runtime. In a benchmark unmerging a 32 TiB sparse virtual address space where only one page was populated, the runtime dropped from 9 minutes to less then 5 seconds. Suggested-by: David Hildenbrand Signed-off-by: Pedro Demarchi Gomes Acked-by: David Hildenbrand (Red Hat) --- mm/ksm.c | 88 ++++++++++++++++++++++++++++++-------------------------- 1 file changed, 48 insertions(+), 40 deletions(-) diff --git a/mm/ksm.c b/mm/ksm.c index 922d2936e206..64d66699133d 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -607,35 +607,55 @@ static inline bool ksm_test_exit(struct mm_struct *mm) return atomic_read(&mm->mm_users) =3D=3D 0; } =20 -static int break_ksm_pmd_entry(pmd_t *pmd, unsigned long addr, unsigned lo= ng next, +struct break_ksm_arg { + unsigned long addr; +}; + +static int break_ksm_pmd_entry(pmd_t *pmdp, unsigned long addr, unsigned l= ong end, struct mm_walk *walk) { - struct folio *folio =3D NULL; + unsigned long *found_addr =3D (unsigned long *) walk->private; + struct mm_struct *mm =3D walk->mm; + pte_t *start_ptep, *ptep; spinlock_t *ptl; - pte_t *pte; - pte_t ptent; - int ret; + int found =3D 0; =20 - pte =3D pte_offset_map_lock(walk->mm, pmd, addr, &ptl); - if (!pte) + if (ksm_test_exit(walk->mm)) return 0; - ptent =3D ptep_get(pte); - if (pte_present(ptent)) { - folio =3D vm_normal_folio(walk->vma, addr, ptent); - } else if (!pte_none(ptent)) { - swp_entry_t entry =3D pte_to_swp_entry(ptent); =20 - /* - * As KSM pages remain KSM pages until freed, no need to wait - * here for migration to end. - */ - if (is_migration_entry(entry)) - folio =3D pfn_swap_entry_folio(entry); + if (signal_pending(current)) + return -ERESTARTSYS; + + start_ptep =3D pte_offset_map_lock(mm, pmdp, addr, &ptl); + if (!start_ptep) + return 0; + + for (ptep =3D start_ptep; addr < end; ptep++, addr +=3D PAGE_SIZE) { + pte_t pte =3D ptep_get(ptep); + struct folio *folio =3D NULL; + + if (pte_present(pte)) { + folio =3D vm_normal_folio(walk->vma, addr, pte); + } else if (!pte_none(pte)) { + swp_entry_t entry =3D pte_to_swp_entry(pte); + + /* + * As KSM pages remain KSM pages until freed, no need to wait + * here for migration to end. + */ + if (is_migration_entry(entry)) + folio =3D pfn_swap_entry_folio(entry); + } + /* return 1 if the page is an normal ksm page or KSM-placed zero page */ + found =3D (folio && folio_test_ksm(folio)) || is_ksm_zero_pte(pte); + if (found) { + *found_addr =3D addr; + goto out_unlock; + } } - /* return 1 if the page is an normal ksm page or KSM-placed zero page */ - ret =3D (folio && folio_test_ksm(folio)) || is_ksm_zero_pte(ptent); - pte_unmap_unlock(pte, ptl); - return ret; +out_unlock: + pte_unmap_unlock(ptep, ptl); + return found; } =20 static const struct mm_walk_ops break_ksm_ops =3D { @@ -661,7 +681,8 @@ static const struct mm_walk_ops break_ksm_lock_vma_ops = =3D { * of the process that owns 'vma'. We also do not want to enforce * protection keys here anyway. */ -static int break_ksm(struct vm_area_struct *vma, unsigned long addr, bool = lock_vma) +static int break_ksm(struct vm_area_struct *vma, unsigned long addr, + unsigned long end, bool lock_vma) { vm_fault_t ret =3D 0; const struct mm_walk_ops *ops =3D lock_vma ? @@ -671,11 +692,9 @@ static int break_ksm(struct vm_area_struct *vma, unsig= ned long addr, bool lock_v int ksm_page; =20 cond_resched(); - ksm_page =3D walk_page_range_vma(vma, addr, addr + 1, ops, NULL); - if (WARN_ON_ONCE(ksm_page < 0)) + ksm_page =3D walk_page_range_vma(vma, addr, end, ops, &addr); + if (ksm_page <=3D 0) return ksm_page; - if (!ksm_page) - return 0; ret =3D handle_mm_fault(vma, addr, FAULT_FLAG_UNSHARE | FAULT_FLAG_REMOTE, NULL); @@ -761,7 +780,7 @@ static void break_cow(struct ksm_rmap_item *rmap_item) mmap_read_lock(mm); vma =3D find_mergeable_vma(mm, addr); if (vma) - break_ksm(vma, addr, false); + break_ksm(vma, addr, addr + PAGE_SIZE, false); mmap_read_unlock(mm); } =20 @@ -1072,18 +1091,7 @@ static void remove_trailing_rmap_items(struct ksm_rm= ap_item **rmap_list) static int unmerge_ksm_pages(struct vm_area_struct *vma, unsigned long start, unsigned long end, bool lock_vma) { - unsigned long addr; - int err =3D 0; - - for (addr =3D start; addr < end && !err; addr +=3D PAGE_SIZE) { - if (ksm_test_exit(vma->vm_mm)) - break; - if (signal_pending(current)) - err =3D -ERESTARTSYS; - else - err =3D break_ksm(vma, addr, lock_vma); - } - return err; + return break_ksm(vma, start, end, lock_vma); } =20 static inline --=20 2.43.0