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Howlett , ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, yosry@kernel.org, kernel-team@meta.com, Usama Arif Subject: [PATCH v6 03/12] mm: add PMD swap entry splitting support Date: Tue, 18 Aug 2026 06:09:44 -0700 Message-ID: <20260818131202.494754-4-usama.arif@linux.dev> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260818131202.494754-1-usama.arif@linux.dev> References: <20260818131202.494754-1-usama.arif@linux.dev> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: B95E81C0013 X-Stat-Signature: 98sqetbtmw6pxh8ee9u79ez14a45c9rd X-HE-Tag: 1787058742-982823 X-HE-Meta: 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 NQcuBJm5 qSgIygZfLl1mLLfrDq5RORfZMG9vqJ9thWmn4itLJZsNmzPSo0wTdkyHO5Pna+qmS5Y7begpLBU/yMDYqOV0fKqRwAfp3+s5jE1aLrCZWG2S35OTZ3GXLVlhoj8HidFLSZtmhZGGSQhBG1/gtDh98fYQoCrj0dEgTDVIKHiP9SOXr7mXqI1ws4BhZhNUA8YgPhGvq+VCl1gHiNI1nF6EMj8bYhWk00ra2K+b8Ntnc8OlPd1eUDdkw9T2Dg78wsKwreAzjw6PGnPmGsQSZ+Z8Gi9r1Xw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Add a swap branch in __split_huge_pmd_locked() that splits a PMD swap entry into 512 PTE swap entries. No folio reference is needed because swap entries point to swap slots rather than pages. Each PTE inherits the correct sub-slot offset and preserves soft_dirty, uffd_wp, and exclusive flags. The folio_remove_rmap_pmd() gate at the end must inspect old_pmd rather than *pmd: for a present THP split, *pmd has already been cleared by pmdp_invalidate(), and that invalidated bit pattern can decode as a plausible swap entry. This branch is reached from the explicit __split_huge_pmd() callers that hit a non-present PMD: partial-range mprotect / munmap, the wp_huge_pmd() PMD-COW fallback, and the swap-in / swapoff fallbacks added in later patches when the cached folio is no longer PMD-sized. page_vma_mapped_walk() does not iterate PMD swap entries, so try_to_unmap_one() and try_to_migrate_one() do not reach this branch and freeze=true cannot occur in this branch today. page and folio are therefore left uninitialized in the swap branch; a VM_WARN_ON_ONCE(freeze) catches any future caller that breaks this invariant before the freeze path dereferences page_to_pfn(page + i) or put_page(page). Signed-off-by: Usama Arif --- mm/huge_memory.c | 29 ++++++++++++++++++++++++++++- 1 file changed, 28 insertions(+), 1 deletion(-) diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 1b6b0aa2baa3b..a473e85d30f51 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -3252,6 +3252,14 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, folio_add_anon_rmap_ptes(folio, page, HPAGE_PMD_NR, vma, haddr, rmap_flags); } + } else if (pmd_is_swap_entry(*pmd)) { + VM_WARN_ON_ONCE(freeze); + /* Swap entries have no page for the migration freeze path. */ + freeze = false; + old_pmd = *pmd; + soft_dirty = pmd_swp_soft_dirty(old_pmd); + uffd_wp = pmd_swp_uffd(old_pmd); + anon_exclusive = pmd_swp_exclusive(old_pmd); } else { /* * Up to this point the pmd is present and huge and userland has @@ -3388,6 +3396,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, VM_WARN_ON(!pte_none(ptep_get(pte + i))); set_pte_at(mm, addr, pte + i, entry); } + } else if (pmd_is_swap_entry(old_pmd)) { + softleaf_t sl_entry = softleaf_from_pmd(old_pmd); + pte_t swp_pte; + swp_entry_t sub_entry; + + for (i = 0, addr = haddr; i < HPAGE_PMD_NR; + i++, addr += PAGE_SIZE) { + sub_entry = swp_entry(swp_type(sl_entry), + swp_offset(sl_entry) + i); + swp_pte = swp_entry_to_pte(sub_entry); + if (soft_dirty) + swp_pte = pte_swp_mksoft_dirty(swp_pte); + if (uffd_wp) + swp_pte = pte_swp_mkuffd(swp_pte); + if (anon_exclusive) + swp_pte = pte_swp_mkexclusive(swp_pte); + VM_WARN_ON(!pte_none(ptep_get(pte + i))); + set_pte_at(mm, addr, pte + i, swp_pte); + } } else { pte_t entry; @@ -3415,7 +3442,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd, } pte_unmap(pte); - if (!pmd_is_migration_entry(*pmd)) + if (!pmd_is_migration_entry(old_pmd) && !pmd_is_swap_entry(old_pmd)) folio_remove_rmap_pmd(folio, page, vma); if (freeze) put_page(page); -- 2.53.0-Meta