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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 References: <20260818131202.494754-1-usama.arif@linux.dev> <20260818131202.494754-4-usama.arif@linux.dev> Content-Language: en-US From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: cof54zs3ss8bhuq8edwf37htg75cge5i X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 949FF1C0008 X-HE-Tag: 1787143175-81182 X-HE-Meta: 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 YoqjEJ38 zndwXvF4RBIY3OaVrYqz7sXdKOFgMv1j04mXZB1qTMULQnDbxKjgibH6eJgi9RyjHWl4RN/YMov92jkxaArvl+pJ/Zzon5v4VbOFtCRgaSgD6r7HJYDG7RrJ0eaBIAxEJTFQTT3SePsNKih5YPnrtGfxDE33/IhHr8a/vgHveY0bYiB4na86y++S1HKS/3zlhOtTSRwEWUppH+fSrTjtEHz3xU2afzu2Tyr7LIsEyNdP3Q4bJ4ezwlfriYMBsq5cCBWnQmuny+sDTURb5Kn4DxpyNwcXaakNTbHFQHF7JoI9kQUk= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 18/08/2026 18:53, David Hildenbrand (Arm) wrote: > On 8/18/26 15:09, Usama Arif wrote: >> 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; > > It's odd to VM_WARN_ON_ONCE() and then set freeze=false; > > I'd just add the comment above the VM_WARN_ON_ONCE() and drop the =false. > > freeze=true really only applies during page migration, where swap entries don't > apply. > Ack > I think it's time to clean that up ... that is > > a) Expose a helper called split_pmd_to_migration_entries() that is only used by > code that installs migration entries. > > b) Hide that "freeze" flag from all other file-external functions > > c) Rename the boolean to "use_migration_entries" > > Then it's rather clear in this code that this should never happen. > Thanks! I have done this now for next revision. The migration choice is now private as use_migration_entries, the normal split APIs no longer expose it, and the rmap migration path uses split_pmd_to_migration_entries(). m > > >> + 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); > > No existing code uses "sl_entry". Maybe just call it "pmd_swp_entry"/"swp_entry" > and below "pte_swp_entry". > Thanks! Will do in next revision > > > Apart from that nothing jumped at me :) > Thanks for the review!