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From: Usama Arif In-Reply-To: <7c8b0e38-f13e-449d-a2a7-de1032b831dd@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Queue-Id: 8113C10000C X-Rspam-User: X-Stat-Signature: m85a4eryt13suaqqruuwb6wbpuaq1rw5 X-Rspamd-Server: rspam04 X-HE-Tag: 1784887540-532755 X-HE-Meta: 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 zQvGK4Sy b5Ws+OSxjpiTpiyqcRJhro5Xc+4UN5Wiv2iI5QZ7iJvBanHAHVGr4BWMpZn5niwsqaojDKMe1soGTdwntZnyWbVxDxcbzpOf9Nb6vmnmQWmVoMfE5/KbrPQqS0A4YrOKo6ysDeNnVSuIrdhAOTxOTXOHtz5t5ibtRWlTjV0GL/QFu+EH4ozHE64pVtUivsfM795gChZoPcnSfdGgnA3lcPClFjMdoXqRLfBMbFOdagbHXR1nh8rVS6++eSv+xD2uXnVB0L08jKoiXnrlOY0GaetVzsjkviDg+VB4rHltqyPfQZuU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 24/07/2026 07:52, Dev Jain wrote: > > > On 22/07/26 8:49 pm, Usama Arif wrote: >> Add a swap branch in __split_huge_pmd_locked() that splits a PMD swap >> entry into 512 PTE swap entries. Unlike migration splits, no folio >> reference is needed because swap entries point to swap slots, not > > Confusing wording. swap entries need reference of swap slots instead > of folios. I would drop the mention of migration here. I added about migration because its the other if part of the if else and was a good comparison. Will remove it. Thanks! > >> 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 04e8a6b55343..9819c0ae228a 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3210,6 +3210,14 @@ 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 >> @@ -3346,6 +3354,25 @@ 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); >> + } > > All these for loops could later benefit from the set_softleaf_ptes() helper: > https://lore.kernel.org/all/20260723070905.3422276-6-dev.jain@arm.com/ > > >> } else { >> pte_t entry; >> >> @@ -3373,7 +3400,7 @@ 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); >