From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id E0F81224B1E for ; Fri, 24 Jul 2026 06:52:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784875979; cv=none; b=sNbmBn97WrqCmqHI/cqNIDRgDujPvfD23KM56OP/eKeF1eXpHg2FYo9Q7x9xJw5qPpZsW94qy4Jz9fl/34sOAWIzPY1cawfokGfiBaRsGO5mkHOYohLLROPlQHJdaQu1vOFZAvfChXAVJ9sISVGSTsibXQQSoPSmR5HHkKkheo0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784875979; c=relaxed/simple; bh=nAITBKBOA7eXQfiFsE4Td1VsUBH8uj5AMHgZrnkOQhc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=k0tLTS/bPU2hjo4HY3TPwIvW9H4ri8K3T1DSZjnolcdYmSVc67+f5hMgdgabdC33Oxcleeb68Gh5yGRPJ0Z6XVOukw9oyuUJ1fZCNPeKfWEEpVtMTz5CgYNtn54lXcZ8HdcnsDJbz5hikYf6N4iWz+E7ZGogyQ+oPs+VgPXSBkE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=qLi8XCCE; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="qLi8XCCE" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7FF4A1477; Thu, 23 Jul 2026 23:52:48 -0700 (PDT) Received: from [10.164.19.44] (unknown [10.164.19.44]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3B1093F66F; Thu, 23 Jul 2026 23:52:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1784875972; bh=nAITBKBOA7eXQfiFsE4Td1VsUBH8uj5AMHgZrnkOQhc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=qLi8XCCEoA2LvCejbJTu+56nK5kjH9Nhn2ahHYJXSen00eQmhmXWctwb46aWktEue AEgO5CEge1tNKb0TgDNqeD+FWNJrGd9a8uKt9TQLU4PZ8yK3yqcv4EXCrR7hRhtAcN 5n+U7QoAFxaRItNOFpCxt5rXw9kN2n037lhZui38= Message-ID: <7c8b0e38-f13e-449d-a2a7-de1032b831dd@arm.com> Date: Fri, 24 Jul 2026 12:22:42 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 02/11] mm: add PMD swap entry splitting support To: Usama Arif , Andrew Morton , david@kernel.org, chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org Cc: ying.huang@linux.alibaba.com, Baoquan He , willy@infradead.org, youngjun.park@lge.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, alex@ghiti.fr, kas@kernel.org, baohua@kernel.org, baolin.wang@linux.alibaba.com, Nico Pache , "Liam R. 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: <20260722152043.2273289-1-usama.arif@linux.dev> <20260722152043.2273289-3-usama.arif@linux.dev> Content-Language: en-US From: Dev Jain In-Reply-To: <20260722152043.2273289-3-usama.arif@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. > 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);