From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-232.mta0.migadu.com [91.218.175.232]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 37DF42DA74C for ; Wed, 19 Aug 2026 12:39:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.232 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787143179; cv=none; b=UoMwaYsdOzGMxeHGmHCFpD9A0kcdbd61JzdRVPPLCct4GzIWnOcDBvsc7oynv2yR9qCRO5alrJiufh4rVwbp35lymZpwoElCW/mBgOd09Otl89tsA/VQqI+wwJdc0FQiYOo+lZELGM7cviPkGBUgft1O8gaNVEjFnCm0XTdEmFk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787143179; c=relaxed/simple; bh=/0QneJV1g4MfxResLf7BWzX48FMJzvzv6CTUChLrIPM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PxYHxUWoN1GgRcqd6DeSkUXVwtWPAwqNXia9gtxT7JSjTRL0gkRlSDU6Zqotw4r31MIg+72n0Yglsjp3RPucceJRvK3778llpzSn8lZ76IaEI9KZLSn4+zvlbRAd3OqSnaHG8MEjiJJoQjGl+PtuKtZlONSYxvkEj6Fe9MqTHJY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=GC5g6UdI; arc=none smtp.client-ip=91.218.175.232 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="GC5g6UdI" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=/0QneJV1g4MfxResLf7BWzX48FMJzvzv6CTUChLrIPM=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1787143174; v=1; x=1787747974; b=GC5g6UdIAH+tCxgWyPzCLKp10meDvd9x6w+B4ACF00UNifqAWZtc2zVy+tcHN3klWrB0Tmax wiD6Mg9P5tzHa2+UwAnPFVfBE70GF/AKNnRWDhvsAAEfujRVJHAKZieFnsCcjMfeICnYZQ26ejT mEQW7ZXV3BfWf4mYRdoYnYQ8= X-Envelope-To: linux-kernel@vger.kernel.org Received: from [IPV6:2a03:83e0:1126:4:9d:a05e:5bd8:c200] (2620:10d:c092:500::4:a428) by smtp.migadu.com with ESMTPS id d88dfc952275f4b9; Wed, 19 Aug 2026 12:39:34 +0000 X-Migadu-Flow: FLOW_OUT Message-ID: <4300a9b5-4423-4299-af54-e88bcca5432d@linux.dev> Date: Wed, 19 Aug 2026 13:39:31 +0100 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 v6 03/12] mm: add PMD swap entry splitting support To: "David Hildenbrand (Arm)" , Andrew Morton , 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, dev.jain@arm.com, 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: <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 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!