From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4CE862EAB61; Mon, 27 Apr 2026 20:13:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777320829; cv=none; b=bZyR1mFIURCe8QzPEkrIjBIcwGv0gfBZQD6Dir2IZjtWpevpPW+bBERNMPnbF95y/NuN9kWl9m73mg0X8L5HVOVl5ts/td1ZB6boIeGEOeJh3YkHpzy+g25GfC0Rp+FuCyBM89aBGQzpHypUcaCvObTLRYrKyJAHpFoZsVbWuWo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777320829; c=relaxed/simple; bh=xRwli7GW6thWpU6EobkQOMk6t9ZJU2wEmi/d4jlIFl4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=itRDWLxTiueySvOf9MWN54wk+iCEZMYKPqz2Sxz27RSRmj1/zCG8+NVAahs+/XnNFiMCvCWvSsmG4R+F/Xdb/IMxxyCCPnd7VRg4KUYgFyjKVHTbGJ1+gXJ6lu55ClndkX5MRWblFaLxXgIVcE7nmPZeUWRJMOIkN+P3lCG5WDg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=cyos5hQ7; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="cyos5hQ7" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5433AC19425; Mon, 27 Apr 2026 20:13:31 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777320828; bh=xRwli7GW6thWpU6EobkQOMk6t9ZJU2wEmi/d4jlIFl4=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=cyos5hQ7hZFe9Z2mkBILoxF+VUCYkf2nLEB1NHVkFWDBTNAcK5xef1Iq/xLiKSXIO jLfo6+LpHwn3NqCk316tOl/kynQgFx/CqR4oEeviuRq/+ITyiBAgj9FqmFJ6rA3saw movh6YS789KqwkB/L5JqwzGF9NRcBMHnEXfJDzVsTUivXWqcXRHvLB+27ZUdQcrSNC e0EGwegRgyTOcPyEkyQBTQ9R0RK7r4yMRgH+UrY117rVaHYMhKPCERBrB0bVxPnRTr oICm0dq4m/O7XjgBLSBZGor4R0hD17ZMrfvyxfoU8nvOjvICk2b0UBbZf5mC3vPStQ ItnxEIRqHHo7Q== Message-ID: <6025c087-378b-4754-b1f2-3c5448b49d6c@kernel.org> Date: Mon, 27 Apr 2026 22:13:27 +0200 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 7.2 v16 05/13] mm/khugepaged: generalize collapse_huge_page for mTHP collapse To: Nico Pache , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org Cc: aarcange@redhat.com, akpm@linux-foundation.org, anshuman.khandual@arm.com, apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, byungchul@sk.com, catalin.marinas@arm.com, cl@gentwo.org, corbet@lwn.net, dave.hansen@linux.intel.com, dev.jain@arm.com, gourry@gourry.net, hannes@cmpxchg.org, hughd@google.com, jack@suse.cz, jackmanb@google.com, jannh@google.com, jglisse@google.com, joshua.hahnjy@gmail.com, kas@kernel.org, lance.yang@linux.dev, Liam.Howlett@oracle.com, ljs@kernel.org, mathieu.desnoyers@efficios.com, matthew.brost@intel.com, mhiramat@kernel.org, mhocko@suse.com, peterx@redhat.com, pfalcato@suse.de, rakie.kim@sk.com, raquini@redhat.com, rdunlap@infradead.org, richard.weiyang@gmail.com, rientjes@google.com, rostedt@goodmis.org, rppt@kernel.org, ryan.roberts@arm.com, shivankg@amd.com, sunnanyong@huawei.com, surenb@google.com, thomas.hellstrom@linux.intel.com, tiwai@suse.de, usamaarif642@gmail.com, vbabka@suse.cz, vishal.moola@gmail.com, wangkefeng.wang@huawei.com, will@kernel.org, willy@infradead.org, yang@os.amperecomputing.com, ying.huang@linux.alibaba.com, ziy@nvidia.com, zokeefe@google.com References: <20260419185750.260784-1-npache@redhat.com> <20260419185750.260784-6-npache@redhat.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/19/26 20:57, Nico Pache wrote: > Pass an order and offset to collapse_huge_page to support collapsing anon > memory to arbitrary orders within a PMD. order indicates what mTHP size we > are attempting to collapse to, and offset indicates were in the PMD to > start the collapse attempt. > > For non-PMD collapse we must leave the anon VMA write locked until after > we collapse the mTHP-- in the PMD case all the pages are isolated, but in > the mTHP case this is not true, and we must keep the lock to prevent > access/changes to the page tables. This can happen if the rmap walkers hit > a pmd_none while the PMD entry is currently unavailable due to being > temporarily removed during the collapse phase. > > Signed-off-by: Nico Pache > --- > mm/khugepaged.c | 103 +++++++++++++++++++++++++++--------------------- > 1 file changed, 57 insertions(+), 46 deletions(-) > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 283bb63854a5..ff6f9f1883ed 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -1198,42 +1198,36 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru > return SCAN_SUCCEED; > } > > -static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long address, > - int referenced, int unmapped, struct collapse_control *cc) > +static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long start_addr, > + int referenced, int unmapped, struct collapse_control *cc, > + unsigned int order) > { > LIST_HEAD(compound_pagelist); > pmd_t *pmd, _pmd; > - pte_t *pte; > + pte_t *pte = NULL; > pgtable_t pgtable; > struct folio *folio; > spinlock_t *pmd_ptl, *pte_ptl; > enum scan_result result = SCAN_FAIL; > struct vm_area_struct *vma; > struct mmu_notifier_range range; > + bool anon_vma_locked = false; > + const unsigned long pmd_addr = start_addr & HPAGE_PMD_MASK; > + const unsigned long end_addr = start_addr + (PAGE_SIZE << order); In general, const read better when they are at the very top of this list. > > - VM_BUG_ON(address & ~HPAGE_PMD_MASK); > - > - /* > - * Before allocating the hugepage, release the mmap_lock read lock. > - * The allocation can take potentially a long time if it involves > - * sync compaction, and we do not need to hold the mmap_lock during > - * that. We will recheck the vma after taking it again in write mode. > - */ > - mmap_read_unlock(mm); > - You should spell out that locking change (moving it to the caller), and why it is required, in the patch description. I'd even have put this into a separate patch, as it's independent to the order-passing changes. [...] > */ > __folio_mark_uptodate(folio); > - pgtable = pmd_pgtable(_pmd); > - > spin_lock(pmd_ptl); > - BUG_ON(!pmd_none(*pmd)); > - pgtable_trans_huge_deposit(mm, pmd, pgtable); > - map_anon_folio_pmd_nopf(folio, pmd, vma, address); > + WARN_ON_ONCE(!pmd_none(*pmd)); > + if (is_pmd_order(order)) { /* PMD collapse */ > + pgtable = pmd_pgtable(_pmd); > + pgtable_trans_huge_deposit(mm, pmd, pgtable); > + map_anon_folio_pmd_nopf(folio, pmd, vma, pmd_addr); > + } else { /* mTHP collapse */ Do both these comments (PMD collapse ...) really add any value? I'd say it's pretty self-documenting code already. > + map_anon_folio_pte_nopf(folio, pte, vma, start_addr, /*uffd_wp=*/ false); > + smp_wmb(); /* make PTEs visible before PMD. See pmd_install() */ > + pmd_populate(mm, pmd, pmd_pgtable(_pmd)); > + } > spin_unlock(pmd_ptl); > > folio = NULL; > > result = SCAN_SUCCEED; > out_up_write: > + if (anon_vma_locked) > + anon_vma_unlock_write(vma->anon_vma); > + if (pte) > + pte_unmap(pte); -- Cheers, David