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b=ZktHXeeVHUdVX84HQm0s5eTwxdjCgV+/1qk2ubVk78nV17YKtFkiFazEoAUDTsoXc M9HCiTHUQ7J69MHTAi4rLJyLVgYuyNbgSZJz+v4NLmOvNpB/bSTDv4lHPBz5Q4Gbdm cw1DRtF+qN9ZW+I8gS7aCogIW950qmkzAO26UzPGSnMSRMFt2p6bBCYM4H0oUIHroz jtcloLZ16zE9tgGKnollzIYQBpif6cPAzGzXl2ECvovYLZTpgksOX/6bxn4RAYnBNk G5I4qUhKiokal3pqf0QgZTJFeUKOfYndJa8L1dZhaKM+4yq8dKa2tMGYzCO83fHsqT OShYOOxxd3Q4Q== Date: Wed, 23 Sep 2026 08:09:58 +0100 From: "Lorenzo Stoakes (ARM)" To: Lance Yang Cc: akpm@linux-foundation.org, david@kernel.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, nico.pache@linux.dev, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, usama.arif@linux.dev, kas@kernel.org, guoren@kernel.org, bcain@kernel.org, geert@linux-m68k.org, dinguyen@kernel.org, schuster.simon@siemens-energy.com, jonas@southpole.se, stefan.kristiansson@saunalahti.fi, shorne@gmail.com, dalias@libc.org, glaubitz@physik.fu-berlin.de, pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu, alex@ghiti.fr, linux@armlinux.org.uk, vgupta@kernel.org, monstr@monstr.eu, chris@zankel.net, jcmvbkbc@gmail.com, will@kernel.org, aneesh.kumar@kernel.org, npiggin@gmail.com, peterz@infradead.org, davem@davemloft.net, andreas@gaisler.com, richard.henderson@linaro.org, mattst88@gmail.com, linmag7@gmail.com, catalin.marinas@arm.com, mark.rutland@arm.com, chenhuacai@kernel.org, kernel@xen0n.name, tsbogend@alpha.franken.de, James.Bottomley@hansenpartnership.com, deller@gmx.de, maddy@linux.ibm.com, mpe@ellerman.id.au, chleroy@kernel.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, richard@nod.at, anton.ivanov@cambridgegreys.com, johannes@sipsolutions.net, tglx@kernel.org, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, arnd@arndb.de, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, jgg@ziepe.ca, jhubbard@nvidia.com, peterx@redhat.com, ysato@users.sourceforge.jp, shakeel.butt@linux.dev, corbet@lwn.net, rdunlap@infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org, linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org, linux-sh@vger.kernel.org, linux-riscv@lists.infradead.org, linux-arm-kernel@lists.infradead.org, linux-snps-arc@lists.infradead.org, linux-arch@vger.kernel.org, sparclinux@vger.kernel.org, linux-alpha@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, linux-um@lists.infradead.org, hughd@google.com, qi.zheng@linux.dev, linux-doc@vger.kernel.org Subject: Re: [PATCH v4 01/12] mm/khugepaged: deposit a newly allocated page table on collapse Message-ID: References: <20260922-rcu-pagetable-freeing-v4-1-fe1ad1f1e303@kernel.org> <20260923022436.61930-1-lance.yang@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260923022436.61930-1-lance.yang@linux.dev> X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 82D32120002 X-Stat-Signature: 9wk636hjispun356cirerg8aw7upfg6s X-HE-Tag: 1790147423-424131 X-HE-Meta: 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 b+e3tPCS 5sYiJCLK+ltAA2Qvmpdm4X/6YLCjD1tyy5noEEHRIZECSCO5/3EhxwPGMvHPnKfoZmDJZmaf81YgN/aQA5viZmPgDcxpf01a9Bw1H3hxv9dfkikuy+z9tkFn6eEE1Iic2LQabmgAvAKWuV4ZGaUf1iqiDi05GllWIy2xHZr9D7x4tZFQXeHHZ+5C15R6Wihskyoo3qOFQ1ATg/bCXK0PyUDYpYc+NhNxQ7CT2FexqsQwna3wLP0zWhnHxpXpgAEGEYgky+YM4EYlB+DZ7UpQ5WtIDg4tFI8cTHggvGmikbsE8B/HpEUYrv4bOkPw5N7ITxQvRb7jKI+YmaKc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 23, 2026 at 10:24:36AM +0800, Lance Yang wrote: > > mm/khugepaged.c | 32 ++++++++++++++++++++++++++++++-- > > 1 file changed, 30 insertions(+), 2 deletions(-) > > > >diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >index f49a6710933b..dab421f8233e 100644 > >--- a/mm/khugepaged.c > >+++ b/mm/khugepaged.c > >@@ -1278,6 +1278,23 @@ static enum scan_result alloc_charge_folio(struct folio **foliop, struct mm_stru > > return SCAN_SUCCEED; > > } > > > >+static pgtable_t alloc_deposit_pte(struct mm_struct *mm) > >+{ > >+ /* > >+ * khugepaged is run from a kernel thread, so need to manually set the > >+ * correct memcg so the allocation gets charged correctly. > >+ */ > >+ struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm); > >+ struct mem_cgroup *old_memcg = set_active_memcg(memcg); > >+ pgtable_t pgtable; > >+ > >+ pgtable = pte_alloc_one(mm); > >+ > >+ set_active_memcg(old_memcg); > >+ mem_cgroup_put(memcg); > >+ return pgtable; > >+} > >+ > > /* > > * collapse_huge_page() expects the mmap_lock to be unlocked before entering and > > * will always return with the lock unlocked, to avoid holding the mmap_lock > >@@ -1293,7 +1310,7 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s > > LIST_HEAD(compound_pagelist); > > pmd_t *pmd, _pmd; > > pte_t *pte = NULL; > >- pgtable_t pgtable; > >+ pgtable_t pgtable = NULL; > > struct folio *folio; > > spinlock_t *pmd_ptl, *pte_ptl; > > enum scan_result result = SCAN_FAIL; > >@@ -1310,6 +1327,12 @@ static enum scan_result collapse_huge_page(struct mm_struct *mm, unsigned long s > > goto out_nolock; > > } > > > >+ if (is_pmd_order(order)) { > >+ pgtable = alloc_deposit_pte(mm); > >+ if (!pgtable) > >+ goto out_nolock; > > One small thing: result is still SCAN_SUCCEED after alloc_charge_folio(), > > result = alloc_charge_folio(&folio, mm, cc, order); > if (result != SCAN_SUCCEED) > goto out_nolock; > > if (folio_memcg_alloc_deferred(folio)) { > result = SCAN_ALLOC_HUGE_PAGE_FAIL; > goto out_nolock; > } > > if (is_pmd_order(order)) { > pgtable = alloc_deposit_pte(mm); > if (!pgtable) > goto out_nolock; > } > > so if alloc_deposit_pte() fails, collapse_huge_page() returns success > without installing a PMD. > > I see sashiko pointed that out too :) > > Cheers, Lance Sigh ack yeah oops :) I think SCAN_ALLOC_HUGE_PAGE_FAIL is the closest though it's not allocating a huge page so much as the page table, but gets the correct MADV_COLLAPSE -ENOMEM back. (Another case of local AI review not catchign something btw! Sashiko does lean towards over-reporting/hallucinating but catches more) Will ask Andrew to update in reply to this patch as a quick thing rather than respin so quick, and fix up on any respin. -- Cheers, Lorenzo