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c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1790130693; v=1; x=1790735493; b=jTR26CvWm9i3THVqQVtxlkEfN2W6uyQGYEufg+iJ5G8jJZzOzPXGL9avii4KnxRHGXrbeaon Q8VrtPoQDphID+IKYhR/qz8ZFobb4JL8OyTyZjkj/vUFF0EPgrk7jEtmkekcHJlAASgYSPPBAbe 9RT9RF6ALfN/g1aIoM/mdmcw= X-Envelope-To: linuxppc-dev@lists.ozlabs.org Received: by smtp.migadu.com with ESMTPS id b86f8e4163d69484; Wed, 23 Sep 2026 02:24:43 +0000 X-Mizu-Trace-ID: b86f8e4163d69484 X-Migadu-Flow: FLOW_OUT From: Lance Yang To: ljs@kernel.org 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, lance.yang@linux.dev, 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 Date: Wed, 23 Sep 2026 10:24:36 +0800 Message-ID: <20260923022436.61930-1-lance.yang@linux.dev> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20260922-rcu-pagetable-freeing-v4-1-fe1ad1f1e303@kernel.org> References: <20260922-rcu-pagetable-freeing-v4-1-fe1ad1f1e303@kernel.org> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On Tue, Sep 22, 2026 at 04:35:32PM +0100, Lorenzo Stoakes (ARM) wrote: >collapse_huge_page() deposits a PTE page table on PMD collapse in order >that it can be utilised for subsequent split operations, meaning that those >operations do not need to perform an allocation (as they are in a context >where it might be unwise). > >However the PTE page table which is deposited is the one which is currently >mapped by the PMD entry that is in the process of being collapsed. > >Once deposited, the PTE page table may be used in a split of any other >unrelated PMD entry. > >This is currently not an issue as this operation is performed with VMA/mmap >write lock + anon rmap locks held, so ordinary page table walkers will >never accidentally end up walking the wrong thing, and GUP-fast is >protected by an IPI via tlb_remove_table_sync_one(). > >However, the series to which this commit belongs implements RCU-safe page >table traversal, at which point this becomes problematic. > >This can be resolved by using pte_offset_map_lock() which gates on a PTE >PTL and a pmd_same() check, but lockless walks are unsafe as things stand. > >Resolve this by simply allocating a new, zeroed, PTE page table to deposit >at the point of collapse. > >This path is already costly and an allocation has already been performed >for the huge folio, so this allocation is statistical noise in terms of >performance and memory usage at this point. > >With this PTE page table deposited, RCU-free the existing PTE page table >so it is safe for page table walkers to traverse within a grace period. > >This also brings this deposit case in line with all other page table >deposit logic which deposit a fresh page table. > >Additionally, this was the only place in the kernel that displaced a page >table like this, so eliminating it also helps consistency. > >An edge case for deposit exists for powerpc and its hash-based MMU - it >stores hash slot data in deposited page tables and zeroes them on withdraw, >so a zeroed deposited page table works correctly for it. > >Since khugepaged runs as a kernel thread, do a little dance in >alloc_deposit_pte() to correctly charge the allocation. > >This is already done for the folio allocation via alloc_charge_folio() but >no such wrapper exists for a page table allocation. > >Signed-off-by: Lorenzo Stoakes (ARM) >--- > 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