From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-185.mta0.migadu.com (out-185.mta0.migadu.com [91.218.175.185]) (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 CE3E8324B1F for ; Fri, 27 Mar 2026 14:34:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.185 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774622081; cv=none; b=qgApwTWo3CKLciRhZVxjusgl0YX/j8dGjAOFf0sMHG59UDf/3vmBrianGnQdcAjaNs9Ddyg6Jv78Tww7yOzutKSpaDyNseUEVjjUOyHZ1+/mioLbuHkWB2W7jX7axhUafNRokc2pYplYDtsily0Z/FnGo5+9UtnBrtdEVgpjh/w= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774622081; c=relaxed/simple; bh=Iz/TReWp9gP0uJWCpmK+DDKJPT4AeIY0TGV49Ip5RdU=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VP2vxdnCV3aJOuoAlgc6qwGuMrenMWSQn8pVGZcbZZW79elN3WW8KDEdb9mxMK3p53Ia5r+QzYb872ORU8tHPwBSC7zkXRtQgddR5L68Bkt1sc8HOYAALGHdgGrXEahZ8WjOGJEpYBq1yTyAtXDO4YLP5tJBDJQ7bnlOvp2MxGs= 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=Ht24yVC3; arc=none smtp.client-ip=91.218.175.185 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="Ht24yVC3" Message-ID: <9dcad942-8888-4b92-b445-c409ae490c03@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1774622076; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=7RxlcroHX4swUzq/nWnXXiWfGa7xs/XFyF8fRtVw6wg=; b=Ht24yVC3pLp17e8idwEujSVYefmRd6nSPN9pv4Cv7PW9LbdCMNzEdxjN8/unn1G3jmzmPe 140Zma49FWsDPlOi7joCdMgPDozMtDs0cQuuXPsZTyFMt3YLzHsn36Pm+6mwYgL0ZwjzWy NwGgmA4DHXFn2T8DW+pI1XvaMSBfw3s= Date: Fri, 27 Mar 2026 10:34:30 -0400 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [v3 00/24] mm: thp: lazy PTE page table allocation at PMD split time Content-Language: en-GB To: "David Hildenbrand (Arm)" , Andrew Morton , Lorenzo Stoakes , willy@infradead.org, linux-mm@kvack.org Cc: fvdl@google.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com, maddy@linux.ibm.com, mpe@ellerman.id.au, linuxppc-dev@lists.ozlabs.org, hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com, borntraeger@linux.ibm.com, svens@linux.ibm.com, linux-s390@vger.kernel.org References: <20260327021403.214713-1-usama.arif@linux.dev> <48d7c810-d219-4346-9e8b-d70243445a91@kernel.org> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: <48d7c810-d219-4346-9e8b-d70243445a91@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 27/03/2026 11:51, David Hildenbrand (Arm) wrote: > On 3/27/26 03:08, Usama Arif wrote: >> When the kernel creates a PMD-level THP mapping for anonymous pages, it >> pre-allocates a PTE page table via pgtable_trans_huge_deposit(). This >> page table sits unused in a deposit list for the lifetime of the THP >> mapping, only to be withdrawn when the PMD is split or zapped. Every >> anonymous THP therefore wastes 4KB of memory unconditionally. On large >> servers where hundreds of gigabytes of memory are mapped as THPs, this >> adds up: roughly 200MB wasted per 100GB of THP memory. This memory >> could otherwise satisfy other allocations, including the very PTE page >> table allocations needed when splits eventually occur. >> >> This series removes the pre-deposit and allocates the PTE page table >> lazily — only when a PMD split actually happens. Since a large number >> of THPs are never split (they are zapped wholesale when processes exit or >> munmap the full range), the allocation is avoided entirely in the common >> case. >> >> The pre-deposit pattern exists because split_huge_pmd was designed as an >> operation that must never fail: if the kernel decides to split, it needs >> a PTE page table, so one is deposited in advance. But "must never fail" >> is an unnecessarily strong requirement. A PMD split is typically triggered >> by a partial operation on a sub-PMD range — partial munmap, partial >> mprotect, COW on a pinned folio, GUP with FOLL_SPLIT_PMD, and similar. >> All of these operations already have well-defined error handling for >> allocation failures (e.g., -ENOMEM, VM_FAULT_OOM). Allowing split to >> fail and propagating the error through these existing paths is the natural >> thing to do. Furthermore, if the system cannot satisfy a single order-0 >> allocation for a page table, it is under extreme memory pressure and >> failing the operation is the correct response. >> >> Designing functions like split_huge_pmd as operations that cannot fail >> has a subtle but real cost to code quality. It forces a pre-allocation >> pattern - every THP creation path must deposit a page table, and every >> split or zap path must withdraw one, creating a hidden coupling between >> widely separated code paths. >> >> This also serves as a code cleanup. On every architecture except powerpc >> with hash MMU, the deposit/withdraw machinery becomes dead code. The >> series removes the generic implementations in pgtable-generic.c and the >> s390/sparc overrides, replacing them with no-op stubs guarded by >> arch_needs_pgtable_deposit(), which evaluates to false at compile time >> on all non-powerpc architectures. >> >> The series is structured as follows: >> >> Patches 1-2: Infrastructure — make split functions return int and >> propagate errors from vma_adjust_trans_huge() through >> __split_vma, vma_shrink, and commit_merge. >> >> Patches 3-15: Handle split failure at every call site — copy_huge_pmd, >> do_huge_pmd_wp_page, zap_pmd_range, wp_huge_pmd, >> change_pmd_range (mprotect), follow_pmd_mask (GUP), >> walk_pmd_range (pagewalk), move_page_tables (mremap), >> move_pages (userfaultfd), device migration, >> pagemap_scan_thp_entry (proc), powerpc subpage_prot, >> and dax_iomap_pmd_fault (DAX). The code will become >> effective in Patch 17 when split functions start >> returning -ENOMEM. >> >> Patch 16: Add __must_check to __split_huge_pmd(), split_huge_pmd() >> and split_huge_pmd_address() so the compiler warns on >> unchecked return values. >> >> Patch 17: The actual change — allocate PTE page tables lazily at >> split time instead of pre-depositing at THP creation. >> This is when split functions will actually start returning >> -ENOMEM. >> >> Patch 18: Remove the now-dead deposit/withdraw code on >> non-powerpc architectures. >> >> Patch 19: Add THP_SPLIT_PMD_FAILED vmstat counter for monitoring >> split failures. >> >> Patches 20-24: Selftests covering partial munmap, mprotect, mlock, >> mremap, and MADV_DONTNEED on THPs to exercise the >> split paths. >> >> The error handling patches are placed before the lazy allocation patch so >> that every call site is already prepared to handle split failures before >> the failure mode is introduced. This makes each patch independently safe >> to apply and bisect through. >> >> The patches were tested with CONFIG_DEBUG_ATOMIC_SLEEP and CONFIG_DEBUG_VM >> enabled. The test results are below: >> >> TAP version 13 >> 1..5 >> # Starting 5 tests from 1 test cases. >> # RUN thp_pmd_split.partial_munmap ... >> # thp_pmd_split_test.c:60:partial_munmap:thp_split_pmd: 0 -> 1 >> # thp_pmd_split_test.c:62:partial_munmap:thp_split_pmd_failed: 0 -> 0 >> # OK thp_pmd_split.partial_munmap >> ok 1 thp_pmd_split.partial_munmap >> # RUN thp_pmd_split.partial_mprotect ... >> # thp_pmd_split_test.c:60:partial_mprotect:thp_split_pmd: 1 -> 2 >> # thp_pmd_split_test.c:62:partial_mprotect:thp_split_pmd_failed: 0 -> 0 >> # OK thp_pmd_split.partial_mprotect >> ok 2 thp_pmd_split.partial_mprotect >> # RUN thp_pmd_split.partial_mlock ... >> # thp_pmd_split_test.c:60:partial_mlock:thp_split_pmd: 2 -> 3 >> # thp_pmd_split_test.c:62:partial_mlock:thp_split_pmd_failed: 0 -> 0 >> # OK thp_pmd_split.partial_mlock >> ok 3 thp_pmd_split.partial_mlock >> # RUN thp_pmd_split.partial_mremap ... >> # thp_pmd_split_test.c:60:partial_mremap:thp_split_pmd: 3 -> 4 >> # thp_pmd_split_test.c:62:partial_mremap:thp_split_pmd_failed: 0 -> 0 >> # OK thp_pmd_split.partial_mremap >> ok 4 thp_pmd_split.partial_mremap >> # RUN thp_pmd_split.partial_madv_dontneed ... >> # thp_pmd_split_test.c:60:partial_madv_dontneed:thp_split_pmd: 4 -> 5 >> # thp_pmd_split_test.c:62:partial_madv_dontneed:thp_split_pmd_failed: 0 -> 0 >> # OK thp_pmd_split.partial_madv_dontneed >> ok 5 thp_pmd_split.partial_madv_dontneed >> # PASSED: 5 / 5 tests passed. >> # Totals: pass:5 fail:0 xfail:0 xpass:0 skip:0 error:0 >> >> The patches are based off of mm-unstable as of 25 Mar >> git hash: d6f51e38433489eb22cb65d1bf72ac7993c5bdec >> >> RFC v2 -> v3: https://lore.kernel.org/all/de0dc7ec-7a8d-4b1a-a419-1d97d2e4d510@linux.dev/ > > Note that we usually go from RFC to v1. ack. > > I'll put this series on my review backlog, but it will take some time > until I get to it (it won't make the next release either way :) ). > No worries and Thanks!