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 46F0E3E929B; Thu, 19 Mar 2026 15:59:07 +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=1773935947; cv=none; b=oUJOo1rPVCER+S8uT3kc1m3Vk94QniYZeLFp6cmGkdjrbyYkXrNSDrwSJXL+p2AtrcgRmQ4oxn+w1esXXOQVt+UYBLASk0k/f2WRCIYezHLBdzItWX9JlOk3rHRA4+aAUCOnUOxD5WBIYtoktH76n5RyUPrc78HIVbhoHm9MLMc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773935947; c=relaxed/simple; bh=Lb4oqU4iPyvZ1i8PhgXMxctkWzAoFzaDDuT6Xm9wAYc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=ZBMBMYJ4cwLFoJ0jL3nYfPKd9J8kFPcCYtaFiJ4FfnJcs17yir6cFM1toxE/4ScaV1+MLBNhTCJYUoQsqB5M+0Arg79pMgolWnYidhcWF6cCpE2/tWWCbrCXgWTTGYkaGh31cvME/Q9iws8avXWNdSqQPSdBnhV/g3eq8puqM14= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=U/qOknp+; 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="U/qOknp+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4D16AC2BCAF; Thu, 19 Mar 2026 15:59:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773935946; bh=Lb4oqU4iPyvZ1i8PhgXMxctkWzAoFzaDDuT6Xm9wAYc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=U/qOknp+NF8zfgg9EcPZ92SUhgK1wewr6thkOtev0OirSgVgo8CxM22mhI9t4olCZ F9QKHOptUpnMSX4t//ajKhF5QZVTFfWiTzSdh88rgLDPci0Uy/5jKPaSIt/ZY+woCQ 1n1O3LjlaVjdze6GVVaNtuWOMomclWrAxCMWuiCtU/Xj6RODTMoR64DPCJ2wrJe3Lu 5XBLLtaNWFj+NSJ+SaQsQ1OkK6AmSV+CFKKwHIvC0xSPoR0rlXgaNxqOHEkmHJfDSu m0EFEZECCKyxoGkvSvJVq8deNghNUoIIkQ3zHPRhsF2x61w5GnxOB6q4xDOrVKDUW3 CYFk6ohEaWLnw== Date: Thu, 19 Mar 2026 15:59:04 +0000 From: "Lorenzo Stoakes (Oracle)" To: "David Hildenbrand (Arm)" Cc: Nico Pache , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-trace-kernel@vger.kernel.org, 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, lorenzo.stoakes@oracle.com, 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 Subject: Re: [PATCH mm-unstable v15 11/13] mm/khugepaged: avoid unnecessary mTHP collapse attempts Message-ID: References: <20260226031741.230674-1-npache@redhat.com> <20260226032631.234234-1-npache@redhat.com> <4774da78-8349-4eda-a09b-7248e82cb26b@kernel.org> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4774da78-8349-4eda-a09b-7248e82cb26b@kernel.org> On Wed, Mar 18, 2026 at 08:48:30PM +0100, David Hildenbrand (Arm) wrote: > On 3/18/26 19:59, Nico Pache wrote: > > > > > > On 3/17/26 4:35 AM, Lorenzo Stoakes (Oracle) wrote: > >> On Wed, Feb 25, 2026 at 08:26:31PM -0700, Nico Pache wrote: > >>> There are cases where, if an attempted collapse fails, all subsequent > >>> orders are guaranteed to also fail. Avoid these collapse attempts by > >>> bailing out early. > >>> > >>> Signed-off-by: Nico Pache > >> > >> With David's concern addressed: > >> > >> Reviewed-by: Lorenzo Stoakes (Oracle) > >> > >>> --- > >>> mm/khugepaged.c | 35 ++++++++++++++++++++++++++++++++++- > >>> 1 file changed, 34 insertions(+), 1 deletion(-) > >>> > >>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c > >>> index 1c3711ed4513..388d3f2537e2 100644 > >>> --- a/mm/khugepaged.c > >>> +++ b/mm/khugepaged.c > >>> @@ -1492,9 +1492,42 @@ static int mthp_collapse(struct mm_struct *mm, unsigned long address, > >>> ret = collapse_huge_page(mm, collapse_address, referenced, > >>> unmapped, cc, mmap_locked, > >>> order); > >>> - if (ret == SCAN_SUCCEED) { > >>> + > >>> + switch (ret) { > >>> + /* Cases were we continue to next collapse candidate */ > >>> + case SCAN_SUCCEED: > >>> collapsed += nr_pte_entries; > >>> + fallthrough; > >>> + case SCAN_PTE_MAPPED_HUGEPAGE: > >>> continue; > >>> + /* Cases were lower orders might still succeed */ > >>> + case SCAN_LACK_REFERENCED_PAGE: > >>> + case SCAN_EXCEED_NONE_PTE: > >>> + case SCAN_EXCEED_SWAP_PTE: > >>> + case SCAN_EXCEED_SHARED_PTE: > >>> + case SCAN_PAGE_LOCK: > >>> + case SCAN_PAGE_COUNT: > >>> + case SCAN_PAGE_LRU: > >>> + case SCAN_PAGE_NULL: > >>> + case SCAN_DEL_PAGE_LRU: > >>> + case SCAN_PTE_NON_PRESENT: > >>> + case SCAN_PTE_UFFD_WP: > >>> + case SCAN_ALLOC_HUGE_PAGE_FAIL: > >>> + goto next_order; > >>> + /* Cases were no further collapse is possible */ > >>> + case SCAN_CGROUP_CHARGE_FAIL: > >>> + case SCAN_COPY_MC: > >>> + case SCAN_ADDRESS_RANGE: > >>> + case SCAN_NO_PTE_TABLE: > >>> + case SCAN_ANY_PROCESS: > >>> + case SCAN_VMA_NULL: > >>> + case SCAN_VMA_CHECK: > >>> + case SCAN_SCAN_ABORT: > >>> + case SCAN_PAGE_ANON: > >>> + case SCAN_PMD_MAPPED: > >>> + case SCAN_FAIL: > >>> + default: > >> > >> Agree with david, let's spell them out please :) > > > > I believe David is arguing for the opposite. To drop all these spelt out cases > > and just leave the default case. > > > > @david is that correct or did I misunderstand that. > > Either spell all out (no default) OR add a default. > > I prefer to just ... use the default :) I mean yup that's fine too I guess, all or nothing, something in between is weird! > > -- > Cheers, > > David Cheers, Lorenzo