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[76.154.99.94]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-89c6b9cea7bsm31502716d6.27.2026.03.18.11.59.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 18 Mar 2026 11:59:38 -0700 (PDT) Message-ID: Date: Wed, 18 Mar 2026 12:59:34 -0600 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v15 11/13] mm/khugepaged: avoid unnecessary mTHP collapse attempts To: "Lorenzo Stoakes (Oracle)" , david@kernel.org Cc: 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 References: <20260226031741.230674-1-npache@redhat.com> <20260226032631.234234-1-npache@redhat.com> From: Nico Pache Content-Language: en-US, en-ZM In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit 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. -- Nico > >> + return collapsed; >> } >> } >> >> -- >> 2.53.0 >> >