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b=Dv40bhexuQMuZDLtQm7he+mWmyxt193jYRqlC6QBz2puRXJzfLOClDBMF03eOc0UG F2CSjew52zK9+MNmUNjFrro4uN/L6kzCnWcbwr93swceQTuKypMchzHD61opvYq1y3 AsSg9CM9rpAN/dHK6e/EkqB77RTVA9qK3h0WBUfh3IEy5myp6AEIlB3MMROVCrA71x 1uT/WWFOkbtjRtN8Eu6oouvUoQmFOK5YjhF0oujTFJm2iAUkMhnkGizS3f8HI0OV1M HlDzgTt2SxWLnKuloDAzgwJQJSygqEcRnbP1QkaLkJv7pqj+d4B3DDynmCJntAe8XG E7MP8Kj4SwThA== Message-ID: <4774da78-8349-4eda-a09b-7248e82cb26b@kernel.org> Date: Wed, 18 Mar 2026 20:48:30 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-unstable v15 11/13] mm/khugepaged: avoid unnecessary mTHP collapse attempts To: Nico Pache , "Lorenzo Stoakes (Oracle)" 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: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 9F77C4000F X-Stat-Signature: rt1doztg84cf88go1im37y6b7ryigkih X-Rspam-User: X-HE-Tag: 1773863328-780380 X-HE-Meta: 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 tnNb4t9a imFGKvVBNMsKOWDzHOizwUPLMpYdmS+uENTXdqERp4JiMbwVJc3u5Z1Wbmy1KpZDABwMfe4uFjzRgp9RSQIiCKyGeGsCON8pZEYzwJD50dhqPwg2jGjoMsDb20r+fAS01W6oLorBiFnIafs4XGgZQ8KxLqY+a/MQOsCPryIVRi+YUVCf9VsyBmuX6ICe/FwD69DJc3eIU95VRJZ4wWKuU9UTK3wAopkq8RnoBfpx5NhYQsAOJD0nhd4nB4gCWwmLwLFtshBjo0+H/1boExLhjSmVt63NjSduWAHAWNMoD3tGK2jtNN+3jjr0FQBR19WtQ0QyjGvR1TLis1b14j8HLDu3BuH2WcImUrP7NKqfrWPVwqc4VNPJTHIYcBQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 :) -- Cheers, David