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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id v16-20020a814810000000b005688deeefc2sm3734453ywa.20.2023.06.05.20.09.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 05 Jun 2023 20:09:05 -0700 (PDT) Date: Mon, 5 Jun 2023 20:08:56 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@ripple.attlocal.net To: "Yin, Fengwei" cc: Hugh Dickins , "Liam R. Howlett" , Peng Zhang , maple-tree@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Liu, Yujie" , Andrew Morton Subject: Re: [PATCH 00/14] Reduce preallocations for maple tree In-Reply-To: Message-ID: References: <20230601021605.2823123-1-Liam.Howlett@oracle.com> <7a5dc9ce-b58f-e1b3-db1a-d00a8a556ae5@intel.com> <4fb5f66d-c8c2-f857-7461-b974154dbc2b@bytedance.com> <20230605140344.66pwpdg5zgb6rfa7@revolver> <9f4c10bb-ee7c-cd41-f1b9-c1a7a1a562fd@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 6 Jun 2023, Yin, Fengwei wrote: > On 6/6/2023 10:41 AM, Hugh Dickins wrote: > > On Mon, 5 Jun 2023, Liam R. Howlett wrote: > >> > >> You mean "mm: update validate_mm() to use vma iterator" here I guess. I > >> have it as a different commit id in my branch. > >> > >> I 'restored' some of the checking because I was able to work around not > >> having the mt_dump() definition with the vma iterator. I'm now > >> wondering how wide spread CONFIG_DEBUG_VM is used and if I should not > >> have added these extra checks. > > > > Most CONFIG_DEBUG_VM checks are quite cheap, mostly VM_BUG_ONs for > Indeed. I had CONFIG_DEBUG_VM enabled and didn't see surprise perf report. > > > > easily checked conditions. If validate_mm() is still the kind of thing > > it used to be, checking through every vma on every mmap operation, please > > don't bring that into CONFIG_DEBUG_VM - it distorts performance too much, > > so always used to be under a separate CONFIG_DEBUG_VM_RB instead. > So does this mean CONFIG_DEBUG_VM is allowed to be enabled for performance > testing? Thanks. I was going to say: No, I did not mean that: I just meant that even developers not doing strict performance testing still like to keep a rough eye on performance changes; and historically CONFIG_DEBUG_VM has not distorted very much. But then I wonder about certain distros which (wrongly or rightly) turn CONFIG_DEBUG_VM on: I expect they do performance testing on their kernels. Hugh