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[91.12.100.243]) by smtp.gmail.com with ESMTPSA id g25sm3943341wmh.3.2019.12.17.10.46.47 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 17 Dec 2019 10:46:47 -0800 (PST) Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable From: David Hildenbrand Mime-Version: 1.0 (1.0) Subject: Re: [PATCH v15 3/7] mm: Add function __putback_isolated_page Date: Tue, 17 Dec 2019 19:46:47 +0100 Message-Id: <08EFF184-E727-4A79-ABEF-52F2463860C3@redhat.com> References: <1a6e4646f570bf193924e099557841eb6e77a80d.camel@linux.intel.com> Cc: David Hildenbrand , Alexander Duyck , kvm@vger.kernel.org, mst@redhat.com, linux-kernel@vger.kernel.org, willy@infradead.org, mhocko@kernel.org, linux-mm@kvack.org, akpm@linux-foundation.org, mgorman@techsingularity.net, vbabka@suse.cz, yang.zhang.wz@gmail.com, nitesh@redhat.com, konrad.wilk@oracle.com, pagupta@redhat.com, riel@surriel.com, lcapitulino@redhat.com, dave.hansen@intel.com, wei.w.wang@intel.com, aarcange@redhat.com, pbonzini@redhat.com, dan.j.williams@intel.com, osalvador@suse.de In-Reply-To: <1a6e4646f570bf193924e099557841eb6e77a80d.camel@linux.intel.com> To: Alexander Duyck X-Mailer: iPhone Mail (17A878) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org > Am 17.12.2019 um 19:25 schrieb Alexander Duyck : >=20 > =EF=BB=BFOn Tue, 2019-12-17 at 18:24 +0100, David Hildenbrand wrote: >>>>> Also there are some scenarios where __page_to_pfn is not that simple a= >>>>> call with us having to get the node ID so we can find the pgdat struct= ure >>>>> to perform the calculation. I'm not sure the compiler would be ble to >>>>> figure out that the result is the same for both calls, so it is better= to >>>>> make it explicit. >>>>=20 >>>> Only in case of CONFIG_SPARSEMEM we have to go via the section - but I >>>> doubt this is really worth optimizing here. >>>>=20 >>>> But yeah, I'm fine with this change, only "IMHO >>>> get_pageblock_migratetype() would be nicer" :) >>>=20 >>> Aren't most distros running with CONFIG_SPARSEMEM enabled? If that is th= e >>> case why not optimize for it? >>=20 >> Because I tend to dislike micro-optimizations without performance >> numbers for code that is not on a hot path. But I mean in this case, as >> you said, you need the pfn either way, so it's completely fine with. >>=20 >> I do wonder, however, if you should just pass in the migratetype from >> the caller. That would be even faster ;) >=20 > The problem is page isolation. We can end up with a page being moved to an= > isolate pageblock while we aren't holding the zone lock, and as such we > likely need to test it again anyway. So there isn't value in storing and > reusing the value for cases like page reporting. >=20 > In addition, the act of isolating the page can cause the migratetype to > change as __isolate_free_page will attempt to change the migratetype to > movable if it is one of the standard percpu types and we are pulling at > least half a pageblock out. So storing the value before we isolate it > would be problematic as well. >=20 > Undoing page isolation is the exception to the issues pointed out above, > but in that case we are overwriting the pageblock migratetype anyway so > the cache lines involved should all be warm from having just set the > value. Nothing would speak against querying the migratetype in the caller and passi= ng it on. After all you=E2=80=98re holding the zone lock, so it can=E2=80=98= t change. >=20