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[109.81.89.238]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-a9a68c27893sm93408266b.195.2024.10.18.06.03.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 18 Oct 2024 06:03:43 -0700 (PDT) Date: Fri, 18 Oct 2024 15:03:43 +0200 From: Michal Hocko To: Suren Baghdasaryan Cc: David Hildenbrand , John Hubbard , Yosry Ahmed , akpm@linux-foundation.org, kent.overstreet@linux.dev, corbet@lwn.net, arnd@arndb.de, mcgrof@kernel.org, rppt@kernel.org, paulmck@kernel.org, thuth@redhat.com, tglx@linutronix.de, bp@alien8.de, xiongwei.song@windriver.com, ardb@kernel.org, vbabka@suse.cz, hannes@cmpxchg.org, roman.gushchin@linux.dev, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, pasha.tatashin@soleen.com, souravpanda@google.com, keescook@chromium.org, dennis@kernel.org, yuzhao@google.com, vvvvvv@google.com, rostedt@goodmis.org, iamjoonsoo.kim@lge.com, rientjes@google.com, minchan@google.com, kaleshsingh@google.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-modules@vger.kernel.org, kernel-team@android.com Subject: Re: [PATCH v3 5/5] alloc_tag: config to store page allocation tag refs in page flags Message-ID: References: <20241014203646.1952505-1-surenb@google.com> <20241014203646.1952505-6-surenb@google.com> <6a2a84f5-8474-432f-b97e-18552a9d993c@redhat.com> <9c81a8bb-18e5-4851-9925-769bf8535e46@redhat.com> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Tue 15-10-24 08:58:59, Suren Baghdasaryan wrote: > On Tue, Oct 15, 2024 at 8:42 AM David Hildenbrand wrote: [...] > > Right, I think what John is concerned about (and me as well) is that > > once a new feature really needs a page flag, there will be objection > > like "no you can't, we need them for allocation tags otherwise that > > feature will be degraded". > > I do understand your concern but IMHO the possibility of degrading a > feature should not be a reason to always operate at degraded capacity > (which is what we have today). If one is really concerned about > possible future regression they can set > CONFIG_PGALLOC_TAG_USE_PAGEFLAGS=n and keep what we have today. That's > why I'm strongly advocating that we do need > CONFIG_PGALLOC_TAG_USE_PAGEFLAGS so that the user has control over how > this scarce resource is used. I really do not think users will know how/why to setup this and I wouldn't even bother them thinking about that at all TBH. This is an implementation detail. It is fine to reuse unused flags space as a storage as a performance optimization but why do you want users to bother with that? Why would they ever want to say N here? -- Michal Hocko SUSE Labs