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[2003:cb:c736:4900:1ef8:ea0b:23cb:a81e]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-429ed79d9a7sm67792725e9.39.2024.08.18.00.45.05 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sun, 18 Aug 2024 00:45:06 -0700 (PDT) Message-ID: <19b1c8e2-f60e-460d-8249-5db71a708dc9@redhat.com> Date: Sun, 18 Aug 2024 09:45:04 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 0/6] mm: split underutilized THPs To: Hugh Dickins , Usama Arif Cc: akpm@linux-foundation.org, linux-mm@kvack.org, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, roman.gushchin@linux.dev, yuzhao@google.com, baohua@kernel.org, ryan.roberts@arm.com, rppt@kernel.org, willy@infradead.org, ryncsn@gmail.com, ak@linux.intel.com, cerasuolodomenico@gmail.com, corbet@lwn.net, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, kernel-team@meta.com References: <20240813120328.1275952-1-usamaarif642@gmail.com> <1e6f3b38-d309-e63f-bca0-5093e152f7d7@google.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: BF031100003 X-Rspamd-Server: rspam01 X-Stat-Signature: tk6u6zs3je79ue685gq7377f3iu8ieqz X-HE-Tag: 1723967110-625046 X-HE-Meta: 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 0piPXNa0 Z4YAreSktYgkps5RVeekImrv1q3NXW2N5HnH6gydEPO07/Pav/NoFhilXOsXFqqa3vVIJYjBy/c5hZOgOeCrUn4m8Ku+Zn4JQBHr/gwGX2FukGuNtMuo5Yp9P5Il3jN8dXtbFYIo+jU0IlsEZ+3l69A4n0jsGdWRcMXte2gery32dTzhbeqJ5xAohjFOH70KEDyb8KJfnA4jSVWSWv/1eVpaSvDnw2W64b+qAMBr7GI9DMrWSTJnPdauroZDVaMAUUL8CsESWwbqH14Cki7tI3ouW7JX66zEF1RCurKaSjo+1pSTsYtp5BF9BfqKOmbBp46AcBFvt8TEn9fGxEiiMhpb/13KY9SLyK8oDzjF0GSDDhHNFaL97jon6hQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Hugh, > 2. I don't understand why there needs to be a new PG_partially_mapped > flag, with all its attendant sets and tests and clears all over. Why > can't deferred_split_scan() detect that case for itself, using the > criteria from __folio_remove_rmap()? I see folio->_nr_pages_mapped > is commented "Do not use outside of rmap and debug code", and > folio_nr_pages_mapped() is currently only used from mm/debug.c; but > using the info already maintained is preferable to adding a PG_flag > (and perhaps more efficient - skips splitting when _nr_pages_mapped > already fell to 0 and folio will soon be freed). No new users of _nr_pages_mapped if easily/cleanly avoidable, please. I'm currently cleaning up the final patches that introduce a new kernel config where we will stop maintaining the page->_mapcount for large folios (and consequently have to stop maintaining folio->_nr_pages_mapped). That's the main reasons for the comment -- at one point in my life I want to be done with that project ;) . folio->_nr_pages_mapped will still exist and be maintained without the new kernel config enabled. But in the new one, once we'll detect a partial mapping we'll have to flag the folio -- for example as done in this series. Having two ways of handling that, depending on the kernel config, will not make the code any better. But I agree that we should look into minimizing the usage of any new such flag: I'd have thought we only have to set the flag once, once we detect a partial mapping ... still have to review that patch more thoroughly. > > 3. Everything in /sys/kernel/mm/transparent_hugepage/ is about THPs, > so please remove the "thp_" from "thp_low_util_shrinker" - > "shrink_underused" perhaps. And it needs a brief description in > Documentation/admin-guide/mm/transhuge.rst. agreed. -- Cheers, David / dhildenb