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[99.254.144.39]) by smtp.gmail.com with ESMTPSA id g2-20020ac87d02000000b0040331f93ee0sm3323969qtb.77.2023.08.14.12.51.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 14 Aug 2023 12:51:31 -0700 (PDT) Date: Mon, 14 Aug 2023 15:51:27 -0400 From: Peter Xu To: Matthew Wilcox Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Mike Kravetz , David Hildenbrand , Andrew Morton , Yu Zhao , Ryan Roberts , Yang Shi , Hugh Dickins , "Kirill A . Shutemov" Subject: Re: [PATCH RFC v2 1/3] mm: Add TAIL_MAPPING_REUSED_MAX Message-ID: References: <20230814184411.330496-1-peterx@redhat.com> <20230814184411.330496-2-peterx@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: EDC4C8000D X-Stat-Signature: d5of3cpywphsyttm545gth98ztdzxod8 X-Rspam-User: X-HE-Tag: 1692042693-730446 X-HE-Meta: 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 akUiwXRd P4iB8mRWSIEpvNhVJEpzRxOGZNuPuZDrzXfeBBVu9c6zUxLnhZit/rICilNunA68B+7IajIA5Trb326I0xI9a48W1RMzTA7XYjxeS1o8Vv97WSlOkqa6MQCYKOdAPNgrDDDrft6gAkDvSBV9VYMABVSL+NFIHBHvrXGxruE2NMa/9BgnXybTAd1ATN4Z2m5RdzEpivW73H/vO0pWVcAOvsxwHjRkDGJ/D6FTraIG2Y69BCrT1L0GPSbEoi+wikfKN17C+9HMYwoHk38XjFlqrSbHxT8+weL52UB75nmWRAaaqv5C9uyCEccMZQz8yXun/1rcnSms9Eo52awseeH1IyWVa+HeN/KA0clXiBusICgOqR4ZIkFIRw7ohgw0I0LJ/n7Jno7oezz/lXeIZ6u5yTnB7pOn0zh+4F29j 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: On Mon, Aug 14, 2023 at 08:08:57PM +0100, Matthew Wilcox wrote: > On Mon, Aug 14, 2023 at 02:44:09PM -0400, Peter Xu wrote: > > +/* > > + * This macro defines the maximum tail pages (of a folio) that can have the > > + * page->mapping field reused (offset 12 for 32bits, or 24 for 64bits). > > No, don't say how many bytes into the structure something is. It'll > only get out of date. If somebody needs to know, use pahole. OK. > > > + * When the tail page's mapping field reused, it'll be exempted from > > + * ->mapping poisoning and checks. Also see the macro TAIL_MAPPING. > > + */ > > +#define TAIL_MAPPING_REUSED_MAX (2) > > More importantly, I think this is over-parametrisation. If you start to > use extra fields in struct folio, just change the code in page_alloc.c > directly. One should at least also need to change __split_huge_page_tail() on the BUG_ON() with the hard-coded "tail > 2"? I wanted to link all these pieces together, and the use case is when anyone would like to e.g. reuse tail page 3 of a folio, hence put the macro here. -- Peter Xu