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h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=UCWHgvdc6YG5HjI3oW+GtrM7KsHZ1JZSco7P5xT0291wv934HLrNOnuOU10OjkhUh cetPaPo4DRZCMdIU4ZvXa1zqAqs2Gck87Ve6KU1FASOib1KQwdYIPonle3pqIeh9FF raLyF2gTVHGw9N2tBKsxgq5d+EZsOcl2MVuMvj90hOrgzd9lmzReEMFSJqVedQlhC1 T0kF5gY0aOWs+e2tZ35e2ptQZtVIHOVxOEH1hOQA5lc703e1L+NYd+I3Awe7gtAC/F Uh+0CgouUViYhGuxIIAi9YgkFGMyeeBE8jrp4Xm1behpZsS3uXDpMr7OD34lzcLb8r k37hJgolXUiWA== Date: Fri, 24 May 2024 11:58:39 +0300 From: Mike Rapoport To: David Hildenbrand Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , "Matthew Wilcox (Oracle)" , Minchan Kim , Sergey Senozhatsky , Hyeonggon Yoo <42.hyeyoo@gmail.com> Subject: Re: [PATCH RFC 1/6] mm: update _mapcount and page_type documentation Message-ID: References: <20240522210341.1030552-1-david@redhat.com> <20240522210341.1030552-2-david@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20240522210341.1030552-2-david@redhat.com> X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 1084E1C002E X-Stat-Signature: xt4d93sj1hycysth3rzu39tfm9tffg3u X-HE-Tag: 1716541231-124318 X-HE-Meta: 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 1/xbAr+Q poHY6GMaWdNxLkezJ9al+2M+Dde7gAcKoxh8M5XTb3vWdw47I3kzSHdE3gv3Qbo/ui1eHone91fAsPIHQQBAugV+G+maAhZ7aYgHHCIMpt0x6a0VmyF9eIAmOss0vu12mPFzSNVI/eib9d7eifWcBDJkeRGnbcW3VXG7F0DM/onBxPtsmHvqdthzQeok8F6o6GbPepqdoSB7KKgwk+QbsSX03v+1+v8DKVRF19mPdJJXZ+grBwD/uBM/Yzpylptjj/6/AjS6M52eb9Q7zp0ZOGJSWFeVrieRpc9cVqhVeVqjHylZvkj95KQG83hzw6wskrqwCE7UNWC8toqg= 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: On Wed, May 22, 2024 at 11:03:36PM +0200, David Hildenbrand wrote: > Let's make it clearer that _mapcount must no longer be used for own > purposes, and how _mapcount and page_type behaves nowadays (also in the > context of hugetlb folios, which are typed folios that will be mapped > to user space). > > Move the documentation regarding "-1" over from page_mapcount_reset(), > which we will remove next. > > Signed-off-by: David Hildenbrand > --- > include/linux/mm.h | 5 ----- > include/linux/mm_types.h | 24 +++++++++++++++--------- > 2 files changed, 15 insertions(+), 14 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 9849dfda44d4..018e7c0265ca 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1206,11 +1206,6 @@ static inline int folio_entire_mapcount(const struct folio *folio) > return atomic_read(&folio->_entire_mapcount) + 1; > } > > -/* > - * The atomic page->_mapcount, starts from -1: so that transitions > - * both from it and to it can be tracked, using atomic_inc_and_test > - * and atomic_add_negative(-1). > - */ > static inline void page_mapcount_reset(struct page *page) > { > atomic_set(&(page)->_mapcount, -1); > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 24323c7d0bd4..532a3030405d 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -46,9 +46,7 @@ struct mem_cgroup; > * which is guaranteed to be aligned. If you use the same storage as > * page->mapping, you must restore it to NULL before freeing the page. > * > - * If your page will not be mapped to userspace, you can also use the four > - * bytes in the mapcount union, but you must call page_mapcount_reset() > - * before freeing it. > + * The mapcount field must not be used for own purposes. > * > * If you want to use the refcount field, it must be used in such a way > * that other CPUs temporarily incrementing and then decrementing the > @@ -152,16 +150,24 @@ struct page { > > union { /* This union is 4 bytes in size. */ > /* > - * If the page can be mapped to userspace, encodes the number > - * of times this page is referenced by a page table. > + * For pages part of non-typed folios for which mappings are Maybe For pages that are part ... > + * tracked via the RMAP, encodes the number of times this page > + * is directly referenced by a page table. > + * > + * Note that the mapcount is always initialized to -1, so that > + * transitions both from it and to it can be tracked, using > + * atomic_inc_and_test() and atomic_add_negative(-1). > */ > atomic_t _mapcount; > > /* > - * If the page is neither PageSlab nor mappable to userspace, > - * the value stored here may help determine what this page > - * is used for. See page-flags.h for a list of page types > - * which are currently stored here. > + * For head pages of typed folios, the value stored here > + * allows for determining what this page is used for. The > + * tail pages of typed folios will not store a type > + * (mapcount == -1). > + * > + * See page-flags.h for a list of page types which are currently > + * stored here. > */ > unsigned int page_type; and maybe move page_type before _mapcount, so that it will be a bit clearer what are "typed" pages and folios are. > }; > -- > 2.45.0 > -- Sincerely yours, Mike.