From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DBF4A1F0E25; Wed, 13 May 2026 13:02:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778677379; cv=none; b=Q9I6LgEXF/pFAfwvclGb8PgwIrBNH5b+VuvHh9mPteHeAj5bKIxF/0wixuGjjZrvgeDKbUB+lB3tOP7/BeczBN6FIBZL5N3ZgLScdz7Y+UZkzjdMQHCAqgK+1eW4Vk8oTTZIoU0pYXM7KjtCa74g6NDa6I+LAD710sVHEAWsNhc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778677379; c=relaxed/simple; bh=w952/StbTzQQpvTv4IVGtb68QWI9Nein9ogKmCl+tws=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=ZnJdgsJLpdJaUJH2OsCmZ+u+ado1A3UnQ5nDbR0QCb0W0PkX1X5ThhBIMPu4Z9oC4D3XCH53O1hfnpGX16O585U3qfADVKFnT/C2J0I/XI3eEpj7dOkmzze+UR9S6qWlmGNZyxjfM5ztmt0hs/COll1NzEYeZYo/wZEs3+Rp7P4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LyZ6H2/H; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="LyZ6H2/H" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F06F4C2BCB7; Wed, 13 May 2026 13:02:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778677379; bh=w952/StbTzQQpvTv4IVGtb68QWI9Nein9ogKmCl+tws=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=LyZ6H2/H0XwhRgDFLwozCZOLN2EiPvTtwDpX/dq/KctJfZrNZU0Yb1NL5rBKULsrZ jbUa1UFigmYYxEslnZPJFSFQ0Q0/9PYrAgAXc+BgPZlQSTLpfqhj/L09CSffY8WVX1 xowoiIuBynVFBoupy5E/b30TuKbfNhMRxtSVhcICglhWbHL+46LEDl5T8W4moytKgf oNcgcEHEbUFBhg/WBMRKpzv+KANUEjI+V5nKtsVgfq4Ks5ZVfYbGwz87B/+lgQBcpU 4LnI7IEFZxJbkOGQe8O5rsSp3qYCmn9uvqs+SUbNosbWfqTsnaBeJORpokAocL2Dbl Nmu/fqxw6905A== Message-ID: <627bb280-be44-4648-8771-5a479cda988f@kernel.org> Date: Wed, 13 May 2026 15:02:43 +0200 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4] mm: introduce a new page type for page pool in page type To: Byungchul Park Cc: Dragos Tatulea , linux-mm@kvack.org, akpm@linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kernel_team@skhynix.com, harry.yoo@oracle.com, ast@kernel.org, daniel@iogearbox.net, davem@davemloft.net, kuba@kernel.org, hawk@kernel.org, john.fastabend@gmail.com, sdf@fomichev.me, saeedm@nvidia.com, leon@kernel.org, tariqt@nvidia.com, mbloch@nvidia.com, andrew+netdev@lunn.ch, edumazet@google.com, pabeni@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, horms@kernel.org, jackmanb@google.com, hannes@cmpxchg.org, ziy@nvidia.com, ilias.apalodimas@linaro.org, willy@infradead.org, brauner@kernel.org, kas@kernel.org, yuzhao@google.com, usamaarif642@gmail.com, baolin.wang@linux.alibaba.com, almasrymina@google.com, toke@redhat.com, asml.silence@gmail.com, bpf@vger.kernel.org, linux-rdma@vger.kernel.org, sfr@canb.auug.org.au, dw@davidwei.uk, ap420073@gmail.com References: <20260224051347.19621-1-byungchul@sk.com> <982b9bc1-0a0a-4fc5-8e3a-3672db2b29a1@nvidia.com> <20260513121805.GA22430@system.software.com> <8348d867-f8a1-432c-be2d-699ad96b2e93@kernel.org> <20260513123920.GA51788@system.software.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/13/26 14:39, Byungchul Park wrote: > On Wed, May 13, 2026 at 02:29:46PM +0200, David Hildenbrand (Arm) wrote: > On 5/13/26 14:18, Byungchul Park wrote: >>> >>> Hi, >>> >>> The problem comes from the fact that page_type and _mapcount are >>> union'ed but there is a case where these two information should be kept >>> at the same time. >>> >>> Why don't we allow these two information can be kept in the 4 bytes at >>> the same time until Zi Yan's work on _mapcount and page_type will be >>> done, instead of taking a step back? >>> >>> It can be more optimized but I suggest the approach I just mentioned: >>> --- >>> diff --git a/fs/proc/internal.h b/fs/proc/internal.h >>> index 64dc44832808..e5ec204866dc 100644 >>> --- a/fs/proc/internal.h >>> +++ b/fs/proc/internal.h >>> @@ -185,8 +185,7 @@ static inline int folio_precise_page_mapcount(struct folio *folio, >>> { >>> int mapcount = atomic_read(&page->_mapcount) + 1; >>> >>> - if (page_mapcount_is_type(mapcount)) >>> - mapcount = 0; >>> + mapcount = page_mapcount_clear_type(mapcount); >>> if (folio_test_large(folio)) >>> mapcount += folio_entire_mapcount(folio); >>> >>> diff --git a/include/linux/mm.h b/include/linux/mm.h >>> index 8260e28205e9..f45064796313 100644 >>> --- a/include/linux/mm.h >>> +++ b/include/linux/mm.h >>> @@ -1865,8 +1865,7 @@ static inline int folio_mapcount(const struct folio *folio) >>> >>> if (likely(!folio_test_large(folio))) { >>> mapcount = atomic_read(&folio->_mapcount) + 1; >>> - if (page_mapcount_is_type(mapcount)) >>> - mapcount = 0; >>> + mapcount = page_mapcount_clear_type(mapcount); >>> return mapcount; >>> } >>> return folio_large_mapcount(folio); >>> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h >>> index 0e03d816e8b9..f3b0d1fa262d 100644 >>> --- a/include/linux/page-flags.h >>> +++ b/include/linux/page-flags.h >>> @@ -934,9 +934,9 @@ static inline bool page_type_has_type(int page_type) >>> } >>> >>> /* This takes a mapcount which is one more than page->_mapcount */ >>> -static inline bool page_mapcount_is_type(unsigned int mapcount) >>> +static inline unsigned int page_mapcount_clear_type(unsigned int mapcount) >>> { >>> - return page_type_has_type(mapcount - 1); >>> + return (unsigned int)(((int)(mapcount << 8)) >> 8); >>> } >>> >>> static inline bool page_has_type(const struct page *page) >>> @@ -953,16 +953,20 @@ static __always_inline void __folio_set_##fname(struct folio *folio) \ >>> { \ >>> if (folio_test_##fname(folio)) \ >>> return; \ >>> - VM_BUG_ON_FOLIO(data_race(folio->page.page_type) != UINT_MAX, \ >>> + VM_BUG_ON_FOLIO(page_type_has_type(data_race(folio->page.page_type)), \ >>> folio); \ >>> - folio->page.page_type = (unsigned int)PGTY_##lname << 24; \ >>> + folio->page.page_type &= ~(PGTY_mapcount_underflow << 24); \ >>> + folio->page.page_type |= (unsigned int)PGTY_##lname << 24; \ >>> } \ >>> static __always_inline void __folio_clear_##fname(struct folio *folio) \ >>> { \ >>> - if (folio->page.page_type == UINT_MAX) \ >>> + int mapcount; \ >>> + \ >>> + if (!page_type_has_type(folio->page.page_type)) \ >>> return; \ >>> VM_BUG_ON_FOLIO(!folio_test_##fname(folio), folio); \ >>> - folio->page.page_type = UINT_MAX; \ >>> + mapcount = atomic_read(&folio->page._mapcount); \ >>> + folio->page.page_type = page_mapcount_clear_type(mapcount); \ >>> } >>> >>> #define PAGE_TYPE_OPS(uname, lname, fname) \ >>> @@ -975,15 +979,20 @@ static __always_inline void __SetPage##uname(struct page *page) \ >>> { \ >>> if (Page##uname(page)) \ >>> return; \ >>> - VM_BUG_ON_PAGE(data_race(page->page_type) != UINT_MAX, page); \ >>> - page->page_type = (unsigned int)PGTY_##lname << 24; \ >>> + VM_BUG_ON_PAGE(page_type_has_type(data_race(page->page_type)), \ >>> + page); \ >>> + page->page_type &= ~(PGTY_mapcount_underflow << 24); \ >>> + page->page_type |= (unsigned int)PGTY_##lname << 24; \ >>> } \ >>> static __always_inline void __ClearPage##uname(struct page *page) \ >>> { \ >>> - if (page->page_type == UINT_MAX) \ >>> + int mapcount; \ >>> + \ >>> + if (!page_type_has_type(page->page_type)) \ >>> return; \ >>> VM_BUG_ON_PAGE(!Page##uname(page), page); \ >>> - page->page_type = UINT_MAX; \ >>> + mapcount = atomic_read(&page->_mapcount); \ >>> + page->page_type = page_mapcount_clear_type(mapcount); \ >>> } >>> >>> /* >>> diff --git a/mm/debug.c b/mm/debug.c >>> index 77fa8fe1d641..9a932ded09d4 100644 >>> --- a/mm/debug.c >>> +++ b/mm/debug.c >>> @@ -74,8 +74,7 @@ static void __dump_folio(const struct folio *folio, const struct page *page, >>> int mapcount = atomic_read(&page->_mapcount) + 1; >>> char *type = ""; >>> >>> - if (page_mapcount_is_type(mapcount)) >>> - mapcount = 0; >>> + mapcount = page_mapcount_clear_type(mapcount); >>> >>> pr_warn("page: refcount:%d mapcount:%d mapping:%p index:%#lx pfn:%#lx\n", >>> folio_ref_count(folio), mapcount, mapping, >>> --- >>> >>> Thoughts? >> >> God no. > > This is not final patch, but for sharing the rough idea *with code* - > maybe there are more points in code that should be adjusted by the > change. I just typed the draft patch quick just for sharing idea. > > If we should allow pp type pages to be used in mapping as well, then > we should allow a page to keep both its type and mapcount at the same > time. Am I missing something? We don't want code to accidentally overflow mapcounts into these bits and have them wrongly be detected as page types. This is just very fragile. -- Cheers, David