From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH] /proc/kpagecount: return 0 for special pages that are never mapped To: Matthew Wilcox Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, adobriyan@gmail.com, akpm@linux-foundation.org, vbabka@suse.cz, sfr@canb.auug.org.au, kirill.shutemov@linux.intel.com, rppt@linux.vnet.ibm.com, mhocko@suse.com, alexander.h.duyck@linux.intel.com, hannes@cmpxchg.org, miles.chen@mediatek.com, n-horiguchi@ah.jp.nec.com References: <1543963526-27917-1-git-send-email-anthony.yznaga@oracle.com> <20181205004836.GU10377@bombadil.infradead.org> From: anthony.yznaga@oracle.com Message-ID: Date: Tue, 4 Dec 2018 17:18:32 -0800 MIME-Version: 1.0 In-Reply-To: <20181205004836.GU10377@bombadil.infradead.org> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: On 12/04/2018 04:48 PM, Matthew Wilcox wrote: > On Tue, Dec 04, 2018 at 02:45:26PM -0800, Anthony Yznaga wrote: >> Certain pages that are never mapped to userspace have a type >> indicated in the page_type field of their struct pages (e.g. PG_buddy). >> page_type overlaps with _mapcount so set the count to 0 and avoid >> calling page_mapcount() for these pages. >> >> Signed-off-by: Anthony Yznaga >> --- >> fs/proc/page.c | 2 +- >> include/linux/page-flags.h | 7 +++++++ >> 2 files changed, 8 insertions(+), 1 deletion(-) >> >> diff --git a/fs/proc/page.c b/fs/proc/page.c >> index 6c517b11acf8..40b05e0d4274 100644 >> --- a/fs/proc/page.c >> +++ b/fs/proc/page.c >> @@ -46,7 +46,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf, >> ppage = pfn_to_page(pfn); >> else >> ppage = NULL; >> - if (!ppage || PageSlab(ppage)) >> + if (!ppage || PageSlab(ppage) || page_has_type(ppage)) >> pcount = 0; >> else >> pcount = page_mapcount(ppage); >> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h >> index 50ce1bddaf56..f9a1c50ccefc 100644 >> --- a/include/linux/page-flags.h >> +++ b/include/linux/page-flags.h >> @@ -673,10 +673,17 @@ static inline int TestClearPageDoubleMap(struct page *page) >> #define PG_balloon 0x00000100 >> #define PG_kmemcg 0x00000200 >> #define PG_table 0x00000400 >> +#define PAGE_TYPE_ALL (PG_buddy | PG_balloon | PG_kmemcg | PG_table) >> >> #define PageType(page, flag) \ >> ((page->page_type & (PAGE_TYPE_BASE | flag)) == PAGE_TYPE_BASE) >> >> +static inline int page_has_type(struct page *page) >> +{ >> + return (PageType(page, 0) && >> + ((page->page_type & PAGE_TYPE_ALL) != PAGE_TYPE_ALL)); >> +} >> + >> #define PAGE_TYPE_OPS(uname, lname) \ > I think this is a bit complex, and a bit of a pain to update as we add > new page types. How about this? > > return (int)page_type < -128; > > (I'm open to appropriate #defines to make this more obvious that it's ~0x7F) I thought about having this: #define PAGE_TYPE_END    0xffffff80 static int inline page_has_type(struct page *page) {     return page->page_type > PAGE_TYPE_BASE &&            page->page_type < PAGE_TYPE_END; } But I opted for the additional complexity to avoid more false-positives from possibly corrupted values.  I'm certainly fine with a simple approach, though.