From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f182.google.com (mail-pd0-f182.google.com [209.85.192.182]) by kanga.kvack.org (Postfix) with ESMTP id 4B5476B0038 for ; Wed, 1 Apr 2015 19:13:59 -0400 (EDT) Received: by pddn5 with SMTP id n5so69675239pdd.2 for ; Wed, 01 Apr 2015 16:13:59 -0700 (PDT) Received: from mail-pa0-x233.google.com (mail-pa0-x233.google.com. [2607:f8b0:400e:c03::233]) by mx.google.com with ESMTPS id n7si4822007pdr.125.2015.04.01.16.13.58 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 01 Apr 2015 16:13:58 -0700 (PDT) Received: by pactp5 with SMTP id tp5so65656159pac.1 for ; Wed, 01 Apr 2015 16:13:58 -0700 (PDT) Date: Wed, 1 Apr 2015 16:13:47 -0700 (PDT) From: Hugh Dickins Subject: Re: [PATCHv4 12/24] thp: PMD splitting without splitting compound page In-Reply-To: <20150401131753.GD17153@node.dhcp.inet.fi> Message-ID: References: <1425486792-93161-1-git-send-email-kirill.shutemov@linux.intel.com> <1425486792-93161-13-git-send-email-kirill.shutemov@linux.intel.com> <87lhicbbf8.fsf@linux.vnet.ibm.com> <20150401131753.GD17153@node.dhcp.inet.fi> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: "Kirill A. Shutemov" Cc: "Aneesh Kumar K.V" , "Kirill A. Shutemov" , Andrew Morton , Andrea Arcangeli , Dave Hansen , Hugh Dickins , Mel Gorman , Rik van Riel , Vlastimil Babka , Christoph Lameter , Naoya Horiguchi , Steve Capper , Johannes Weiner , Michal Hocko , Jerome Marchand , linux-kernel@vger.kernel.org, linux-mm@kvack.org On Wed, 1 Apr 2015, Kirill A. Shutemov wrote: > On Wed, Apr 01, 2015 at 12:08:35PM +0530, Aneesh Kumar K.V wrote: > > > > With this we now have pte mapping part of a compound page(). Now the > > gneric gup implementation does > > > > gup_pte_range() > > ptem = ptep = pte_offset_map(&pmd, addr); > > do { > > > > .... > > ... > > if (!page_cache_get_speculative(page)) > > goto pte_unmap; > > ..... > > } > > > > That page_cache_get_speculative will fail in our case because it does > > if (unlikely(!get_page_unless_zero(page))) on a tail page. ?? > > Nice catch, thanks. Indeed; but it's not the generic gup implementation, it's just the generic fast gup implementation. > > But the problem is not exclusive to my patchset. In current kernel some > drivers (sound, for instance) map compound pages with PTEs. Nobody has cared whether fast gup works on those, just so long as slow gup works on those without VM_IO | VM_PFNMAP. Whereas people did care that fast gup work on THPs, so gave them more complicated handling. > > We can try to get page_cache_get_speculative() work on PTE-mapped tail > pages. Untested patch is below. I didn't check through; but we'll agree that it's sad to see the complexity you've managed to reduce elsewhere now popping up again in other places. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org