From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965517AbbBCLnD (ORCPT ); Tue, 3 Feb 2015 06:43:03 -0500 Received: from cantor2.suse.de ([195.135.220.15]:53423 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965429AbbBCLm4 (ORCPT ); Tue, 3 Feb 2015 06:42:56 -0500 Message-ID: <54D0B43D.8000209@suse.cz> Date: Tue, 03 Feb 2015 12:42:53 +0100 From: Vlastimil Babka User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0 MIME-Version: 1.0 To: "Kirill A. Shutemov" CC: Dave Hansen , Mel Gorman , linux-mm@kvack.org, Minchan Kim , Andrew Morton , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, mtk.manpages@gmail.com, linux-man@vger.kernel.org Subject: Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints References: <20150202165525.GM2395@suse.de> <54CFF8AC.6010102@intel.com> <54D08483.40209@suse.cz> <20150203105301.GC14259@node.dhcp.inet.fi> In-Reply-To: <20150203105301.GC14259@node.dhcp.inet.fi> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/03/2015 11:53 AM, Kirill A. Shutemov wrote: > On Tue, Feb 03, 2015 at 09:19:15AM +0100, Vlastimil Babka wrote: >> [CC linux-api, man pages] >> >> On 02/02/2015 11:22 PM, Dave Hansen wrote: >> > On 02/02/2015 08:55 AM, Mel Gorman wrote: >> >> This patch identifies when a thread is frequently calling MADV_DONTNEED >> >> on the same region of memory and starts ignoring the hint. On an 8-core >> >> single-socket machine this was the impact on ebizzy using glibc 2.19. >> > >> > The manpage, at least, claims that we zero-fill after MADV_DONTNEED is >> > called: >> > >> >> MADV_DONTNEED >> >> Do not expect access in the near future. (For the time being, the application is finished with the given range, so the kernel can free resources >> >> associated with it.) Subsequent accesses of pages in this range will succeed, but will result either in reloading of the memory contents from the >> >> underlying mapped file (see mmap(2)) or zero-fill-on-demand pages for mappings without an underlying file. >> > >> > So if we have anything depending on the behavior that it's _always_ >> > zero-filled after an MADV_DONTNEED, this will break it. >> >> OK, so that's a third person (including me) who understood it as a zero-fill >> guarantee. I think the man page should be clarified (if it's indeed not >> guaranteed), or we have a bug. >> >> The implementation actually skips MADV_DONTNEED for >> VM_LOCKED|VM_HUGETLB|VM_PFNMAP vma's. > > It doesn't skip. It fails with -EINVAL. Or I miss something. No, I missed that. Thanks for pointing out. The manpage also explains EINVAL in this case: * The application is attempting to release locked or shared pages (with MADV_DONTNEED). - that covers mlocking ok, not sure if the rest fits the "shared pages" case though. I dont see any check for other kinds of shared pages in the code. >> - The word "will result" did sound as a guarantee at least to me. So here it >> could be changed to "may result (unless the advice is ignored)"? > > It's too late to fix documentation. Applications already depends on the > beheviour. Right, so as long as they check for EINVAL, it should be safe. It appears that jemalloc does. I still wouldnt be sure just by reading the man page that the clearing is guaranteed whenever I dont get an error return value, though, From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vlastimil Babka Subject: Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints Date: Tue, 03 Feb 2015 12:42:53 +0100 Message-ID: <54D0B43D.8000209@suse.cz> References: <20150202165525.GM2395@suse.de> <54CFF8AC.6010102@intel.com> <54D08483.40209@suse.cz> <20150203105301.GC14259@node.dhcp.inet.fi> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20150203105301.GC14259@node.dhcp.inet.fi> Sender: owner-linux-mm@kvack.org To: "Kirill A. Shutemov" Cc: Dave Hansen , Mel Gorman , linux-mm@kvack.org, Minchan Kim , Andrew Morton , linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, mtk.manpages@gmail.com, linux-man@vger.kernel.org List-Id: linux-man@vger.kernel.org On 02/03/2015 11:53 AM, Kirill A. Shutemov wrote: > On Tue, Feb 03, 2015 at 09:19:15AM +0100, Vlastimil Babka wrote: >> [CC linux-api, man pages] >> >> On 02/02/2015 11:22 PM, Dave Hansen wrote: >> > On 02/02/2015 08:55 AM, Mel Gorman wrote: >> >> This patch identifies when a thread is frequently calling MADV_DONTNEED >> >> on the same region of memory and starts ignoring the hint. On an 8-core >> >> single-socket machine this was the impact on ebizzy using glibc 2.19. >> > >> > The manpage, at least, claims that we zero-fill after MADV_DONTNEED is >> > called: >> > >> >> MADV_DONTNEED >> >> Do not expect access in the near future. (For the time being, the application is finished with the given range, so the kernel can free resources >> >> associated with it.) Subsequent accesses of pages in this range will succeed, but will result either in reloading of the memory contents from the >> >> underlying mapped file (see mmap(2)) or zero-fill-on-demand pages for mappings without an underlying file. >> > >> > So if we have anything depending on the behavior that it's _always_ >> > zero-filled after an MADV_DONTNEED, this will break it. >> >> OK, so that's a third person (including me) who understood it as a zero-fill >> guarantee. I think the man page should be clarified (if it's indeed not >> guaranteed), or we have a bug. >> >> The implementation actually skips MADV_DONTNEED for >> VM_LOCKED|VM_HUGETLB|VM_PFNMAP vma's. > > It doesn't skip. It fails with -EINVAL. Or I miss something. No, I missed that. Thanks for pointing out. The manpage also explains EINVAL in this case: * The application is attempting to release locked or shared pages (with MADV_DONTNEED). - that covers mlocking ok, not sure if the rest fits the "shared pages" case though. I dont see any check for other kinds of shared pages in the code. >> - The word "will result" did sound as a guarantee at least to me. So here it >> could be changed to "may result (unless the advice is ignored)"? > > It's too late to fix documentation. Applications already depends on the > beheviour. Right, so as long as they check for EINVAL, it should be safe. It appears that jemalloc does. I still wouldnt be sure just by reading the man page that the clearing is guaranteed whenever I dont get an error return value, though, -- 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