Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759452AbbBIGqO (ORCPT ); Mon, 9 Feb 2015 01:46:14 -0500 Received: from mail-pa0-f46.google.com ([209.85.220.46]:48594 "EHLO mail-pa0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751660AbbBIGqL (ORCPT ); Mon, 9 Feb 2015 01:46:11 -0500 Date: Mon, 9 Feb 2015 15:46:00 +0900 From: Minchan Kim To: "Michael Kerrisk (man-pages)" Cc: Vlastimil Babka , "Kirill A. Shutemov" , Dave Hansen , Mel Gorman , "linux-mm@kvack.org" , Andrew Morton , lkml , Linux API , linux-man , Hugh Dickins Subject: Re: MADV_DONTNEED semantics? Was: [RFC PATCH] mm: madvise: Ignore repeated MADV_DONTNEED hints Message-ID: <20150209064600.GA32300@blaptop> References: <54D08483.40209@suse.cz> <20150203105301.GC14259@node.dhcp.inet.fi> <54D0B43D.8000209@suse.cz> <54D0F56A.9050003@gmail.com> <54D22298.3040504@suse.cz> <54D2508A.9030804@suse.cz> <20150205010757.GA20996@blaptop> <54D4E098.8050004@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <54D4E098.8050004@gmail.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5433 Lines: 147 Hello, Michael On Fri, Feb 06, 2015 at 04:41:12PM +0100, Michael Kerrisk (man-pages) wrote: > On 02/05/2015 02:07 AM, Minchan Kim wrote: > > Hello, > > > > On Wed, Feb 04, 2015 at 08:24:27PM +0100, Michael Kerrisk (man-pages) wrote: > >> On 4 February 2015 at 18:02, Vlastimil Babka wrote: > >>> On 02/04/2015 03:00 PM, Michael Kerrisk (man-pages) wrote: > >>>> > >>>> Hello Vlastimil, > >>>> > >>>> On 4 February 2015 at 14:46, Vlastimil Babka wrote: > >>>>>>> > >>>>>>> - 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. > >>>>>> > >>>>>> > >>>>>> Agreed. "shared" here seems confused. I've removed it. And I've > >>>>>> added mention of "Huge TLB pages" for this error. > >>>>> > >>>>> > >>>>> Thanks. > >>>> > >>>> > >>>> I also added those cases for MADV_REMOVE, BTW. > >>> > >>> > >>> Right. There's also the following for MADV_REMOVE that needs updating: > >>> > >>> "Currently, only shmfs/tmpfs supports this; other filesystems return with > >>> the error ENOSYS." > >>> > >>> - it's not just shmem/tmpfs anymore. It should be best to refer to > >>> fallocate(2) option FALLOC_FL_PUNCH_HOLE which seems to be (more) up to > >>> date. > >>> > >>> - AFAICS it doesn't return ENOSYS but EOPNOTSUPP. Also neither error code is > >>> listed in the ERRORS section. > >> > >> Yup, I recently added that as well, based on a patch from Jan Chaloupka. > >> > >>>>>>>>> - 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. > >>>>>> > >>>>>> So, first a brief question: in the cases where the call does not error > >>>>>> out, > >>>>>> are we agreed that in the current implementation, MADV_DONTNEED will > >>>>>> always result in zero-filled pages when the region is faulted back in > >>>>>> (when we consider pages that are not backed by a file)? > >>>>> > >>>>> I'd agree at this point. > >>>> > >>>> Thanks for the confirmation. > >>>> > >>>>> Also we should probably mention anonymously shared pages (shmem). I think > >>>>> they behave the same as file here. > >>>> > >>>> You mean tmpfs here, right? (I don't keep all of the synonyms straight.) > >>> > >>> shmem is tmpfs (that by itself would fit under "files" just fine), but also > >>> sys V segments created by shmget(2) and also mappings created by mmap with > >>> MAP_SHARED | MAP_ANONYMOUS. I'm not sure if there's a single manpage to > >>> refer to the full list. > >> > >> So, how about this text: > >> > >> After a successful MADV_DONTNEED operation, the seman‐ > >> tics of memory access in the specified region are > >> changed: subsequent accesses of pages in the range > >> will succeed, but will result in either reloading of > >> the memory contents from the underlying mapped file > >> (for shared file mappings, shared anonymous mappings, > >> and shmem-based techniques such as System V shared > >> memory segments) or zero-fill-on-demand pages for > >> anonymous private mappings. > > > > Hmm, I'd like to clarify. > > > > Whether it was intention or not, some of userspace developers thought > > about that syscall drop pages instantly if was no-error return so that > > they will see more free pages(ie, rss for the process will be decreased) > > with keeping the VMA. Can we rely on it? > > I do not know. Michael? It's important to identify difference between MADV_DONTNEED and MADV_FREE so it would be better to clear out in this chance. > > > And we should make error section, too. > > "locked" covers mlock(2) and you said you will add hugetlb. Then, > > VM_PFNMAP? In that case, it fails. How can we say about VM_PFNMAP? > > special mapping for some drivers? > > I'm open for offers on what to add. I suggests from quote "LWN" http://lwn.net/Articles/162860/ "*special mapping* which is not made up of "normal" pages. It is usually created by device drivers which map special memory areas into user space" > > > One more thing, "The kernel is free to ignore the advice". > > It conflicts "This call does not influence the semantics of the > > application (except in the case of MADV_DONTNEED)" so > > is it okay we can believe "The kernel is free to ingmore the advise > > except MADV_DONTNEED"? > > I decided to just drop the sentence > > The kernel is free to ignore the advice. > > It creates misunderstandings, and does not really add information. Sounds good. > > Cheers, > > Michael > > -- > Michael Kerrisk > Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ > Linux/UNIX System Programming Training: http://man7.org/training/ -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/