Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756327AbbBFPl0 (ORCPT ); Fri, 6 Feb 2015 10:41:26 -0500 Received: from mail-we0-f176.google.com ([74.125.82.176]:37042 "EHLO mail-we0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754337AbbBFPlX (ORCPT ); Fri, 6 Feb 2015 10:41:23 -0500 Message-ID: <54D4E098.8050004@gmail.com> Date: Fri, 06 Feb 2015 16:41:12 +0100 From: "Michael Kerrisk (man-pages)" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: Minchan Kim CC: mtk.manpages@gmail.com, 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 References: <20150202165525.GM2395@suse.de> <54CFF8AC.6010102@intel.com> <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> In-Reply-To: <20150205010757.GA20996@blaptop> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4714 Lines: 127 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? > 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. > 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. Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/ -- 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/