From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f182.google.com (mail-wi0-f182.google.com [209.85.212.182]) by kanga.kvack.org (Postfix) with ESMTP id B3C3C6B0032 for ; Fri, 27 Mar 2015 05:47:38 -0400 (EDT) Received: by wiaa2 with SMTP id a2so23569032wia.0 for ; Fri, 27 Mar 2015 02:47:38 -0700 (PDT) Received: from mx2.suse.de (cantor2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id fu19si2408115wjc.14.2015.03.27.02.47.36 for (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Fri, 27 Mar 2015 02:47:37 -0700 (PDT) Message-ID: <55152737.6060404@suse.cz> Date: Fri, 27 Mar 2015 10:47:35 +0100 From: Vlastimil Babka MIME-Version: 1.0 Subject: Re: [patch][resend] MAP_HUGETLB munmap fails with size not 2MB aligned References: In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: David Rientjes , Davide Libenzi Cc: Hugh Dickins , Andrew Morton , KOSAKI Motohiro , Andrea Arcangeli , Joern Engel , Jianguo Wu , Eric B Munson , linux-mm@kvack.org, Linux Kernel Mailing List , Linux API , linux-man@vger.kernel.org, Michael Kerrisk Might be too late in this thread, but in case you are going to continue and/or repost: [CC += linux-api@vger.kernel.org] (also linux-man and Michael to match my other reply) Since this is a kernel-user-space API change, please CC linux-api@. The kernel source file Documentation/SubmitChecklist notes that all Linux kernel patches that change userspace interfaces should be CCed to linux-api@vger.kernel.org, so that the various parties who are interested in API changes are informed. For further information, see https://www.kernel.org/doc/man-pages/linux-api-ml.html On 03/26/2015 09:03 PM, David Rientjes wrote: > On Thu, 26 Mar 2015, Davide Libenzi wrote: > >> > Yes, this munmap() behavior of lengths <= hugepage_size - PAGE_SIZE for a >> > hugetlb vma is long standing and there may be applications that break as a >> > result of changing the behavior: a database that reserves all allocated >> > hugetlb memory with mmap() so that it always has exclusive access to those >> > hugepages, whether they are faulted or not, and maintains its own hugepage >> > pool (which is common), may test the return value of munmap() and depend >> > on it returning -EINVAL to determine if it is freeing memory that was >> > either dynamically allocated or mapped from the hugetlb reserved pool. >> >> You went a long way to create such a case. >> But, in your case, that application will erroneously considering hugepage >> mmaped memory, as dynamically allocated, since it will always get EINVAL, >> unless it passes an aligned size. Aligned size, which a fix like the one >> posted in the patch will still leave as success. > > There was a patch proposed last week to add reserved pools to the > hugetlbfs mount option specifically for the case where a large database > wants sole reserved access to the hugepage pool. This is why hugetlbfs > pages become reserved on mmap(). In that case, the database never wants > to do munmap() and instead maintains its own hugepage pool. > > That makes the usual database case, mmap() all necessary hugetlb pages to > reserve them, even easier since they have historically had to maintain > this pool amongst various processes. > > Is there a process out there that tests for munmap(ptr) == EINVAL and, if > true, returns ptr to its hugepage pool? I can't say for certain that none > exist, that's why the potential for breakage exists. > >> OTOH, an application, which might be more common than the one you posted, >> which calls munmap() to release a pointer which it validly got from a >> previous mmap(), will leak huge pages as all the issued munmaps will fail. >> > > That application would have to be ignoring an EINVAL return value. > >> > If we were to go back in time and decide this when the munmap() behavior >> > for hugetlb vmas was originally introduced, that would be valid. The >> > problem is that it could lead to userspace breakage and that's a >> > non-starter. >> > >> > What we can do is improve the documentation and man-page to clearly >> > specify the long-standing behavior so that nobody encounters unexpected >> > results in the future. >> >> This way you will leave the mmap API with broken semantics. >> In any case, I am done arguing. >> I will leave to Andrew to sort it out, and to Michael Kerrisk to update >> the mmap man pages with the new funny behaviour. >> > > The behavior is certainly not new, it has always been the case for > munmap() on hugetlb vmas. > > In a strict POSIX interpretation, it refers only to pages in the sense of > what is returned by sysconf(_SC_PAGESIZE). Such vmas are not backed by > any pages of size sysconf(_SC_PAGESIZE), so this behavior is undefined. > It would be best to modify the man page to explicitly state this for > MAP_HUGETLB. > > -- > 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 > -- 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