From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f197.google.com (mail-pf1-f197.google.com [209.85.210.197]) by kanga.kvack.org (Postfix) with ESMTP id 45C228E0001 for ; Thu, 27 Sep 2018 12:59:56 -0400 (EDT) Received: by mail-pf1-f197.google.com with SMTP id s1-v6so3366324pfm.22 for ; Thu, 27 Sep 2018 09:59:56 -0700 (PDT) Received: from out30-130.freemail.mail.aliyun.com (out30-130.freemail.mail.aliyun.com. [115.124.30.130]) by mx.google.com with ESMTPS id r73-v6si2674788pfk.83.2018.09.27.09.59.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 27 Sep 2018 09:59:54 -0700 (PDT) From: Yang Shi Subject: [v3 PATCH 2/2 -mm] mm: brk: downgrade mmap_sem to read when shrinking Date: Fri, 28 Sep 2018 00:59:42 +0800 Message-Id: <1538067582-60038-2-git-send-email-yang.shi@linux.alibaba.com> In-Reply-To: <1538067582-60038-1-git-send-email-yang.shi@linux.alibaba.com> References: <1538067582-60038-1-git-send-email-yang.shi@linux.alibaba.com> Sender: owner-linux-mm@kvack.org List-ID: To: mhocko@kernel.org, kirill@shutemov.name, willy@infradead.org, ldufour@linux.vnet.ibm.com, vbabka@suse.cz, akpm@linux-foundation.org Cc: yang.shi@linux.alibaba.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org brk might be used to shrink memory mapping too other than munmap(). So, it may hold write mmap_sem for long time when shrinking large mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in munmap") described. The brk() will not manipulate vmas anymore after __do_munmap() call for the mapping shrink use case. But, it may set mm->brk after __do_munmap(), which needs hold write mmap_sem. However, a simple trick can workaround this by setting mm->brk before __do_munmap(). Then restore the original value if __do_munmap() fails. With this trick, it is safe to downgrade to read mmap_sem. So, the same optimization, which downgrades mmap_sem to read for zapping pages, is also feasible and reasonable to this case. The period of holding exclusive mmap_sem for shrinking large mapping would be reduced significantly with this optimization. Acked-by: Vlastimil Babka Acked-by: Kirill A. Shutemov Cc: Michal Hocko Cc: Matthew Wilcox Cc: Laurent Dufour Cc: Andrew Morton Signed-off-by: Yang Shi --- v3: Fixed the comments from Vlastimil and Kirill. And, added their Acked-by. Thanks. v2: Rephrase the commit log per Michal mm/mmap.c | 43 ++++++++++++++++++++++++++++++++----------- 1 file changed, 32 insertions(+), 11 deletions(-) diff --git a/mm/mmap.c b/mm/mmap.c index 017bcfa..68dc4fb 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -191,16 +191,19 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long SYSCALL_DEFINE1(brk, unsigned long, brk) { unsigned long retval; - unsigned long newbrk, oldbrk; + unsigned long newbrk, oldbrk, origbrk; struct mm_struct *mm = current->mm; struct vm_area_struct *next; unsigned long min_brk; bool populate; + bool downgraded = false; LIST_HEAD(uf); if (down_write_killable(&mm->mmap_sem)) return -EINTR; + origbrk = mm->brk; + #ifdef CONFIG_COMPAT_BRK /* * CONFIG_COMPAT_BRK can still be overridden by setting @@ -229,14 +232,29 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long newbrk = PAGE_ALIGN(brk); oldbrk = PAGE_ALIGN(mm->brk); - if (oldbrk == newbrk) - goto set_brk; + if (oldbrk == newbrk) { + mm->brk = brk; + goto success; + } - /* Always allow shrinking brk. */ + /* + * Always allow shrinking brk. + * __do_munmap() may downgrade mmap_sem to read. + */ if (brk <= mm->brk) { - if (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf)) - goto set_brk; - goto out; + /* + * mm->brk need to be protected by write mmap_sem, update it + * before downgrading mmap_sem. + * When __do_munmap fail, it will be restored from origbrk. + */ + mm->brk = brk; + retval = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true); + if (retval < 0) { + mm->brk = origbrk; + goto out; + } else if (retval == 1) + downgraded = true; + goto success; } /* Check against existing mmap mappings. */ @@ -247,18 +265,21 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long /* Ok, looks good - let it rip. */ if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0) goto out; - -set_brk: mm->brk = brk; + +success: populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0; - up_write(&mm->mmap_sem); + if (downgraded) + up_read(&mm->mmap_sem); + else + up_write(&mm->mmap_sem); userfaultfd_unmap_complete(mm, &uf); if (populate) mm_populate(oldbrk, newbrk - oldbrk); return brk; out: - retval = mm->brk; + retval = origbrk; up_write(&mm->mmap_sem); return retval; } -- 1.8.3.1