From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pf1-f198.google.com (mail-pf1-f198.google.com [209.85.210.198]) by kanga.kvack.org (Postfix) with ESMTP id 68CC68E0001 for ; Thu, 27 Sep 2018 12:06:11 -0400 (EDT) Received: by mail-pf1-f198.google.com with SMTP id d22-v6so3124000pfn.3 for ; Thu, 27 Sep 2018 09:06:11 -0700 (PDT) Received: from out4437.biz.mail.alibaba.com (out4437.biz.mail.alibaba.com. [47.88.44.37]) by mx.google.com with ESMTPS id v3-v6si2294734ply.358.2018.09.27.09.06.08 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 27 Sep 2018 09:06:10 -0700 (PDT) Subject: Re: [v2 PATCH 2/2 -mm] mm: brk: dwongrade mmap_sem to read when shrinking References: <1537985434-22655-1-git-send-email-yang.shi@linux.alibaba.com> <1537985434-22655-2-git-send-email-yang.shi@linux.alibaba.com> <33d52132-546f-3c8a-3445-cdbc9068589a@suse.cz> From: Yang Shi Message-ID: <4bcdb16d-902e-55ed-5f5d-12c5ba7cbdfd@linux.alibaba.com> Date: Thu, 27 Sep 2018 09:05:37 -0700 MIME-Version: 1.0 In-Reply-To: <33d52132-546f-3c8a-3445-cdbc9068589a@suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka , mhocko@kernel.org, kirill@shutemov.name, willy@infradead.org, ldufour@linux.vnet.ibm.com, akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org On 9/27/18 5:14 AM, Vlastimil Babka wrote: > On 9/26/18 8:10 PM, Yang Shi wrote: > > Again, "downgrade" in the subject > >> brk might be used to shinrk memory mapping too other than munmap(). > ^ shrink > >> 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. >> >> Cc: Michal Hocko >> Cc: Kirill A. Shutemov >> Cc: Matthew Wilcox >> Cc: Laurent Dufour >> Cc: Vlastimil Babka >> Cc: Andrew Morton >> Signed-off-by: Yang Shi > Acked-by: Vlastimil Babka > > Same nit for the "bool downgrade" name as for patch 1/2. Will solve in next version. Thanks, Yang > > Thanks, > Vlastimil