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Date: Fri, 22 Jun 2018 18:01:08 -0700 From: Yang Shi <yang.shi@...ux.alibaba.com> To: Michal Hocko <mhocko@...nel.org>, Nadav Amit <nadav.amit@...il.com> Cc: Matthew Wilcox <willy@...radead.org>, ldufour@...ux.vnet.ibm.com, Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, acme@...nel.org, alexander.shishkin@...ux.intel.com, jolsa@...hat.com, namhyung@...nel.org, "open list:MEMORY MANAGEMENT" <linux-mm@...ck.org>, linux-kernel@...r.kernel.org Subject: Re: [RFC v2 PATCH 2/2] mm: mmap: zap pages with read mmap_sem for large mapping Yes, this is true but I guess what Yang Shi meant was that an userspace >> access racing with munmap is not well defined. You never know whether >> you get your data, #PTF or SEGV because it depends on timing. The user >> visible change might be that you lose content and get zero page instead >> if you hit the race window while we are unmapping which was not possible >> before. But whouldn't such an access pattern be buggy anyway? You need >> some form of external synchronization AFAICS. >> >> But maybe some userspace depends on "getting right data or get SEGV" >> semantic. If we have to preserve that then we can come up with a VM_DEAD >> flag set before we tear it down and force the SEGV on the #PF path. >> Something similar we already do for MMF_UNSTABLE. > > Set VM_DEAD with read mmap_sem held? It should be ok since this is the > only place to set this flag for this unique special case. BTW, it looks the vm flags have used up in 32 bit. If we really need VM_DEAD, it should be for both 32-bit and 64-bit. Any suggestion? Thanks, Yang > > Yang > >
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