linux-kernel - Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap

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Date:   Tue, 7 Aug 2018 07:45:24 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     willy@...radead.org, ldufour@...ux.vnet.ibm.com,
        kirill@...temov.name, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC v6 PATCH 2/2] mm: mmap: zap pages with read mmap_sem in
 munmap

On Mon 06-08-18 15:19:06, Yang Shi wrote:
> 
> 
> On 8/6/18 1:52 PM, Michal Hocko wrote:
> > On Mon 06-08-18 13:48:35, Yang Shi wrote:
> > > 
> > > On 8/6/18 1:41 PM, Michal Hocko wrote:
> > > > On Mon 06-08-18 09:46:30, Yang Shi wrote:
> > > > > On 8/6/18 2:40 AM, Michal Hocko wrote:
> > > > > > On Fri 03-08-18 14:01:58, Yang Shi wrote:
> > > > > > > On 8/3/18 2:07 AM, Michal Hocko wrote:
> > > > > > > > On Fri 27-07-18 02:10:14, Yang Shi wrote:
> > > > [...]
> > > > > > > > > If the vma has VM_LOCKED | VM_HUGETLB | VM_PFNMAP or uprobe, they are
> > > > > > > > > considered as special mappings. They will be dealt with before zapping
> > > > > > > > > pages with write mmap_sem held. Basically, just update vm_flags.
> > > > > > > > Well, I think it would be safer to simply fallback to the current
> > > > > > > > implementation with these mappings and deal with them on top. This would
> > > > > > > > make potential issues easier to bisect and partial reverts as well.
> > > > > > > Do you mean just call do_munmap()? It sounds ok. Although we may waste some
> > > > > > > cycles to repeat what has done, it sounds not too bad since those special
> > > > > > > mappings should be not very common.
> > > > > > VM_HUGETLB is quite spread. Especially for DB workloads.
> > > > > Wait a minute. In this way, it sounds we go back to my old implementation
> > > > > with special handling for those mappings with write mmap_sem held, right?
> > > > Yes, I would really start simple and add further enhacements on top.
> > > If updating vm_flags with read lock is safe in this case, we don't have to
> > > do this. The only reason for this special handling is about vm_flags update.
> > Yes, maybe you are right that this is safe. I would still argue to have
> > it in a separate patch for easier review, bisectability etc...
> 
> Sorry, I'm a little bit confused. Do you mean I should have the patch
> *without* handling the special case (just like to assume it is safe to
> update vm_flags with read lock), then have the other patch on top of it,
> which simply calls do_munmap() to deal with the special cases?

Just skip those special cases in the initial implementation and handle
each special case in its own patch on top.
-- 
Michal Hocko
SUSE Labs