linux-kernel - Re: [RFC v8 PATCH 3/5] mm: mmap: zap pages with read mmap

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Date:   Wed, 22 Aug 2018 14:56:34 -0700
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Vlastimil Babka <vbabka@...e.cz>, mhocko@...nel.org,
        willy@...radead.org, ldufour@...ux.vnet.ibm.com,
        akpm@...ux-foundation.org, peterz@...radead.org, mingo@...hat.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC v8 PATCH 3/5] mm: mmap: zap pages with read mmap_sem in
 munmap



On 8/22/18 2:42 PM, Dave Hansen wrote:
> On 08/22/2018 02:10 PM, Kirill A. Shutemov wrote:
>>> For x86, mpx_notify_unmap() looks finally zap the VM_MPX vmas in bound table
>>> range with zap_page_range() and doesn't update vm flags, so it sounds ok to
>>> me since vmas have been detached, nobody can find those vmas. But, I'm not
>>> familiar with the details of mpx, maybe Kirill could help to confirm this?
>> I don't see anything obviously dependent on down_write() in
>> mpx_notify_unmap(), but Dave should know better.
> We need mmap_sem for write in mpx_notify_unmap().
>
> Its job is to clean up bounds tables, but bounds tables are dynamically
> allocated and destroyed by the kernel.  When we destroy a table, we also
> destroy the VMA for the bounds table *itself*, separate from the VMA
> being unmapped.

Thanks for confirming this. I didn't realize there is VMA for bounds 
table itself.

>
> But, this code is very likely to go away soon.  If it's causing a
> problem for you, let me know and I'll see if I can get to removing it
> faster.

Does it depends on unmap_region()? Or IOW, does it has to be called 
after unmap_region()? Now the calling sequence is:

detach vmas
unmap_region()
mpx_notify_unmap()

I'm wondering if it is safe to move it up before unmap_region() like:

detach vmas
mpx_notify_unmap()
unmap_region()

With this change we also can do our optimization to do unmap_region() 
with read mmap_sem. Otherwise it does cause problem.

Thanks,
Yang