lkml.org 
[lkml]   [2018]   [Aug]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [RFC v8 PATCH 3/5] mm: mmap: zap pages with read mmap_sem in munmap
From
Date


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


\
 
 \ /
  Last update: 2018-08-22 23:57    [W:0.048 / U:1.656 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site