lkml.org 
[lkml]   [2015]   [Aug]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v7 3/6] mm: Introduce VM_LOCKONFAULT
On Tue, 25 Aug 2015, Michal Hocko wrote:

> On Fri 21-08-15 14:31:32, Eric B Munson wrote:
> [...]
> > I am in the middle of implementing lock on fault this way, but I cannot
> > see how we will hanlde mremap of a lock on fault region. Say we have
> > the following:
> >
> > addr = mmap(len, MAP_ANONYMOUS, ...);
> > mlock(addr, len, MLOCK_ONFAULT);
> > ...
> > mremap(addr, len, 2 * len, ...)
> >
> > There is no way for mremap to know that the area being remapped was lock
> > on fault so it will be locked and prefaulted by remap. How can we avoid
> > this without tracking per vma if it was locked with lock or lock on
> > fault?
>
> Yes mremap is a problem and it is very much similar to mmap(MAP_LOCKED).
> It doesn't guarantee the full mlock semantic because it leaves partially
> populated ranges behind without reporting any error.

This was not my concern. Instead, I was wondering how to keep lock on
fault sematics with mremap if we do not have a VMA flag. As a user, it
would surprise me if a region I mlocked with lock on fault and then
remapped to a larger size was fully populated and locked by the mremap
call.

>
> Considering the current behavior I do not thing it would be terrible
> thing to do what Konstantin was suggesting and populate only the full
> ranges in a best effort mode (it is done so anyway) and document the
> behavior properly.
> "
> If the memory segment specified by old_address and old_size is
> locked (using mlock(2) or similar), then this lock is maintained
> when the segment is resized and/or relocated. As a consequence,
> the amount of memory locked by the process may change.
>
> If the range is already fully populated and the range is
> enlarged the new range is attempted to be fully populated
> as well to preserve the full mlock semantic but there is no
> guarantee this will succeed. Partially populated (e.g. created by
> mlock(MLOCK_ONFAULT)) ranges do not have the full mlock semantic
> so they are not populated on resize.
> "

You are proposing that mremap would scan the PTEs as Vlastimil has
suggested?

>
> So what we have as a result is that partially populated ranges are
> preserved and fully populated ones work in the best effort mode the same
> way as they are now.
>
> Does that sound at least remotely reasonably?
>
>
> --
> Michal Hocko
> SUSE Labs
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2015-08-25 16:41    [W:0.174 / U:0.064 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site