Re: [RFC] Tracking mlocked pages and moving them off the LRU

[Martin J. Bligh]

Christoph Hellwig wrote:
> On Fri, Feb 02, 2007 at 10:20:12PM -0800, Christoph Lameter wrote:
> > This is a new variation on the earlier RFC for tracking mlocked pages.
> > We now mark a mlocked page with a bit in the page flags and remove
> > them from the LRU. Pages get moved back when no vma that references
> > the page has VM_LOCKED set anymore.
> >
> > This means that vmscan no longer uselessly cycles over large amounts
> > of mlocked memory should someone attempt to mlock large amounts of
> > memory (may even result in a livelock on large systems).
> >
> > Synchronization is build around state changes of the PageMlocked bit.
> > The NR_MLOCK counter is incremented and decremented based on
> > state transitions of PageMlocked. So the count is accurate.
> >
> > There is still some unfinished business:
> >
> > 1. We use the 21st page flag and we only have 20 on 32 bit NUMA platforms.
> >
> > 2. Since mlocked pages are now off the LRU page migration will no longer
> >    move them.
> >
> > 3. Use NR_MLOCK to tune various VM behaviors so that the VM does not 
> >    longer fall due to too many mlocked pages in certain areas.
>
> This patch seems to not handle the cases where more than one process mlocks
> a page and you really need a pincount in the page to not release it before
> all processes have munlock it or died.  I did a similar patch a while
> ago and tried to handle it by overloading the lru lists pointers with
> a pincount, but at some point I gave up because I couldn't get that part
> right.

Doesn't matter - you can just do it lazily. If you find a page that is
locked, move it to the locked list. when unlocking a page you *always*
move it back to the normal list. If someone else is still locking it,
we'll move it back to the lock list on next reclaim pass.

I have a half-finished patch from 6 months ago that does this, but never
found time to complete it ;-(

M.
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