# Re: \[PATCH V7\] Allow compaction of unevictable pages

- From: Andrew Morton \<akpm@linux-foundation.org\>
- Date: 2015-03-20 22:17:07 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2015/03/4456081
- Thread: https://ratatoskr.run/linux-rt-users/2015/03/4455740/t.md

```text
On Fri, 20 Mar 2015 09:49:50 -0400 Eric B Munson [ref] wrote:

> Currently, pages which are marked as unevictable are protected from
> compaction, but not from other types of migration.  The POSIX real time
> extension explicitly states that mlock() will prevent a major page
> fault, but the spirit of this is that mlock() should give a process the
> ability to control sources of latency, including minor page faults.
> However, the mlock manpage only explicitly says that a locked page will
> not be written to swap and this can cause some confusion.  The
> compaction code today does not give a developer who wants to avoid swap
> but wants to have large contiguous areas available any method to achieve
> this state.  This patch introduces a sysctl for controlling compaction
> behavior with respect to the unevictable lru.  Users that demand no page
> faults after a page is present can set compact_unevictable_allowed to 0
> and users who need the large contiguous areas can enable compaction on
> locked memory by leaving the default value of 1.

Do we really really really need the /proc knob?  We're already
migrating these pages so users of mlock will occasionally see some
latency - how likely is it that this patch will significantly damage
anyone?

```