[PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize direct map fragmentation

[Matthew Wilcox]

On Wed, Sep 30, 2020 at 01:27:45PM +0300, Mike Rapoport wrote:
> On Tue, Sep 29, 2020 at 05:15:52PM +0200, Peter Zijlstra wrote:
> > On Tue, Sep 29, 2020 at 05:58:13PM +0300, Mike Rapoport wrote:
> > > On Tue, Sep 29, 2020 at 04:12:16PM +0200, Peter Zijlstra wrote:
> > 
> > > > It will drop them down to 4k pages. Given enough inodes, and allocating
> > > > only a single sekrit page per pmd, we'll shatter the directmap into 4k.
> > > 
> > > Why? Secretmem allocates PMD-size page per inode and uses it as a pool
> > > of 4K pages for that inode. This way it ensures that
> > > __kernel_map_pages() is always called on PMD boundaries.
> > 
> > Oh, you unmap the 2m page upfront? I read it like you did the unmap at
> > the sekrit page alloc, not the pool alloc side of things.
> > 
> > Then yes, but then you're wasting gobs of memory. Basically you can pin
> > 2M per inode while only accounting a single page.
> 
> Right, quite like THP :)

Huh?  THP accounts every page it allocates.  If you allocate 2MB,
it accounts 512 pages.  And THP are reclaimable by vmscan, this is
obviously not.