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

[Peter Zijlstra]

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.