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

[James Bottomley]

On Wed, 2020-09-30 at 13:27 +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 :)
> 
> I considered using a global pool of 2M pages for secretmem and
> handing 4K pages to each inode from that global pool. But I've
> decided to waste memory in favor of simplicity.

I can also add that the user space consumer of this we wrote does its
user pool allocation at a 2M granularity, so nothing is actually
wasted.

https://git.kernel.org/pub/scm/linux/kernel/git/jejb/secret-memory-preloader.git/

James