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

On Tue, Sep 29, 2020 at 04:12:16PM +0200, Peter Zijlstra wrote:

On Tue, Sep 29, 2020 at 04:05:29PM +0300, Mike Rapoport wrote:

quoted

On Fri, Sep 25, 2020 at 09:41:25AM +0200, Peter Zijlstra wrote:

quoted

On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote:

quoted

From: Mike Rapoport <redacted>

Removing a PAGE_SIZE page from the direct map every time such page is
allocated for a secret memory mapping will cause severe fragmentation of
the direct map. This fragmentation can be reduced by using PMD-size pages
as a pool for small pages for secret memory mappings.

Add a gen_pool per secretmem inode and lazily populate this pool with
PMD-size pages.

What's the actual efficacy of this? Since the pmd is per inode, all I
need is a lot of inodes and we're in business to destroy the directmap,
no?

Afaict there's no privs needed to use this, all a process needs is to
stay below the mlock limit, so a 'fork-bomb' that maps a single secret
page will utterly destroy the direct map.

This indeed will cause 1G pages in the direct map to be split into 2M
chunks, but I disagree with 'destroy' term here. Citing the cover letter
of an earlier version of this series:

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.

-- 
Sincerely yours,
Mike.