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:
On Fri, Sep 25, 2020 at 09:41:25AM +0200, Peter Zijlstra wrote:
On Thu, Sep 24, 2020 at 04:29:03PM +0300, Mike Rapoport wrote:
From: Mike Rapoport email@example.com
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.