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:

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.

  I've tried to find some numbers that show the benefit of using larger
  pages in the direct map, but I couldn't find anything so I've run a
  couple of benchmarks from phoronix-test-suite on my laptop (i7-8650U
  with 32G RAM).

Existing benchmarks suck at this, but FB had a load that had a
deterministic enough performance regression to bisect to a directmap
issue, fixed by:

  7af0145067bc ("x86/mm/cpa: Prevent large page split when ftrace flips RW on kernel text")

  I've tested three variants: the default with 28G of the physical
  memory covered with 1G pages, then I disabled 1G pages using
  "nogbpages" in the kernel command line and at last I've forced the
  entire direct map to use 4K pages using a simple patch to
  arch/x86/mm/init.c.  I've made runs of the benchmarks with SSD and
  tmpfs.
  
  Surprisingly, the results does not show huge advantage for large
  pages. For instance, here the results for kernel build with
  'make -j8', in seconds:

Your benchmark should stress the TLB of your uarch, such that additional
pressure added by the shattered directmap shows up.

And no, I don't have one either.

                        |  1G    |  2M    |  4K
  ----------------------+--------+--------+---------
  ssd, mitigations=on	| 308.75 | 317.37 | 314.9
  ssd, mitigations=off	| 305.25 | 295.32 | 304.92
  ram, mitigations=on	| 301.58 | 322.49 | 306.54
  ram, mitigations=off	| 299.32 | 288.44 | 310.65

These results lack error data, but assuming the reults are significant,
then this very much makes a case for 1G mappings. 5s on a kernel builds
is pretty good.