Subject: Re: [PATCH v11 1/7] mm: ksm: Export ksm_madvise()

From: Bharata B Rao <[email-masked]>
Date: Wed, 27 Nov 2019 12:23:00 +0530

On Tue, Nov 26, 2019 at 07:59:49PM -0800, Hugh Dickins wrote:
> On Mon, 25 Nov 2019, Bharata B Rao wrote:
> 
> > On PEF-enabled POWER platforms that support running of secure guests,
> > secure pages of the guest are represented by device private pages
> > in the host. Such pages needn't participate in KSM merging. This is
> > achieved by using ksm_madvise() call which need to be exported
> > since KVM PPC can be a kernel module.
> > 
> > Signed-off-by: Bharata B Rao <[email-masked]>
> > Acked-by: Paul Mackerras <[email-masked]>
> > Cc: Andrea Arcangeli <[email-masked]>
> > Cc: Hugh Dickins <[email-masked]>
> 
> I can say
> Acked-by: Hugh Dickins <[email-masked]>
> to this one.
> 
> But not to your 2/7 which actually makes use of it: because sadly it
> needs down_write(&kvm->mm->mmap_sem) for the case when it switches off
> VM_MERGEABLE in vma->vm_flags.  That's frustrating, since I think it's
> the only operation for which down_read() is not good enough.

Oh ok! Thanks for pointing this out.

> 
> I have no idea how contended that mmap_sem is likely to be, nor how
> many to-be-secured pages that vma is likely to contain: you might find
> it okay simply to go with it down_write throughout, or you might want
> to start out with it down_read, and only restart with down_write (then
> perhaps downgrade_write later) when you see VM_MERGEABLE is set.

Using down_write throughtout is not easy as we do migrate_vma_pages()
from fault path (->migrate_to_ram()) too. Here we come with down_read
already held.

Starting with down_read and restarting with down_write if VM_MERGEABLE
is set -- this also looks a bit difficult as we will have challenges
with locking order if we release mmap_sem in between and re-acquire.

So I think I will start with down_write in this particular case
and will downgrade_write as soon as ksm_madvise() is complete.

> 
> The crash you got (thanks for the link): that will be because your
> migrate_vma_pages() had already been applied to a page that was
> already being shared via KSM.
> 
> But if these secure pages are expected to be few and far between,
> maybe you'd prefer to keep VM_MERGEABLE, and add per-page checks
> of some kind into mm/ksm.c, to skip over these surprising hybrids.

I did bail out from a few routines in mm/ksm.c with
is_device_private_page(page) check, but that wasn't good enough and
I encountered crashes in different code paths. Guess a bit more
understanding of KSM internals would be required before retrying that.

However since all the pages of the guest except for a few will be turned
into secure pages early during boot, it appears better if secure guests
don't participate in in KSM merging at all.

Regards,
Bharata.