Re: [PATCH v3 2/5] x86/sgx: Use sgx_free_epc_page() in sgx_reclaim_pages()

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, Mar 11, 2021 at 09:36:15AM +1300, Kai Huang wrote:
> On Wed, 2021-03-10 at 17:11 +0200, Jarkko Sakkinen wrote:
> > On Wed, Mar 03, 2021 at 08:59:17AM -0800, Dave Hansen wrote:
> > > On 3/3/21 7:03 AM, Jarkko Sakkinen wrote:
> > > > diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
> > > > index 52d070fb4c9a..ed99c60024dc 100644
> > > > --- a/arch/x86/kernel/cpu/sgx/main.c
> > > > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > > > @@ -305,7 +305,6 @@ static void sgx_reclaim_pages(void)
> > > >  {
> > > >  	struct sgx_epc_page *chunk[SGX_NR_TO_SCAN];
> > > >  	struct sgx_backing backing[SGX_NR_TO_SCAN];
> > > > -	struct sgx_epc_section *section;
> > > >  	struct sgx_encl_page *encl_page;
> > > >  	struct sgx_epc_page *epc_page;
> > > >  	pgoff_t page_index;
> > > > @@ -378,11 +377,7 @@ static void sgx_reclaim_pages(void)
> > > >  		kref_put(&encl_page->encl->refcount, sgx_encl_release);
> > > >  		epc_page->flags &= ~SGX_EPC_PAGE_RECLAIMER_TRACKED;
> > > >  
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > 
> > > > -		section = &sgx_epc_sections[epc_page->section];
> > > > -		spin_lock(&section->lock);
> > > > -		list_add_tail(&epc_page->list, &section->page_list);
> > > > -		section->free_cnt++;
> > > > -		spin_unlock(&section->lock);
> > > > +		sgx_free_epc_page(epc_page);
> > > >  	}
> > > >  }
> > > 
> > > In current upstream (3fb6d0e00e), sgx_free_epc_page() calls __eremove().
> > >  This code does not call __eremove().  That seems to be changing
> > > behavior where none was intended.
> > 
> > EREMOVE does not matter here, as it doesn't in almost all most of the sites
> > where sgx_free_epc_page() is used in the driver. It does nothing to an
> > uninitialized pages.
> 
> Right. EREMOVE on uninitialized pages does nothing, so a more reasonable way is to
> just NOT call EREMOVE (your original code), since it is absolutely unnecessary.
> 
> I don't see ANY reason we should call EREMOVE here. 
> 
> Actually w/o my patch to split EREMOVE out of sgx_free_epc_page(), it then makes
> perfect sense to have new sgx_free_epc_page() here.
> 
> > 
> > The two patches that I posted originally for Kai's series took EREMOVE out
> > of sgx_free_epc_page() and put an explicit EREMOVE where it is actually
> > needed, but for reasons unknown to me, that change is gone.
> > 
> 
> It's not gone. It goes into a new sgx_encl_free_epc_page(), which is exactly the same
> as current sgx_free_epc_page() which as EREMOVE, instead of putting EREMOVE into a
> dedicated sgx_reset_epc_page(), as you did in your series:
> 
> https://lore.kernel.org/linux-sgx/20210113233541.17669-1-jarkko@kernel.org/
> 
> However, your change has side effort: it always put page back into free pool, even
> EREMOVE fails. To make your change w/o having any functional change, it has to be:
> 
> 	if(!sgx_reset_epc_page())
> 		sgx_free_epc_page();

OK, great, your patch set uses the wrapper only in the necessary call
sites. Sorry, I overlooked this part.

Anyway, it knowingly does that. I considered either as equally harmful
side-ffects when I implemented. Either can only trigger, when there is a
bug in the kernel code.

It *could* do what that snippet suggest but it's like "out of the frying pan,
into the fire" kind of change.

Since NUMA patch set anyway requires to have a global dirty list, I think
the better way to deal with this, would be to declare a new global in the
patch under discussion:

static struct list_head sgx_dirty_list;

And sgx_encl_free_epc_page() could simply put the pages in this list. In
some cases you could possibly even reset the system state using kexec for
debugging purposes, so it could potentially bring a tiny bit of value.

I can rebase then my NUMA patches on top of SGX specific KVM patches, once
Boris have applied them.

> And for this, Dave raised one concern we should add a WARN() to let user know EPC
> page is leaked, and reboot is requied to get them back.
> 
> However with sgx_reset_epc_page(), there's no place to add such WARN(), and
> implementing original sgx_free_epc_page() as sgx_encl_free_epc_page() looks very
> reasonable to me:
> 
> https://www.spinics.net/lists/linux-sgx/msg04631.html

/Jarkko