Re: [PATCH] x86/sgx: fix a NULL pointer

[Haitao Huang]

On Tue, 18 Jul 2023 15:56:27 -0500, Dave Hansen <dave.hansen@xxxxxxxxx>  
wrote:

> On 7/18/23 13:32, Haitao Huang wrote:
> ...
> > Ignore VA pages for now. Say for a system with 10 page EPC, 2 enclaves,
> > each needs 5 pages non-SECS so total demand would be 12 pages. The ksgxd
> > would only need to swap out 2 pages at the most to get one enclave fully
> > loaded with 6 pages, and the other one with 4 pages. There is no chance
> > the ksgxd would swap any one of two SECS pages.
> >
> > We would need at least one enclave A of 10 pages total to squeeze out
> > the other B completely. For that to happen B pretty much has to be
> > sleeping all the time so the LRU based reclaiming would hit it but not
> > pages of A. So no chance to hit #PF on pages of B still.
> >
> > So some minimal pressure is needed to ensure SECS swapped. The higher
> > the pressure the higher the chance to hit #PF while SECS is swapped.
>
> What would the second-to-last non-SECS page be?  A thread control page?
> VA page?
>
> As long as *that* page can generate a page fault, then you only need two
> pages for this scenario to happen:
>
> 1. Reclaimer takes encl->lock
> 2. #PF occurs from another thread, blocks on encl->lock
> 3. SECS is reclaimed
> 4. encl->lock released
> 5. #PF sees reclaimed SECS
I agree this is the race. But for this to happen, that is at #1 you have  
only one non-SECS page left so #3 can happen. That means it is already  
high pressure because reclaimer has swapped all other non-SECS.
In my example of two enclaves of 5 non-EPC pages. #3 won't happen because  
you don't reach #1 with only one non-SECS left.

Thanks
Haitao