Re: [PATCH v29 00/20] Intel SGX foundations

[Jarkko Sakkinen]

On Thu, May 07, 2020 at 05:25:55PM -0700, Sean Christopherson wrote:
> Ah, fudge.  shmem_zero_setup() triggers shmem_acct_size() and thus
> __vm_enough_memory().  Which I should have rememered because I've stared
> at that code several times when dealing with the enclave's backing store.
> I wasn't seeing the issue because I happened to use MAP_PRIVATE.
> 
> So, bad analysis, good conclusion, i.e. the kernel is still doing the
> right thing, it's just not ideal for userspace.
> 
> 
> Jarkko, we should update the docs and selftest to recommend and use
> 
>   PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS
> 
> or
> 
>   PROT_NONE, MAP_SHARED | MAP_NORESERVE | MAP_ANONYMOUS"
> 
> when carving out ELRANGE, with an explicit comment that all the normal
> rules for mapping memory still apply.

Ugh, had forgotten this.

OK, I guess this comment explains it all:

"
/*
 * shmem_file_setup pre-accounts the whole fixed size of a VM object,
 * for shared memory and for shared anonymous (/dev/zero) mappings
 * (unless MAP_NORESERVE and sysctl_overcommit_memory <= 1),
 * consistent with the pre-accounting of private mappings ...
 */
static inline int shmem_acct_size(unsigned long flags, loff_t size)
"

/Jarkko