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Date:   Wed, 4 May 2022 16:05:16 -0700
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Subject: Re: [RFC PATCH 0/4] SGX shmem backing store issue
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To:     Reinette Chatre <reinette.chatre@intel.com>,
        dave.hansen@linux.intel.com, jarkko@kernel.org,
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From:   Dave Hansen <dave.hansen@intel.com>
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On 5/4/22 15:13, Reinette Chatre wrote:
> -	if (pcmd_page_empty) {
> +	if (pcmd_page_empty && !pcmd_page_in_use(encl, pcmd_first_page)) {
>  		ida_free(&encl->pcmd_in_backing, PFN_DOWN(page_pcmd_off));
>  		sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
>  	}

One other thing.  The role of encl->lock here is very important.
Without it, two concurrent page faults could do their individual
memset(), each see !pcmd_page_empty, then decline to truncate the page.

Also, given the challenges here, I do think we should check the
pcmd_page after truncate to ensure it is still all zero's.