linux-kernel - Re: [PATCH v3 2/5] x86/sgx: Use sgx_free_epc_page() in sgx_reclaim

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Message-ID: <YEjhjhBpYJ6i6EFD@kernel.org>
Date:   Wed, 10 Mar 2021 17:11:10 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     linux-sgx@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/5] x86/sgx: Use sgx_free_epc_page() in
 sgx_reclaim_pages()

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.

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.

Replacing the ad-hoc code with sgx_free_epc_page() is absolutely the right
action to take because it follows the pattern how sgx_free_epc_page() is
used in the driver.

For reference:

https://lore.kernel.org/linux-sgx/20210113233541.17669-1-jarkko@kernel.org/

> Was this, perhaps, based on top of Kai's series that changes the
> behavior of sgx_free_epc_page()?

I did not refer to that patch series.

/Jarkko