linux-kernel - Re: [PATCH v3 03/28] x86/sgx: Add 'struct sgx_epc_lru

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Message-ID: <4f5496d2e0ea8edba430e7de7304bdd840616146.camel@intel.com>
Date:   Mon, 24 Jul 2023 10:04:48 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "hpa@...or.com" <hpa@...or.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "bp@...en8.de" <bp@...en8.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "x86@...nel.org" <x86@...nel.org>,
        "haitao.huang@...ux.intel.com" <haitao.huang@...ux.intel.com>,
        "tj@...nel.org" <tj@...nel.org>,
        "mingo@...hat.com" <mingo@...hat.com>
CC:     "kristen@...ux.intel.com" <kristen@...ux.intel.com>,
        "Chatre, Reinette" <reinette.chatre@...el.com>,
        "Li, Zhiquan1" <zhiquan1.li@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH v3 03/28] x86/sgx: Add 'struct sgx_epc_lru_lists' to
 encapsulate lru list(s)

On Mon, 2023-07-17 at 08:23 -0500, Haitao Huang wrote:
> On Mon, 17 Jul 2023 07:45:36 -0500, Jarkko Sakkinen <jarkko@...nel.org>  
> wrote:
> 
> > On Wed Jul 12, 2023 at 11:01 PM UTC, Haitao Huang wrote:
> > > From: Kristen Carlson Accardi <kristen@...ux.intel.com>
> > > 
> > > Introduce a data structure to wrap the existing reclaimable list
> > > and its spinlock in a struct to minimize the code changes needed
> > > to handle multiple LRUs as well as reclaimable and non-reclaimable
> > > lists. The new structure will be used in a following set of patches to
> > > implement SGX EPC cgroups.

Although briefly mentioned in the first patch, it would be better to put more
background about the "reclaimable" and "non-reclaimable" thing here, focusing on
_why_ we need multiple LRUs (presumably you mean two lists: reclaimable and non-
reclaimable).

> > > 
> > > The changes to the structure needed for unreclaimable lists will be
> > > added in later patches.
> > > 
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > > Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> > > Signed-off-by: Haitao Huang <haitao.huang@...ux.intel.com>
> > > Cc: Sean Christopherson <seanjc@...gle.com>
> > > 
> > > V3:
> > > Removed the helper functions and revised commit messages

Please put change history into:

---
  change history
---

So it can be stripped away when applying the patch.

> > > ---
> > >  arch/x86/kernel/cpu/sgx/sgx.h | 17 +++++++++++++++++
> > >  1 file changed, 17 insertions(+)
> > > 
> > > diff --git a/arch/x86/kernel/cpu/sgx/sgx.h  
> > > b/arch/x86/kernel/cpu/sgx/sgx.h
> > > index f6e3c5810eef..77fceba73a25 100644
> > > --- a/arch/x86/kernel/cpu/sgx/sgx.h
> > > +++ b/arch/x86/kernel/cpu/sgx/sgx.h
> > > @@ -92,6 +92,23 @@ static inline void *sgx_get_epc_virt_addr(struct  
> > > sgx_epc_page *page)
> > >  	return section->virt_addr + index * PAGE_SIZE;
> > >  }
> > > 
> > > +/*
> > > + * This data structure wraps a list of reclaimable EPC pages, and a  
> > > list of
> > > + * non-reclaimable EPC pages and is used to implement a LRU policy  
> > > during
> > > + * reclamation.
> > > + */

I'd prefer to not mention the "non-reclaimable" thing in this patch, but defer
to the one actually introduces the "non-reclaimable" list.  Actually, I don't
think we even need this comment, given you have this in the structure:

	struct list_head reclaimable;

Which already explains the "reclaimable" list.  I suppose the non-reclaimable
list would be named similarly thus need no comment either.

Also, I am wondering why you need to split this out as a separate patch.  It
basically does nothing.  To me you should just merge this to the next patch,
which actually does what you claimed in the changelog:

	Introduce a data structure to wrap the existing reclaimable list and 
	its spinlock ...

Then this can be an infrastructure change patch, which doesn't bring any
functional change, to support the non-reclaimable list.


> > > +struct sgx_epc_lru_lists {
> > > +	/* Must acquire this lock to access */
> > > +	spinlock_t lock;
> > 
> > Isn't this self-explanatory, why the inline comment?
> 
> I got a warning from the checkpatch script complaining this lock needs  
> comments.

I suspected this, so I applied this patch, removed the comment, generated a new
patch, and run checkpatch.pl for it.  It didn't report any warning/error in my
testing.

Are you sure you got a warning?