linux-kernel - Re: [PATCH v2 09/17] powerpc/mm: Add new hash_page

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Message-ID: <1412235563.6143.33.camel@ale.ozlabs.ibm.com>
Date:	Thu, 02 Oct 2014 17:39:23 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	greg@...ah.com, arnd@...db.de, benh@...nel.crashing.org,
	anton@...ba.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...abs.org, jk@...abs.org, imunsie@...ibm.com,
	cbe-oss-dev@...ts.ozlabs.org,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 09/17] powerpc/mm: Add new hash_page_mm()

On Thu, 2014-10-02 at 13:48 +1000, Michael Ellerman wrote:
> On Tue, 2014-30-09 at 10:34:58 UTC, Michael Neuling wrote:
> > From: Ian Munsie <imunsie@....ibm.com>
> > 
> > This adds a new function hash_page_mm() based on the existing hash_page().
> > This version allows any struct mm to be passed in, rather than assuming
> > current.  This is useful for servicing co-processor faults which are not in the
> > context of the current running process.
> 
> I'm not a big fan. hash_page() is already a train wreck, and this doesn't make
> it any better.

I can document it to make the situation a bit better.  It's certainly
not clear which one to use here and under what circumstances.  It's
basically ask benh territory.  

> > diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
> > index bbdb054..0a5c8c0 100644
> > --- a/arch/powerpc/mm/hash_utils_64.c
> > +++ b/arch/powerpc/mm/hash_utils_64.c
> > @@ -904,7 +904,7 @@ void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
> >  		return;
> >  	slice_set_range_psize(mm, addr, 1, MMU_PAGE_4K);
> >  	copro_flush_all_slbs(mm);
> > -	if (get_paca_psize(addr) != MMU_PAGE_4K) {
> > +	if ((get_paca_psize(addr) != MMU_PAGE_4K) && (current->mm == mm)) {
> >  		get_paca()->context = mm->context;
> >  		slb_flush_and_rebolt();
> 
> This is a bit fishy.
> 
> If that mm is currently running on another cpu you just failed to update it's
> paca. But I think the call to check_paca_psize() in hash_page() will save you
> on that cpu.
> 
> In fact we might be able to remove that synchronisation from
> demote_segment_4k() and always leave it up to check_paca_psize()?

Aneesh asked the same thing for v1 and we convinced ourselves it was ok.
I said this at the time...

I had a chat to benh offline about this and he thinks it's fine.  A
running process in the same mm context will either have hit this mapping
or not.  If it's hit it, the page will be invalidated and it'll come in
via hash_page and have it's segment demoted also (and paca updated).  If
it hasn't hit, again it'll come into hash_page() and get demoted also.

> > @@ -989,26 +989,24 @@ static void check_paca_psize(unsigned long ea, struct mm_struct *mm,
> >   * -1 - critical hash insertion error
> >   * -2 - access not permitted by subpage protection mechanism
> >   */
> > -int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
> > +int hash_page_mm(struct mm_struct *mm, unsigned long ea, unsigned long access, unsigned long trap)
> >  {
> >  	enum ctx_state prev_state = exception_enter();
> >  	pgd_t *pgdir;
> >  	unsigned long vsid;
> > -	struct mm_struct *mm;
> >  	pte_t *ptep;
> >  	unsigned hugeshift;
> >  	const struct cpumask *tmp;
> >  	int rc, user_region = 0, local = 0;
> >  	int psize, ssize;
> >  
> > -	DBG_LOW("hash_page(ea=%016lx, access=%lx, trap=%lx\n",
> > -		ea, access, trap);
> > +	DBG_LOW("%s(ea=%016lx, access=%lx, trap=%lx\n",
> > +		__func__, ea, access, trap);
> >  
> >  	/* Get region & vsid */
> >   	switch (REGION_ID(ea)) {
> >  	case USER_REGION_ID:
> >  		user_region = 1;
> > -		mm = current->mm;
> >  		if (! mm) {
> >  			DBG_LOW(" user region with no mm !\n");
> >  			rc = 1;
> 
> What about the VMALLOC case where we do:
> 		mm = &init_mm;
> 		
> Is that what you want? It seems odd that you pass an mm to the routine, but
> then potentially it ends up using a different mm after all depending on the
> address.

Good point.  We have hash_page() still.  I can make that check in there
and decide which mm to use and pass that to hash_page_mm().   Then we
always use mm in hash_page_mm().  hash_page() will then look like this: 

int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
{
	struct mm_struct *mm = current->mm;

	if (REGION_ID(ea) == VMALLOC_REGION_ID)
		mm = &init_mm;

	return hash_page_mm(mm, ea, access, trap);
}

Mikey
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