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:

quoted

From: Ian Munsie <redacted>
=20
This adds a new function hash_page_mm() based on the existing hash_page=

().

quoted

This version allows any struct mm to be passed in, rather than assuming
current.  This is useful for servicing co-processor faults which are no=

t in the

quoted

context of the current running process.

=20
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. =20

quoted

diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_uti=

ls_64.c

quoted

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, unsign=

ed long addr)

quoted

 		return;
 	slice_set_range_psize(mm, addr, 1, MMU_PAGE_4K);
 	copro_flush_all_slbs(mm);
-	if (get_paca_psize(addr) !=3D MMU_PAGE_4K) {
+	if ((get_paca_psize(addr) !=3D MMU_PAGE_4K) && (current->mm =3D=3D mm=

)) {

quoted

 		get_paca()->context =3D mm->context;
 		slb_flush_and_rebolt();

=20
This is a bit fishy.
=20
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.
=20
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.

quoted

@@ -989,26 +989,24 @@ static void check_paca_psize(unsigned long ea, st=

ruct mm_struct *mm,

quoted

  * -1 - critical hash insertion error
  * -2 - access not permitted by subpage protection mechanism
  */
-int hash_page(unsigned long ea, unsigned long access, unsigned long tr=

ap)

quoted

+int hash_page_mm(struct mm_struct *mm, unsigned long ea, unsigned long=

 access, unsigned long trap)

quoted

 {
 	enum ctx_state prev_state =3D 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 =3D 0, local =3D 0;
 	int psize, ssize;
=20
-	DBG_LOW("hash_page(ea=3D%016lx, access=3D%lx, trap=3D%lx\n",
-		ea, access, trap);
+	DBG_LOW("%s(ea=3D%016lx, access=3D%lx, trap=3D%lx\n",
+		__func__, ea, access, trap);
=20
 	/* Get region & vsid */
  	switch (REGION_ID(ea)) {
 	case USER_REGION_ID:
 		user_region =3D 1;
-		mm =3D current->mm;
 		if (! mm) {
 			DBG_LOW(" user region with no mm !\n");
 			rc =3D 1;

=20
What about the VMALLOC case where we do:
		mm =3D &init_mm;
	=09
Is that what you want? It seems odd that you pass an mm to the routine, b=

ut

then potentially it ends up using a different mm after all depending on t=

he

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:=20

int hash_page(unsigned long ea, unsigned long access, unsigned long trap)
{
	struct mm_struct *mm =3D current->mm;

	if (REGION_ID(ea) =3D=3D VMALLOC_REGION_ID)
		mm =3D &init_mm;

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

Mikey