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pc64: don't panic when running out of page-tables

We used to just panic if we'r out of kernel memory
for MMU page tables.

Instead, call flushmmu() and retry, waiting until
we can allocate the memory.

--- a/sys/src/9/pc64/mmu.c

+++ b/sys/src/9/pc64/mmu.c

@@ -179,10 +179,12 @@

 			n = 256;

 			p = malloc(n * sizeof(MMU));

 			if(p == nil)

-				panic("mmualloc: out of memory for MMU");

+				return nil;

 			p->page = mallocalign(n * PTSZ, BY2PG, 0, 0);

-			if(p->page == nil)

-				panic("mmualloc: out of memory for MMU pages");

+			if(p->page == nil){

+				free(p);

+				return nil;

+			}

 			for(i=1; i<n; i++){

 				p[i].page = p[i-1].page + (1<<PTSHIFT);

 				p[i-1].next = &p[i];

@@ -210,8 +212,8 @@

 	if(va < VMAP){

 		assert(up != nil);

 		assert((va < USTKTOP) || (va >= KMAP && va < KMAP+KMAPSIZE));

-

-		p = mmualloc();

+		if((p = mmualloc()) == nil)

+			return nil;

 		p->index = index;

 		p->level = level;

 		if(va < USTKTOP){

@@ -246,7 +248,7 @@

 }

 uintptr*

-mmuwalk(uintptr* table, uintptr va, int level, int create)

+mmuwalk(uintptr *table, uintptr va, int level, int create)

 {

 	uintptr pte;

 	int i, x;

@@ -256,7 +258,7 @@

 		pte = table[x];

 		if(pte & PTEVALID){

 			if(pte & PTESIZE)

-				return 0;

+				return nil;

 			pte = PPN(pte);

 			if(pte >= (uintptr)-KZERO)

 				table = (void*)(pte + VMAP);

@@ -264,8 +266,10 @@

 				table = (void*)(pte + KZERO);

 		} else {

 			if(!create)

-				return 0;

+				return nil;

 			table = mmucreate(table, va, i, x);

+			if(table == nil)

+				return nil;

 		}

 		x = PTLX(va, i);

 	}

@@ -272,6 +276,22 @@

 	return &table[x];

 }

+static uintptr*

+getpte(uintptr va)

+{

+	uintptr *pte;

+

+	if((pte = mmuwalk(m->pml4, va, 0, 1)) == nil){

+		flushmmu();

+		while((pte = mmuwalk(m->pml4, va, 0, 1)) == nil){

+			int x = spllo();

+			resrcwait("out of MMU pages");

+			splx(x);

+		}

+	}

+	return pte;

+}

+

 static int

 ptecount(uintptr va, int level)

 {

@@ -500,9 +520,7 @@

 	int x;

 	x = splhi();

-	pte = mmuwalk(m->pml4, va, 0, 1);

-	if(pte == 0)

-		panic("putmmu: bug: va=%#p pa=%#p", va, pa);

+	pte = getpte(va);

 	old = *pte;

 	*pte = pa | PTEACCESSED|PTEDIRTY|PTEUSER;

 	splx(x);

@@ -522,7 +540,7 @@

 	x = splhi();

 	pte = mmuwalk(m->pml4, va, 0, 0);

-	if(pte == 0 || ((old = *pte) & PTEVALID) == 0 || PPN(old) == pa){

+	if(pte == nil || ((old = *pte) & PTEVALID) == 0 || PPN(old) == pa){

 		splx(x);

 		return;

 	}

@@ -550,8 +568,8 @@

 	x = splhi();

 	va = KMAP + (((uintptr)up->kmapindex++ << PGSHIFT) & (KMAPSIZE-1));

-	pte = mmuwalk(m->pml4, va, 0, 1);

-	if(pte == 0 || (*pte & PTEVALID) != 0)

+	pte = getpte(va);

+	if((*pte & PTEVALID) != 0)

 		panic("kmap: pa=%#p va=%#p", pa, va);

 	*pte = pa | PTEACCESSED|PTEDIRTY|PTEWRITE|PTENOEXEC|PTEVALID;

 	splx(x);

@@ -571,7 +589,7 @@

 	x = splhi();

 	pte = mmuwalk(m->pml4, va, 0, 0);

-	if(pte == 0 || (*pte & PTEVALID) == 0)

+	if(pte == nil || (*pte & PTEVALID) == 0)

 		panic("kunmap: va=%#p", va);

 	*pte = 0;

 	splx(x);

@@ -632,9 +650,9 @@

 	for(va = (uintptr)a; n > 0; n -= z, va += z){

 		l = 0;

 		pte = mmuwalk(m->pml4, va, l, 0);

-		if(pte == 0)

+		if(pte == nil)

 			pte = mmuwalk(m->pml4, va, ++l, 0);

-		if(pte == 0 || (*pte & PTEVALID) == 0)

+		if(pte == nil || (*pte & PTEVALID) == 0)

 			panic("patwc: va=%#p", va);

 		z = PGLSZ(l);

 		z -= va & (z-1);

-- 

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