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To date, we have assumed that the TLB will only set the PG_M bit in a

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PTE if that PTE has the PG_RW bit set.  However, this assumption does
not hold on recent processors from Intel.  For example, consider a PTE
that has the PG_RW bit set but the PG_M bit clear.  Suppose this PTE
is cached in the TLB and later the PG_RW bit is cleared in the PTE,
but the corresponding TLB entry is not (yet) invalidated.
Historically, upon a write access using this (stale) TLB entry, the
TLB would observe that the PG_RW bit had been cleared and initiate a
page fault, aborting the setting of the PG_M bit in the PTE.  Now,
however, P4- and Core2-family processors will set the PG_M bit before
observing that the PG_RW bit is clear and initiating a page fault.  In
other words, the write does not occur but the PG_M bit is still set.

The real impact of this difference is not that great.  Specifically,
we should no longer assert that any PTE with the PG_M bit set must
also have the PG_RW bit set, and we should ignore the state of the
PG_M bit unless the PG_RW bit is set.  However, these changes enable
me to remove a work-around from pmap_promote_pde(), the superpage
promotion procedure.

(Note: The AMD processors that we have tested, including the latest,
the Phenom, still exhibit the historical behavior.)

Acknowledgments: After I observed the problem, Stephan (ups) was
instrumental in characterizing the exact behavior of Intel's recent
TLBs.

Tested by: Peter Holm
This commit is contained in:
alc 2008-03-23 20:38:01 +00:00
parent 5ddf5664cc
commit e702727e2c
1 changed files with 17 additions and 38 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

55
sys/amd64/amd64/pmap.c
View File

@ -1861,12 +1861,8 @@ pmap_collect(pmap_t locked_pmap, struct vpgqueues *vpq)
("pmap_collect: wired pte %#lx", tpte));
if (tpte & PG_A)
vm_page_flag_set(m, PG_REFERENCED);
if (tpte & PG_M) {
KASSERT((tpte & PG_RW),
("pmap_collect: modified page not writable: va: %#lx, pte: %#lx",
va, tpte));
if ((tpte & (PG_M | PG_RW)) == (PG_M | PG_RW))
vm_page_dirty(m);
}
free = NULL;
pmap_unuse_pt(pmap, va, *pde, &free);
pmap_invalidate_page(pmap, va);
@ -2311,12 +2307,8 @@ pmap_remove_pde(pmap_t pmap, pd_entry_t *pdq, vm_offset_t sva,
eva = sva + NBPDR;
for (va = sva, m = PHYS_TO_VM_PAGE(oldpde & PG_FRAME);
va < eva; va += PAGE_SIZE, m++) {
if (oldpde & PG_M) {
KASSERT((oldpde & PG_RW) != 0,
("pmap_remove_pde: modified 2mpage not writable: va: %#lx, pde: %#lx",
va, oldpde));
if ((oldpde & (PG_M | PG_RW)) == (PG_M | PG_RW))
vm_page_dirty(m);
}
if (oldpde & PG_A)
vm_page_flag_set(m, PG_REFERENCED);
if (TAILQ_EMPTY(&m->md.pv_list) &&
@ -2364,12 +2356,8 @@ pmap_remove_pte(pmap_t pmap, pt_entry_t *ptq, vm_offset_t va,
pmap->pm_stats.resident_count -= 1;
if (oldpte & PG_MANAGED) {
m = PHYS_TO_VM_PAGE(oldpte & PG_FRAME);
if (oldpte & PG_M) {
KASSERT((oldpte & PG_RW),
("pmap_remove_pte: modified page not writable: va: %#lx, pte: %#lx",
va, oldpte));
if ((oldpte & (PG_M | PG_RW)) == (PG_M | PG_RW))
vm_page_dirty(m);
}
if (oldpte & PG_A)
vm_page_flag_set(m, PG_REFERENCED);
pmap_remove_entry(pmap, m, va);
@ -2581,12 +2569,8 @@ pmap_remove_all(vm_page_t m)
/*
* Update the vm_page_t clean and reference bits.
*/
if (tpte & PG_M) {
KASSERT((tpte & PG_RW),
("pmap_remove_all: modified page not writable: va: %#lx, pte: %#lx",
pv->pv_va, tpte));
if ((tpte & (PG_M | PG_RW)) == (PG_M | PG_RW))
vm_page_dirty(m);
}
free = NULL;
pmap_unuse_pt(pmap, pv->pv_va, *pde, &free);
pmap_invalidate_page(pmap, pv->pv_va);
@ -2627,7 +2611,7 @@ pmap_protect_pde(pmap_t pmap, pd_entry_t *pde, vm_offset_t sva, vm_prot_t prot)
* page mapping with a stored page table page has PG_A
* set.
*/
if ((oldpde & PG_M) != 0)
if ((oldpde & (PG_M | PG_RW)) == (PG_M | PG_RW))
vm_page_dirty(m);
}
}
@ -2745,7 +2729,7 @@ pmap_protect(pmap_t pmap, vm_offset_t sva, vm_offset_t eva, vm_prot_t prot)
vm_page_flag_set(m, PG_REFERENCED);
pbits &= ~PG_A;
}
if ((pbits & PG_M) != 0) {
if ((pbits & (PG_M | PG_RW)) == (PG_M | PG_RW)) {
if (m == NULL)
m = PHYS_TO_VM_PAGE(pbits &
PG_FRAME);
@ -2818,12 +2802,15 @@ pmap_promote_pde(pmap_t pmap, pd_entry_t *pde, vm_offset_t va)
return;
}
if ((oldpte & (PG_M | PG_RW)) == PG_RW) {
/*
* When PG_M is already clear, PG_RW can be cleared
* without a TLB invalidation.
*/
if (!atomic_cmpset_long(pte, oldpte, oldpte & ~PG_RW))
goto retry;
oldpte &= ~PG_RW;
oldpteva = (oldpte & PG_FRAME & PDRMASK) |
(va & ~PDRMASK);
pmap_invalidate_page(pmap, oldpteva);
CTR2(KTR_PMAP, "pmap_promote_pde: protect for va %#lx"
" in pmap %p", oldpteva, pmap);
}
@ -3035,10 +3022,7 @@ pmap_enter(pmap_t pmap, vm_offset_t va, vm_prot_t access, vm_page_t m,
PG_NX) == 0 && (newpte & PG_NX)))
invlva = TRUE;
}
if (origpte & PG_M) {
KASSERT((origpte & PG_RW),
("pmap_enter: modified page not writable: va: %#lx, pte: %#lx",
va, origpte));
if ((origpte & (PG_M | PG_RW)) == (PG_M | PG_RW)) {
if ((origpte & PG_MANAGED) != 0)
vm_page_dirty(om);
if ((newpte & PG_RW) == 0)
@ -3096,7 +3080,7 @@ pmap_enter_pde(pmap_t pmap, vm_offset_t va, vm_page_t m, vm_prot_t prot)
newpde |= PG_MANAGED;
/*
* Create a PV entry for each of the managed pages.
* Abort this mapping if its PV entry could not be created.
*/
if (!pmap_pv_insert_pde(pmap, va, m)) {
free = NULL;
@ -3815,10 +3799,7 @@ pmap_remove_pages(pmap_t pmap)
/*
* Update the vm_page_t clean/reference bits.
*/
if (tpte & PG_M) {
KASSERT((tpte & PG_RW) != 0,
("pmap_remove_pages: modified page not writable: va: %#lx, pte: %#lx",
pv->pv_va, tpte));
if ((tpte & (PG_M | PG_RW)) == (PG_M | PG_RW)) {
if ((tpte & PG_PS) != 0) {
for (mt = m; mt < &m[NBPDR / PAGE_SIZE]; mt++)
vm_page_dirty(mt);
@ -3916,7 +3897,7 @@ pmap_is_modified_pvh(struct md_page *pvh)
pmap = PV_PMAP(pv);
PMAP_LOCK(pmap);
pte = pmap_pte(pmap, pv->pv_va);
rv = (*pte & PG_M) != 0;
rv = (*pte & (PG_M | PG_RW)) == (PG_M | PG_RW);
PMAP_UNLOCK(pmap);
if (rv)
break;
@ -4124,9 +4105,7 @@ pmap_clear_modify(vm_page_t m)
}
}
}
} else
KASSERT((oldpde & PG_M) == 0,
("pmap_clear_modify: modified page not writable"));
}
PMAP_UNLOCK(pmap);
}
TAILQ_FOREACH(pv, &m->md.pv_list, pv_list) {
@ -4136,7 +4115,7 @@ pmap_clear_modify(vm_page_t m)
KASSERT((*pde & PG_PS) == 0, ("pmap_clear_modify: found"
" a 2mpage in page %p's pv list", m));
pte = pmap_pde_to_pte(pde, pv->pv_va);
if (*pte & PG_M) {
if ((*pte & (PG_M | PG_RW)) == (PG_M | PG_RW)) {
atomic_clear_long(pte, PG_M);
pmap_invalidate_page(pmap, pv->pv_va);
}
@ -4415,7 +4394,7 @@ pmap_mincore(pmap_t pmap, vm_offset_t addr)
/*
* Modified by us
*/
if (pte & PG_M)
if ((pte & (PG_M | PG_RW)) == (PG_M | PG_RW))
val |= MINCORE_MODIFIED|MINCORE_MODIFIED_OTHER;
else {
/*