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Fix two problems with pmap_clear_modify().

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First, pmap_clear_modify() is write protecting all mappings to the specified
page, not just clearing the modified bit.  Specifically, it sets PTE_RO on
the PTE, which is wrong.  Moreover, it is calling vm_page_dirty(), which is
not the expected behavior for pmap_clear_modify().  Generally speaking, the
machine-independent VM layer masks these mistakes.  For example, setting
PTE_RO will result in additional soft faults, but not a catastrophe.

Second, pmap_clear_modify() may not clear the modified bits because it only
iterates over the PV list when the page has the PV_TABLE_MOD flag set and
elsewhere the pmap clears the PV_TABLE_MOD flag anytime a modified mapping
is write protected or destroyed.  However, the page may still have other
mappings with the modified bit set.

Eliminate a stale comment.
This commit is contained in:
Alan Cox 2012-08-17 05:02:29 +00:00
parent 8c09f7b626
commit f274a47134
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=239352
1 changed files with 13 additions and 51 deletions
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64
sys/mips/mips/pmap.c
View File

@ -179,7 +179,6 @@ static vm_page_t pmap_pv_reclaim(pmap_t locked_pmap);
static void pmap_pvh_free(struct md_page *pvh, pmap_t pmap, vm_offset_t va);
static pv_entry_t pmap_pvh_remove(struct md_page *pvh, pmap_t pmap,
vm_offset_t va);
static __inline void pmap_changebit(vm_page_t m, int bit, boolean_t setem);
static vm_page_t pmap_enter_quick_locked(pmap_t pmap, vm_offset_t va,
vm_page_t m, vm_prot_t prot, vm_page_t mpte);
static int pmap_remove_pte(struct pmap *pmap, pt_entry_t *ptq, vm_offset_t va,
@ -2664,8 +2663,6 @@ pmap_remove_pages(pmap_t pmap)
/*
* pmap_testbit tests bits in pte's
* note that the testbit/changebit routines are inline,
* and a lot of things compile-time evaluate.
*/
static boolean_t
pmap_testbit(vm_page_t m, int bit)
@ -2691,51 +2688,6 @@ pmap_testbit(vm_page_t m, int bit)
return (rv);
}
/*
* this routine is used to clear dirty bits in ptes
*/
static __inline void
pmap_changebit(vm_page_t m, int bit, boolean_t setem)
{
pv_entry_t pv;
pmap_t pmap;
pt_entry_t *pte;
if (m->oflags & VPO_UNMANAGED)
return;
rw_assert(&pvh_global_lock, RA_WLOCKED);
/*
* Loop over all current mappings setting/clearing as appropos If
* setting RO do we need to clear the VAC?
*/
TAILQ_FOREACH(pv, &m->md.pv_list, pv_list) {
pmap = PV_PMAP(pv);
PMAP_LOCK(pmap);
pte = pmap_pte(pmap, pv->pv_va);
if (setem) {
*pte |= bit;
pmap_update_page(pmap, pv->pv_va, *pte);
} else {
pt_entry_t pbits = *pte;
if (pbits & bit) {
if (bit == PTE_D) {
if (pbits & PTE_D)
vm_page_dirty(m);
*pte = (pbits & ~PTE_D) | PTE_RO;
} else {
*pte = pbits & ~bit;
}
pmap_update_page(pmap, pv->pv_va, *pte);
}
}
PMAP_UNLOCK(pmap);
}
if (!setem && bit == PTE_D)
vm_page_aflag_clear(m, PGA_WRITEABLE);
}
/*
* pmap_page_wired_mappings:
*
@ -2896,6 +2848,9 @@ pmap_is_prefaultable(pmap_t pmap, vm_offset_t addr)
void
pmap_clear_modify(vm_page_t m)
{
pmap_t pmap;
pt_entry_t *pte;
pv_entry_t pv;
KASSERT((m->oflags & VPO_UNMANAGED) == 0,
("pmap_clear_modify: page %p is not managed", m));
@ -2911,10 +2866,17 @@ pmap_clear_modify(vm_page_t m)
if ((m->aflags & PGA_WRITEABLE) == 0)
return;
rw_wlock(&pvh_global_lock);
if (m->md.pv_flags & PV_TABLE_MOD) {
pmap_changebit(m, PTE_D, FALSE);
m->md.pv_flags &= ~PV_TABLE_MOD;
TAILQ_FOREACH(pv, &m->md.pv_list, pv_list) {
pmap = PV_PMAP(pv);
PMAP_LOCK(pmap);
pte = pmap_pte(pmap, pv->pv_va);
if (pte_test(pte, PTE_D)) {
pte_clear(pte, PTE_D);
pmap_update_page(pmap, pv->pv_va, *pte);
}
PMAP_UNLOCK(pmap);
}
m->md.pv_flags &= ~PV_TABLE_MOD;
rw_wunlock(&pvh_global_lock);
}