| Message ID | 20210323004912.35132-3-peterx@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | userfaultfd-wp: Support shmem and hugetlbfs | expand |
Hi: On 2021/3/23 8:48, Peter Xu wrote: > pte_unmap_same() will always unmap the pte pointer. After the unmap, vmf->pte > will not be valid any more. We should clear it. > > It was safe only because no one is accessing vmf->pte after pte_unmap_same() > returns, since the only caller of pte_unmap_same() (so far) is do_swap_page(), > where vmf->pte will in most cases be overwritten very soon. > > pte_unmap_same() will be used in other places in follow up patches, so that > vmf->pte will not always be re-written. This patch enables us to call > functions like finish_fault() because that'll conditionally unmap the pte by > checking vmf->pte first. Or, alloc_set_pte() will make sure to allocate a new > pte even after calling pte_unmap_same(). > > Since we'll need to modify vmf->pte, directly pass in vmf into pte_unmap_same() > and then we can also avoid the long parameter list. > > Signed-off-by: Peter Xu <peterx@redhat.com> Good cleanup! Thanks. Reviewed-by: Miaohe Lin <linmiaohe@huawei.com> > --- > mm/memory.c | 13 +++++++------ > 1 file changed, 7 insertions(+), 6 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index a458a595331f..d534eba85756 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -2607,19 +2607,20 @@ EXPORT_SYMBOL_GPL(apply_to_existing_page_range); > * proceeding (but do_wp_page is only called after already making such a check; > * and do_anonymous_page can safely check later on). > */ > -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd, > - pte_t *page_table, pte_t orig_pte) > +static inline int pte_unmap_same(struct vm_fault *vmf) > { > int same = 1; > #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPTION) > if (sizeof(pte_t) > sizeof(unsigned long)) { > - spinlock_t *ptl = pte_lockptr(mm, pmd); > + spinlock_t *ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd); > spin_lock(ptl); > - same = pte_same(*page_table, orig_pte); > + same = pte_same(*vmf->pte, vmf->orig_pte); > spin_unlock(ptl); > } > #endif > - pte_unmap(page_table); > + pte_unmap(vmf->pte); > + /* After unmap of pte, the pointer is invalid now - clear it. */ > + vmf->pte = NULL; > return same; > } > > @@ -3308,7 +3309,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > vm_fault_t ret = 0; > void *shadow = NULL; > > - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte)) > + if (!pte_unmap_same(vmf)) > goto out; > > entry = pte_to_swp_entry(vmf->orig_pte); >
On Tue, Mar 23, 2021 at 10:34:45AM +0800, Miaohe Lin wrote: > Hi: > On 2021/3/23 8:48, Peter Xu wrote: > > pte_unmap_same() will always unmap the pte pointer. After the unmap, vmf->pte > > will not be valid any more. We should clear it. > > > > It was safe only because no one is accessing vmf->pte after pte_unmap_same() > > returns, since the only caller of pte_unmap_same() (so far) is do_swap_page(), > > where vmf->pte will in most cases be overwritten very soon. > > > > pte_unmap_same() will be used in other places in follow up patches, so that > > vmf->pte will not always be re-written. This patch enables us to call > > functions like finish_fault() because that'll conditionally unmap the pte by > > checking vmf->pte first. Or, alloc_set_pte() will make sure to allocate a new > > pte even after calling pte_unmap_same(). > > > > Since we'll need to modify vmf->pte, directly pass in vmf into pte_unmap_same() > > and then we can also avoid the long parameter list. > > > > Signed-off-by: Peter Xu <peterx@redhat.com> > > Good cleanup! Thanks. > Reviewed-by: Miaohe Lin <linmiaohe@huawei.com> Just a note that this is not a pure cleanup - the latter patches may start to depend on the clearing of vmf->pte in their logic. Thanks for the quick review!
diff --git a/mm/memory.c b/mm/memory.c index a458a595331f..d534eba85756 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -2607,19 +2607,20 @@ EXPORT_SYMBOL_GPL(apply_to_existing_page_range); * proceeding (but do_wp_page is only called after already making such a check; * and do_anonymous_page can safely check later on). */ -static inline int pte_unmap_same(struct mm_struct *mm, pmd_t *pmd, - pte_t *page_table, pte_t orig_pte) +static inline int pte_unmap_same(struct vm_fault *vmf) { int same = 1; #if defined(CONFIG_SMP) || defined(CONFIG_PREEMPTION) if (sizeof(pte_t) > sizeof(unsigned long)) { - spinlock_t *ptl = pte_lockptr(mm, pmd); + spinlock_t *ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd); spin_lock(ptl); - same = pte_same(*page_table, orig_pte); + same = pte_same(*vmf->pte, vmf->orig_pte); spin_unlock(ptl); } #endif - pte_unmap(page_table); + pte_unmap(vmf->pte); + /* After unmap of pte, the pointer is invalid now - clear it. */ + vmf->pte = NULL; return same; } @@ -3308,7 +3309,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) vm_fault_t ret = 0; void *shadow = NULL; - if (!pte_unmap_same(vma->vm_mm, vmf->pmd, vmf->pte, vmf->orig_pte)) + if (!pte_unmap_same(vmf)) goto out; entry = pte_to_swp_entry(vmf->orig_pte);
pte_unmap_same() will always unmap the pte pointer. After the unmap, vmf->pte will not be valid any more. We should clear it. It was safe only because no one is accessing vmf->pte after pte_unmap_same() returns, since the only caller of pte_unmap_same() (so far) is do_swap_page(), where vmf->pte will in most cases be overwritten very soon. pte_unmap_same() will be used in other places in follow up patches, so that vmf->pte will not always be re-written. This patch enables us to call functions like finish_fault() because that'll conditionally unmap the pte by checking vmf->pte first. Or, alloc_set_pte() will make sure to allocate a new pte even after calling pte_unmap_same(). Since we'll need to modify vmf->pte, directly pass in vmf into pte_unmap_same() and then we can also avoid the long parameter list. Signed-off-by: Peter Xu <peterx@redhat.com> --- mm/memory.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)