[PATCH 1/2] mm: bring in additional flag for fixup_user_fault to signal unlock

Dominik Dingel
2015-11-26T17:27:20Z

With the introduction of userfaultfd, kvm on s390 needs fixup_user_fault to
pass in FAULT_FLAG_ALLOW_RETRY and give feedback if during the faulting we
ever unlocked mmap_sem.

This patch brings in the logic to handle retries as well as it cleans up
the current documentation. fixup_user_fault was not having the same
semantics as filemap_fault. It never indicated if a retry happened and so
a caller wasn't able to handle that case. So we now changed the behaviour
to always retry a locked mmap_sem.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
---
arch/s390/mm/pgtable.c | 8 +++++---
include/linux/mm.h | 5 +++--
kernel/futex.c | 2 +-
mm/gup.c | 25 +++++++++++++++++++++----
4 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index 54ef3bc..b15759c 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -585,7 +585,7 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
rc = vmaddr;
goto out_up;
}
- if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags)) {
+ if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags, NULL)) {
rc = -EFAULT;
goto out_up;
}
@@ -730,7 +730,8 @@ int gmap_ipte_notify(struct gmap *gmap, unsigned long gaddr, unsigned long len)
break;
}
/* Get the page mapped */
- if (fixup_user_fault(current, gmap->mm, addr, FAULT_FLAG_WRITE)) {
+ if (fixup_user_fault(current, gmap->mm, addr, FAULT_FLAG_WRITE,
+ NULL)) {
rc = -EFAULT;
break;
}
@@ -805,7 +806,8 @@ retry:
if (!(pte_val(*ptep) & _PAGE_INVALID) &&
(pte_val(*ptep) & _PAGE_PROTECT)) {
pte_unmap_unlock(ptep, ptl);
- if (fixup_user_fault(current, mm, addr, FAULT_FLAG_WRITE)) {
+ if (fixup_user_fault(current, mm, addr, FAULT_FLAG_WRITE,
+ NULL)) {
up_read(&mm->mmap_sem);
return -EFAULT;
}
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 00bad77..7783073 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1163,7 +1163,8 @@ int invalidate_inode_page(struct page *page);
extern int handle_mm_fault(struct mm_struct *mm, struct vm_area_struct *vma,
unsigned long address, unsigned int flags);
extern int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
- unsigned long address, unsigned int fault_flags);
+ unsigned long address, unsigned int fault_flags,
+ bool *unlocked);
#else
static inline int handle_mm_fault(struct mm_struct *mm,
struct vm_area_struct *vma, unsigned long address,
@@ -1175,7 +1176,7 @@ static inline int handle_mm_fault(struct mm_struct *mm,
}
static inline int fixup_user_fault(struct task_struct *tsk,
struct mm_struct *mm, unsigned long address,
- unsigned int fault_flags)
+ unsigned int fault_flags, bool *unlocked)
{
/* should never happen if there's no MMU */
BUG();
diff --git a/kernel/futex.c b/kernel/futex.c
index 684d754..fb640c5 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -639,7 +639,7 @@ static int fault_in_user_writeable(u32 __user *uaddr)

down_read(&mm->mmap_sem);
ret = fixup_user_fault(current, mm, (unsigned long)uaddr,
- FAULT_FLAG_WRITE);
+ FAULT_FLAG_WRITE, NULL);
up_read(&mm->mmap_sem);

return ret < 0 ? ret : 0;
diff --git a/mm/gup.c b/mm/gup.c
index deafa2c..4ed35a3 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -564,6 +564,8 @@ EXPORT_SYMBOL(__get_user_pages);
* @mm: mm_struct of target mm
* @address: user address
* @fault_flags:flags to pass down to handle_mm_fault()
+ * @unlocked: did we unlock the mmap_sem while retrying, maybe NULL if caller
+ * does not allow retry
*
* This is meant to be called in the specific scenario where for locking reasons
* we try to access user memory in atomic context (within a pagefault_disable()
@@ -575,17 +577,19 @@ EXPORT_SYMBOL(__get_user_pages);
* The main difference with get_user_pages() is that this function will
* unconditionally call handle_mm_fault() which will in turn perform all the
* necessary SW fixup of the dirty and young bits in the PTE, while
- * handle_mm_fault() only guarantees to update these in the struct page.
+ * get_user_pages() only guarantees to update these in the struct page.
*
* This is important for some architectures where those bits also gate the
* access permission to the page because they are maintained in software. On
* such architectures, gup() will not be enough to make a subsequent access
* succeed.
*
- * This has the same semantics wrt the @mm->mmap_sem as does filemap_fault().
+ * This function will not return with an unlocked mmap_sem. So it has not the
+ * same semantics wrt the @mm->mmap_sem as does filemap_fault().
*/
int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
- unsigned long address, unsigned int fault_flags)
+ unsigned long address, unsigned int fault_flags,
+ bool *unlocked)
{
struct vm_area_struct *vma;
vm_flags_t vm_flags;
@@ -599,6 +603,10 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
if (!(vm_flags & vma->vm_flags))
return -EFAULT;

+ if (unlocked)
+ fault_flags |= FAULT_FLAG_ALLOW_RETRY;
+
+retry:
ret = handle_mm_fault(mm, vma, address, fault_flags);
if (ret & VM_FAULT_ERROR) {
if (ret & VM_FAULT_OOM)
@@ -609,12 +617,21 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm,
return -EFAULT;
BUG();
}
- if (tsk) {
+ if (tsk && !(fault_flags & FAULT_FLAG_TRIED)) {
if (ret & VM_FAULT_MAJOR)
tsk->maj_flt++;
else
tsk->min_flt++;
}
+ if (ret & VM_FAULT_RETRY) {
+ down_read(&mm->mmap_sem);
+ if (!(fault_flags & FAULT_FLAG_TRIED)) {
+ *unlocked = true;
+ fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
+ fault_flags |= FAULT_FLAG_TRIED;
+ goto retry;
+ }
+ }
return 0;
}

--
2.3.9