[PATCH v3 03/11] s390/mm/gmap: Refactor gmap_fault() and add support for pfault

Claudio Imbrenda
2024-10-15T16:43:32Z

When specifying FAULT_FLAG_RETRY_NOWAIT as flag for gmap_fault(), the
gmap fault will be processed only if it can be resolved quickly and
without sleeping. This will be needed for pfault.

Refactor gmap_fault() to improve readability.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
---
arch/s390/mm/gmap.c | 119 +++++++++++++++++++++++++++++++++++++-------
1 file changed, 100 insertions(+), 19 deletions(-)

diff --git a/arch/s390/mm/gmap.c b/arch/s390/mm/gmap.c
index eb0b51a36be0..f51ad948ba53 100644
--- a/arch/s390/mm/gmap.c
+++ b/arch/s390/mm/gmap.c
@@ -637,44 +637,125 @@ int __gmap_link(struct gmap *gmap, unsigned long gaddr, unsigned long vmaddr)
}

/**
- * gmap_fault - resolve a fault on a guest address
+ * fixup_user_fault_nowait - manually resolve a user page fault without waiting
+ * @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_lock while retrying
+ *
+ * This function behaves similarly to fixup_user_fault(), but it guarantees
+ * that the fault will be resolved without waiting. The function might drop
+ * and re-acquire the mm lock, in which case @unlocked will be set to true.
+ *
+ * The guarantee is that the fault is handled without waiting, but the
+ * function itself might sleep, due to the lock.
+ *
+ * Context: Needs to be called with mm->mmap_lock held in read mode, and will
+ * return with the lock held in read mode; @unlocked will indicate whether
+ * the lock has been dropped and re-acquired. This is the same behaviour as
+ * fixup_user_fault().
+ *
+ * Return: 0 on success, -EAGAIN if the fault cannot be resolved without
+ * waiting, -EFAULT if the fault cannot be resolved, -ENOMEM if out of
+ * memory.
+ */
+static int fixup_user_fault_nowait(struct mm_struct *mm, unsigned long address,
+ unsigned int fault_flags, bool *unlocked)
+{
+ struct vm_area_struct *vma;
+ unsigned int test_flags;
+ vm_fault_t fault;
+ int rc;
+
+ fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_RETRY_NOWAIT;
+ test_flags = fault_flags & FAULT_FLAG_WRITE ? VM_WRITE : VM_READ;
+
+ vma = find_vma(mm, address);
+ if (unlikely(!vma || address < vma->vm_start))
+ return -EFAULT;
+ if (unlikely(!(vma->vm_flags & test_flags)))
+ return -EFAULT;
+
+ fault = handle_mm_fault(vma, address, fault_flags, NULL);
+ /* the mm lock has been dropped, take it again */
+ if (fault & VM_FAULT_COMPLETED) {
+ *unlocked = true;
+ mmap_read_lock(mm);
+ return 0;
+ }
+ /* the mm lock has not been dropped */
+ if (fault & VM_FAULT_ERROR) {
+ rc = vm_fault_to_errno(fault, 0);
+ BUG_ON(!rc);
+ return rc;
+ }
+ /* the mm lock has not been dropped because of FAULT_FLAG_RETRY_NOWAIT */
+ if (fault & VM_FAULT_RETRY)
+ return -EAGAIN;
+ /* nothing needed to be done and the mm lock has not been dropped */
+ return 0;
+}
+
+/**
+ * __gmap_fault - resolve a fault on a guest address
* @gmap: pointer to guest mapping meta data structure
* @gaddr: guest address
* @fault_flags: flags to pass down to handle_mm_fault()
*
- * Returns 0 on success, -ENOMEM for out of memory conditions, and -EFAULT
- * if the vm address is already mapped to a different guest segment.
+ * Context: Needs to be called with mm->mmap_lock held in read mode. Might
+ * drop and re-acquire the lock. Will always return with the lock held.
*/
-int gmap_fault(struct gmap *gmap, unsigned long gaddr,
- unsigned int fault_flags)
+static int __gmap_fault(struct gmap *gmap, unsigned long gaddr, unsigned int fault_flags)
{
unsigned long vmaddr;
- int rc;
bool unlocked;
-
- mmap_read_lock(gmap->mm);
+ int rc = 0;

retry:
unlocked = false;
+
vmaddr = __gmap_translate(gmap, gaddr);
- if (IS_ERR_VALUE(vmaddr)) {
- rc = vmaddr;
- goto out_up;
- }
- if (fixup_user_fault(gmap->mm, vmaddr, fault_flags,
- &unlocked)) {
- rc = -EFAULT;
- goto out_up;
+ if (IS_ERR_VALUE(vmaddr))
+ return vmaddr;
+
+ if (fault_flags & FAULT_FLAG_RETRY_NOWAIT) {
+ rc = fixup_user_fault_nowait(gmap->mm, vmaddr, fault_flags, &unlocked);
+ if (rc)
+ return rc;
+ } else if (fixup_user_fault(gmap->mm, vmaddr, fault_flags, &unlocked)) {
+ return -EFAULT;
}
/*
* In the case that fixup_user_fault unlocked the mmap_lock during
- * faultin redo __gmap_translate to not race with a map/unmap_segment.
+ * fault-in, redo __gmap_translate() to avoid racing with a
+ * map/unmap_segment.
+ * In particular, __gmap_translate(), fixup_user_fault{,_nowait}(),
+ * and __gmap_link() must all be called atomically in one go; if the
+ * lock had been dropped in between, a retry is needed.
*/
if (unlocked)
goto retry;

- rc = __gmap_link(gmap, gaddr, vmaddr);
-out_up:
+ return __gmap_link(gmap, gaddr, vmaddr);
+}
+
+/**
+ * gmap_fault - resolve a fault on a guest address
+ * @gmap: pointer to guest mapping meta data structure
+ * @gaddr: guest address
+ * @fault_flags: flags to pass down to handle_mm_fault()
+ *
+ * Returns 0 on success, -ENOMEM for out of memory conditions, -EFAULT if the
+ * vm address is already mapped to a different guest segment, and -EAGAIN if
+ * FAULT_FLAG_RETRY_NOWAIT was specified and the fault could not be processed
+ * immediately.
+ */
+int gmap_fault(struct gmap *gmap, unsigned long gaddr, unsigned int fault_flags)
+{
+ int rc;
+
+ mmap_read_lock(gmap->mm);
+ rc = __gmap_fault(gmap, gaddr, fault_flags);
mmap_read_unlock(gmap->mm);
return rc;
}
--
2.47.0