it seems that nobody is going to review this patch ;)
So I tried to read mirror_vm_fault() and the usage of mmap_sem doesn't
look right to me. But let me repeat, this is not my area I can be easily
wrong, please correct me.
On 09/04, Adalbert Lazăr wrote:
quoted hunk ↗ jump to hunk
+static vm_fault_t mirror_vm_fault(struct vm_fault *vmf)
+{
+ struct vm_area_struct *vma = vmf->vma;
+ struct mm_struct *mm = vma->vm_mm;
+ struct remote_vma_context *ctx = vma->vm_private_data;
+ struct remote_view *view = ctx->view;
+ struct file *file = vma->vm_file;
+ struct remote_file_context *fctx = file->private_data;
+ unsigned long req_addr;
+ unsigned int gup_flags;
+ struct page *req_page;
+ vm_fault_t result = VM_FAULT_SIGBUS;
+ struct mm_struct *src_mm = fctx->mm;
+ unsigned long seq;
+ int idx;
+
+fault_retry:
+ seq = mmu_interval_read_begin(&view->mmin);
+
+ idx = srcu_read_lock(&fctx->fault_srcu);
+
+ /* check if view was invalidated */
+ if (unlikely(!READ_ONCE(view->valid))) {
+ pr_debug("%s: region [%lx-%lx) was invalidated!!\n", __func__,
+ view->offset, view->offset + view->size);
+ goto out_invalid; /* VM_FAULT_SIGBUS */
+ }
+
+ /* drop current mm semapchore */
+ up_read(¤t->mm->mmap_sem);
Please use mmap_read_lock/unlock(mm) instead of down/up_read(mmap_sem).
But why is it safe to drop ->mmap_sem without checking
FAULT_FLAG_ALLOW_RETRY/RETRY_NOWAIT ?
quoted hunk ↗ jump to hunk
+ /* take remote mm semaphore */
+ if (vmf->flags & FAULT_FLAG_ALLOW_RETRY) {
+ if (!down_read_trylock(&src_mm->mmap_sem)) {
I don't understand this... perhaps you meant FAULT_FLAG_RETRY_NOWAIT ?
quoted hunk ↗ jump to hunk
+ * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_sem must be released
+ * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
+ * not set.
Well, iiuc FAULT_FLAG_ALLOW_RETRY means that ->fault() _can_ drop mmap_sem
and return VM_FAULT_RETRY (unless NOWAIT).
quoted hunk ↗ jump to hunk
+ if (vmf->flags & (FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_TRIED)) {
+ if (!(vmf->flags & FAULT_FLAG_KILLABLE))
+ if (current && fatal_signal_pending(current)) {
+ up_read(¤t->mm->mmap_sem);
+ return VM_FAULT_RETRY;
I fail to understand this. But in any case, you do not need to check
current != NULL and up_read() looks wrong if RETRY_NOWAIT?
Oleg.