Re: [PATCH v3 5/8] mm: Device exclusive memory access

On Tuesday, 2 March 2021 11:05:59 AM AEDT Jason Gunthorpe wrote:

On Fri, Feb 26, 2021 at 06:18:29PM +1100, Alistair Popple wrote:

quoted

+/**
+ * make_device_exclusive_range() - Mark a range for exclusive use by a 

device

quoted

+ * @mm: mm_struct of assoicated target process
+ * @start: start of the region to mark for exclusive device access
+ * @end: end address of region
+ * @pages: returns the pages which were successfully mark for exclusive 

acces

quoted

+ *
+ * Returns: number of pages successfully marked for exclusive access
+ *
+ * This function finds the ptes mapping page(s) to the given address 

range and

quoted

+ * replaces them with special swap entries preventing userspace CPU 

access. On

quoted

+ * fault these entries are replaced with the original mapping after 

calling MMU

quoted

+ * notifiers.
+ */
+int make_device_exclusive_range(struct mm_struct *mm, unsigned long 

start,

quoted

+				unsigned long end, struct page **pages)
+{
+	long npages = (end - start) >> PAGE_SHIFT;
+	long i;
+
+	npages = get_user_pages_remote(mm, start, npages,
+				       FOLL_GET | FOLL_WRITE | FOLL_SPLIT_PMD,
+				       pages, NULL, NULL);
+	for (i = 0; i < npages; i++) {
+		if (!trylock_page(pages[i])) {
+			put_page(pages[i]);
+			pages[i] = NULL;
+			continue;
+		}
+
+		if (!try_to_protect(pages[i])) {

Isn't this racy? get_user_pages returns the ptes at an instant in
time, they could have already been changed to something else?

Right. On it's own this does not guarantee that the page is mapped at the 
given location, only that a mapping won't get established without an mmu 
notifier callback to clear the swap entry.

The intent was a driver could use HMM or some other mechanism to keep PTEs 
synchronised if required. However I just looked at patch 8 in the series again 
and it appears I got this wrong when converting from the old migration 
approach:

+               mutex_unlock(&svmm->mutex);
+               ret = nouveau_atomic_range_fault(svmm, drm, args,
+                                               size, hmm_flags, mm);

The mutex needs to be unlocked after the range fault to ensure the PTE hasn't 
changed. But this ends up being a problem because try_to_protect() calls 
notifiers which need to take that mutex and hence deadlocks.

I would think you'd want to switch to the swap entry atomically under
th PTLs?

That is one approach, but the reuse of get_user_pages() to walk the page 
tables and fault/gather the pages is a nice simplification and adding a new 
FOLL flag/mode to atomically swap entries doesn't seem right.

However try_to_protect() scans the PTEs again under the PTL so checking the 
mapping of interest actually gets replaced during the rmap walk seems like a 
reasonable solution. Thanks for the comments.

 - Alistair

Jason