On Wed, Feb 02, 2022 at 06:27:47PM -0300, Mauricio Faria de Oliveira wrote:

On Wed, Feb 2, 2022 at 4:56 PM Yu Zhao [off-list ref] wrote:

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On Mon, Jan 31, 2022 at 08:02:55PM -0300, Mauricio Faria de Oliveira wrote:

quoted

Problem:
=======

Thanks for the update. A couple of quick questions:

quoted

Userspace might read the zero-page instead of actual data from a
direct IO read on a block device if the buffers have been called
madvise(MADV_FREE) on earlier (this is discussed below) due to a
race between page reclaim on MADV_FREE and blkdev direct IO read.

1) would page migration be affected as well?

Could you please elaborate on the potential problem you considered?

I checked migrate_pages() -> try_to_migrate() holds the page lock,
thus shouldn't race with shrink_page_list() -> with try_to_unmap()
(where the issue with MADV_FREE is), but maybe I didn't get you
correctly.

Could the race exist between DIO and migration? While DIO is writing
to a page, could migration unmap it and copy the data from this page
to a new page?

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@@ -1599,7 +1599,30 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,

                      /* MADV_FREE page check */
                      if (!PageSwapBacked(page)) {
-                             if (!PageDirty(page)) {
+                             int ref_count, map_count;
+
+                             /*
+                              * Synchronize with gup_pte_range():
+                              * - clear PTE; barrier; read refcount
+                              * - inc refcount; barrier; read PTE
+                              */
+                             smp_mb();
+
+                             ref_count = page_count(page);
+                             map_count = page_mapcount(page);
+
+                             /*
+                              * Order reads for page refcount and dirty flag;
+                              * see __remove_mapping().
+                              */
+                             smp_rmb();

2) why does it need to order against __remove_mapping()? It seems to
   me that here (called from the reclaim path) it can't race with
   __remove_mapping() because both lock the page.

I'll improve that comment in v4.  The ordering isn't against __remove_mapping(),
but actually because of an issue described in __remove_mapping()'s comments
(something else that doesn't hold the page lock, just has a page reference, that
may clear the page dirty flag then drop the reference; thus check ref,
then dirty).

Got it. IIRC, get_user_pages() doesn't imply a write barrier. If so,
there should be a smp_wmb() on the other side:

	 * get_user_pages(&page);

	smp_wmb()

	 * SetPageDirty(page);
	 * put_page(page);

(__remove_mapping() doesn't need smp_[rw]mb() on either side because
it relies on page refcnt freeze and retesting.)

Thanks.