Re: [PATCH v3 08/23] vfio, mm: fix get_user_pages_remote() and FOLL_LONGTERM

On Tue, Nov 12, 2019 at 02:45:51PM -0800, Dan Williams wrote:

On Tue, Nov 12, 2019 at 2:43 PM John Hubbard [off-list ref] wrote:

quoted

On 11/12/19 12:43 PM, Jason Gunthorpe wrote:
...

quoted

quoted

-            }
+    ret = get_user_pages_remote(NULL, mm, vaddr, 1, flags | FOLL_LONGTERM,
+                                page, vmas, NULL);
+    /*
+     * The lifetime of a vaddr_get_pfn() page pin is
+     * userspace-controlled. In the fs-dax case this could
+     * lead to indefinite stalls in filesystem operations.
+     * Disallow attempts to pin fs-dax pages via this
+     * interface.
+     */
+    if (ret > 0 && vma_is_fsdax(vmas[0])) {
+            ret = -EOPNOTSUPP;
+            put_page(page[0]);
     }

AFAIK this chunk is redundant now as it is some hack to emulate
FOLL_LONGTERM? So vmas can be deleted too.

Let me first make sure I understand what Dan has in mind for the vma
checking, in the other thread...

It's not redundant relative to upstream which does not do anything the
FOLL_LONGTERM in the gup-slow path... but I have not looked at patches
1-7 to see if something there made it redundant.

Oh, the hunk John had below for get_user_pages_remote() also needs to
call __gup_longterm_locked() when FOLL_LONGTERM is specified, then
that calls check_dax_vmas() which duplicates the vma_is_fsdax() check
above.

Certainly no caller of FOLL_LONGTERM should have to do dax specific
VMA checking.

Jason