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

On Tue, Nov 12, 2019 at 5:08 PM John Hubbard [off-list ref] wrote:

On 11/12/19 4:58 PM, Dan Williams wrote:
...

quoted

quoted

quoted

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.

Oh true, good eye. It is redundant if it does additionally call
__gup_longterm_locked(), and it needs to do that otherwises it undoes
the CMA migration magic that Aneesh added.

OK. So just to be clear, I'll be removing this from the patch:

        /*
         * 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]);
        }

(and the declaration of "vmas", as well).

...and add a call to __gup_longterm_locked internal to
get_user_pages_remote(), right?