On Wed, Feb 2, 2022 at 4:56 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote:
>
> On Mon, Jan 31, 2022 at 08:02:55PM -0300, Mauricio Faria de Oliveira wrote:
> > Problem:
> > =======
>
> Thanks for the update. A couple of quick questions:
>
> > 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.
>
> > @@ -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).
Hope this clarifies the question.
Thanks!
>
> > + /*
> > + * The only page refs must be from the isolation
> > + * plus one or more rmap's (dropped by discard:).
> > + */
> > + if ((ref_count == 1 + map_count) &&
> > + !PageDirty(page)) {
> > /* Invalidate as we cleared the pte */
> > mmu_notifier_invalidate_range(mm,
> > address, address + PAGE_SIZE);
--
Mauricio Faria de Oliveira
