Re: [PATCH] mm: fix race between MADV_FREE reclaim and blkdev direct IO read

[Mauricio Faria de Oliveira <mfo@canonical.com>]

On Thu, Dec 16, 2021 at 3:17 PM Minchan Kim <minchan@kernel.org> wrote:
...
> Hi Mauricio,
>
> Thanks for catching the bug. There is some comment before I would
> look the problem in more detail. Please see below.
>

Hey! Thanks for looking into this. Sorry for the delay; I've been out
a few weeks.

> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 163ac4e6bcee..f04151aae03b 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1570,7 +1570,18 @@ 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 refcount = page_ref_count(page);
> > +
> > +                             /*
> > +                              * The only page refs must be from the isolation
> > +                              * (checked by the caller shrink_page_list() too)
> > +                              * and the (single) rmap (dropped by discard:).
> > +                              *
> > +                              * Check the reference count before dirty flag
> > +                              * with memory barrier; see __remove_mapping().
> > +                              */
> > +                             smp_rmb();
> > +                             if (refcount == 2 && !PageDirty(page)) {
>
> A madv_free marked page could be mapped at several processes so
> it wouldn't be refcount two all the time, I think.
> Shouldn't we check it with page_mapcount with page_refcount?
>
>     page_ref_count(page) - 1  > page_mapcount(page)
>

It's the other way around, isn't it? The madvise(MADV_FREE) call only clears
the page dirty flag if page_mapcount() == 1 (ie not mapped by more processes).

@ madvise_free_pte_range()

                        /*
                         * If page is shared with others, we couldn't clear
                         * PG_dirty of the page.
                         */
                        if (page_mapcount(page) != 1) {
                                unlock_page(page);
                                continue;
                        }
...
                        ClearPageDirty(page);
                        unlock_page(page);


If that's right, the refcount of 2 should be OK (one from the isolation,
another one from the single map/one process.)

Does that make sense?  I might be missing something.

Thanks!

-- 
Mauricio Faria de Oliveira