Re: [PATCH] zsmalloc: Fix races between asynchronous zspage free and page migration

[Minchan Kim]

On Wed, May 11, 2022 at 02:45:30PM -0700, Sultan Alsawaf wrote:
> On Wed, May 11, 2022 at 02:07:19PM -0700, Minchan Kim wrote:
> > Then, how about this?
> 
> Your proposal is completely wrong still. My original patch is fine; we can stick
> with that.
> 
> > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> > index 9152fbde33b5..2f205c18aee4 100644
> > --- a/mm/zsmalloc.c
> > +++ b/mm/zsmalloc.c
> > @@ -1716,12 +1716,31 @@ static enum fullness_group putback_zspage(struct size_class *class,
> >   * To prevent zspage destroy during migration, zspage freeing should
> >   * hold locks of all pages in the zspage.
> >   */
> > -static void lock_zspage(struct zspage *zspage)
> > +static void lock_zspage(struct zs_pool *pool, struct zspage *zspage)
> >  {
> > -       struct page *page = get_first_page(zspage);
> > -
> > +       struct page *page;
> > +       int nr_locked;
> > +       struct page *locked_pages[ZS_MAX_PAGES_PER_ZSPAGE];
> > +       struct address_space *mapping;
> > +retry:
> > +       nr_locked = 0;
> > +       memset(locked_pages, 0, sizeof(struct page) * ARRAY_SIZE(locked_pages));
> 
> This memset() zeroes out memory past the end of the array because it is an array
> of pointers, not an array of page structs; the sizeof() is incorrect.
> 
> > +       page = get_first_page(zspage);
> 
> You can't use get_first_page() outside of the migrate lock.
> 
> >         do {
> >                 lock_page(page);
> 
> You can't lock a page that you don't own.

That's key point what my idea was wrong.

Thanks for correction!