Re: [PATCHv4 0/4] zsmalloc: fine-grained fullness and new compaction algorithm

[Yosry Ahmed]

On Mon, Apr 17, 2023 at 4:12 AM Sergey Senozhatsky
<senozhatsky@xxxxxxxxxxxx> wrote:
>
> On (23/04/17 01:29), Yosry Ahmed wrote:
> > > @@ -2239,8 +2241,8 @@ static unsigned long __zs_compact(struct zs_pool *pool,
> > >                 if (fg == ZS_INUSE_RATIO_0) {
> > >                         free_zspage(pool, class, src_zspage);
> > >                         pages_freed += class->pages_per_zspage;
> > > -                       src_zspage = NULL;
> > >                 }
> > > +               src_zspage = NULL;
> > >
> > >                 if (get_fullness_group(class, dst_zspage) == ZS_INUSE_RATIO_100
> > >                     || spin_is_contended(&pool->lock)) {
> >
> > For my own education, how can this result in the "next is NULL" debug
> > error Yu Zhao is seeing?
> >
> > IIUC if we do not set src_zspage to NULL properly after putback, then
> > we will attempt to putback again after the main loop in some cases.
> > This can result in a zspage being present more than once in the
> > per-class fullness list, right?
> >
> > I am not sure how this can lead to "next is NULL", which sounds like a
> > corrupted list_head, because the next ptr should never be NULL as far
> > as I can tell. I feel like I am missing something.
>
> That's a good question to which I don't have an answer. We can list_add()
> the same zspage twice, unlocking the pool after first list_add() so that
> another process (including another zs_compact()) can do something to that
> zspage. The answer is somewhere between these lines, I guess.

But the first list_add() is (in this case) the correct add, so we
expect other processes to be able to access the zspage after the first
list_add() anyway, right?

>
> I can see how, for example, another DEBUG_LIST check can be triggered:
> "list_add double add", because we basically can do
>
>         list_add(page, list)
>         list_add(page, list)
>
> I can also see how lockdep can be unhappy with us doing
>
>         write_unlock(&zspage->lock);
>         write_unlock(&zspage->lock);
>
> But I don't think I see how "next is NULL" happens (I haven't observed
> it).

Yeah I reached the same conclusion. Couldn't figure out how we can
reach the NULL scenario.