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.
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).
