Re: [PATCH mmotm] mm: ksm: do not block on page lock when searching stable tree fix

[Yang Shi]

On 2/18/19 9:26 PM, Hugh Dickins wrote:
> I hit the kernel BUG at mm/ksm.c:809! quite easily under KSM swapping
> load.  That's the BUG_ON(age > 1) in remove_rmap_item_from_tree().
>
> There is a comment above it, but explaining in more detail: KSM saves
> effort by not fully maintaining the unstable tree like a proper RB
> tree throughout, but at the start of each pass forgetting the old tree
> and rebuilding anew from scratch. But that means that whenever it looks
> like we need to remove an item from the unstable tree, we have to check
> whether it has already been linked into the new tree this time around
> (hence rb_erase needed), or it's just a free-floating leftover from the
> previous tree.
>
> "age" 0 or 1 says which: but if it's more than 1, then something has
> gone wrong: cmp_and_merge_page() was forgetting to remove the item
> in the new EBUSY case.
>
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
> Fix to fold into
> mm-ksm-do-not-block-on-page-lock-when-searching-stable-tree.patch

Thanks for catching this. The fix looks good to me.

> I like that patch better now it has the mods suggested by John Hubbard;
> but what I'd still really prefer to do is to make the patch unnecessary,
> by reworking that window of KSM page migration so that there's just no
> need for stable_tree_search() to take page lock.  We would all prefer
> that.  However, each time I've gone to do so, it's turned out to need
> more care than I expected, and I run out of time.  So, let's go with
> what we have, and one day I might perhaps get back to it.

I agree it needs extra scrutiny to make the code lockless.

Regards,
Yang

>   mm/ksm.c |    7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
>
> --- mmotm/mm/ksm.c	2019-02-14 15:16:13.000000000 -0800
> +++ linux/mm/ksm.c	2019-02-18 20:36:44.707310427 -0800
> @@ -2082,10 +2082,6 @@ static void cmp_and_merge_page(struct pa
>   
>   	/* We first start with searching the page inside the stable tree */
>   	kpage = stable_tree_search(page);
> -
> -	if (PTR_ERR(kpage) == -EBUSY)
> -		return;
> -
>   	if (kpage == page && rmap_item->head == stable_node) {
>   		put_page(kpage);
>   		return;
> @@ -2094,6 +2090,9 @@ static void cmp_and_merge_page(struct pa
>   	remove_rmap_item_from_tree(rmap_item);
>   
>   	if (kpage) {
> +		if (PTR_ERR(kpage) == -EBUSY)
> +			return;
> +
>   		err = try_to_merge_with_ksm_page(rmap_item, page, kpage);
>   		if (!err) {
>   			/*