Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split

[Hugh Dickins]

On Tue, 25 May 2021, Yang Shi wrote:

> When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
> return false positive for PTE-mapped THP since page_mapcount() is used
> to check if the THP is unmapped, but it just checks compound mapount and
> head page's mapcount.  If the THP is PTE-mapped and head page is not
> mapped, it may return false positive.

But those false positives did not matter because there was a separate
DEBUG_VM check later.

It's good to have the link to Wang Yugui's report, but that paragraph
is not really about this patch, as it has evolved now: this patch
consolidates the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE.

> 
> The try_to_unmap() has been changed to void function, so check
> page_mapped() after it.  And changed BUG_ON to WARN_ON since it is not a
> fatal issue.

The change from DEBUG_VM BUG to VM_WARN_ON_ONCE is the most important
part of this, and the reason it's good for stable: and the patch title
ought to highlight that, not the page_mapcount business.

> 
> [1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@xxxxxxxxxxxx/
> 
> Reviewed-by: Zi Yan <ziy@xxxxxxxxxx>
> Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>

This will be required Cc: stable@xxxxxxxxxxxxxxx
(but we don't want to Cc them on this mail).

As I said on the other, I think this should be 1/2 not 2/2.

> ---
> v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
>     since there is no fundamental change against v2.
> v2: Removed dead code and updated the comment of try_to_unmap() per Zi
>     Yan.
>  mm/huge_memory.c | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 80fe642d742d..72d81d8e01b1 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2343,6 +2343,8 @@ static void unmap_page(struct page *page)
>  		ttu_flags |= TTU_SPLIT_FREEZE;
>  
>  	try_to_unmap(page, ttu_flags);
> +
> +	VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);

There is one useful piece of information that dump_page() will not show:
total_mapcount(page).  Is there a way of crafting that into the output?

Not with the macros available, I think.  Maybe we should be optimistic
and assume I already have the fixes, so not worth trying to refine the
message (but I'm not entirely convinced of that!).

The trouble with
	if (VM_WARN_ON_ONCE_PAGE(page_mapped(page), page))
		pr_warn("total_mapcount:%d\n", total_mapcount(page));
is that it's printed regardless of the ONCEness.  Another "trouble"
is that it's printed so long after the page_mapped(page) check that
it may be 0 by now - but one can see that as itself informative.

I guess leave it as you have it, I don't see an easy better
(without going back to something like the old contortions).

>  }
>  
>  static void remap_page(struct page *page, unsigned int nr)
> @@ -2653,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	struct deferred_split *ds_queue = get_deferred_split_queue(head);
>  	struct anon_vma *anon_vma = NULL;
>  	struct address_space *mapping = NULL;
> -	int count, mapcount, extra_pins, ret;
> +	int mapcount, extra_pins, ret;

Remove mapcount too.

>  	pgoff_t end;
>  
>  	VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
> @@ -2712,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  	}
>  
>  	unmap_page(head);
> -	VM_BUG_ON_PAGE(compound_mapcount(head), head);
>  
>  	/* block interrupt reentry in xa_lock and spinlock */
>  	local_irq_disable();
> @@ -2730,7 +2731,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  
>  	/* Prevent deferred_split_scan() touching ->_refcount */
>  	spin_lock(&ds_queue->split_queue_lock);
> -	count = page_count(head);
>  	mapcount = total_mapcount(head);
>  	if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {

mapcount was useful for printing in the hand-crafted message deleted,
but serves no purpose now: just do the page_ref_freeze() without it.

>  		if (!list_empty(page_deferred_list(head))) {
> @@ -2752,16 +2752,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
>  		__split_huge_page(page, list, end);
>  		ret = 0;
>  	} else {
> -		if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
> -			pr_alert("total_mapcount: %u, page_count(): %u\n",
> -					mapcount, count);
> -			if (PageTail(page))
> -				dump_page(head, NULL);
> -			dump_page(page, "total_mapcount(head) > 0");
> -			BUG();
> -		}
>  		spin_unlock(&ds_queue->split_queue_lock);
> -fail:		if (mapping)
> +fail:
> +		if (mapping)
>  			xa_unlock(&mapping->i_pages);
>  		local_irq_enable();
>  		remap_page(head, thp_nr_pages(head));
> -- 
> 2.26.2