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SubjectRe: [PATCH v18 31/32] mm: Add explicit page decrement in exception path for isolate_lru_pages
On Mon, Aug 24, 2020 at 08:55:04PM +0800, Alex Shi wrote:
> +++ b/mm/vmscan.c
> @@ -1688,10 +1688,13 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
>
> if (!TestClearPageLRU(page)) {
> /*
> - * This page may in other isolation path,
> - * but we still hold lru_lock.
> + * This page is being isolated in another
> + * thread, but we still hold lru_lock. The
> + * other thread must be holding a reference
> + * to the page so this should never hit a
> + * reference count of 0.
> */
> - put_page(page);
> + WARN_ON(put_page_testzero(page));
> goto busy;

I read Hugh's review and that led me to take a look at this. We don't
do it like this. Use the same pattern as elsewhere in mm:

page_ref_sub(page, nr);
VM_BUG_ON_PAGE(page_count(page) <= 0, page);

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