On Wed, Mar 10, 2021 at 04:54:49PM +0000, David Howells wrote:
> Add a function, unlock_page_private_2(), to unlock PG_private_2 analogous
> to that of PG_lock. Add a kerneldoc banner to that indicating the example
> usage case.
This isn't a problem with this patch per se, but I'm concerned about
private2 and expected page refcounts.
static inline int is_page_cache_freeable(struct page *page)
{
/*
* A freeable page cache page is referenced only by the caller
* that isolated the page, the page cache and optional buffer
* heads at page->private.
*/
int page_cache_pins = thp_nr_pages(page);
return page_count(page) - page_has_private(page) == 1 + page_cache_pins;
}
static inline int page_has_private(struct page *page)
{
return !!(page->flags & PAGE_FLAGS_PRIVATE);
}
#define PAGE_FLAGS_PRIVATE \
(1UL << PG_private | 1UL << PG_private_2)
So ... a page with both flags cleared should have a refcount of N.
A page with one or both flags set should have a refcount of N+1.
How is a poor filesystem supposed to make that true? Also btrfs has this
problem since it uses private_2 for its own purposes.
