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SubjectRe: [RFC] [PATCH] Include LRU in page count
On Sat, Aug 31, 2002 at 01:03:07AM +0200, Daniel Phillips wrote:
> This rare race happened to become not so rare in 2.5 recently, and was
> analyzed by Christian Ehrhardt, who also proposed a solution based on a new
> approach to locking, essentially put_page_testone. We went on to check 2.4

Just a little correction: The key function implemented in my solution
is an atomic GET_page_testone which is called if the page might have
a zero refcount. The idea I had in mind is to distinguish heavy and weak
references to pages. Your solution is probably the better way to go.

> I proposed an alternate solution using the traditional put_page_testzero
> primitive, which relies on assigning a page count of one for membership on
> the lru list. A slightly racy heuristic is used for efficient lru list
> removal. The resulting incarnation of lru_cache_release is:
> static inline void page_cache_release(struct page *page)
> {
> if (page_count(page) == 2 && spin_trylock(&pagemap_lru_lock)) {
> if (PageLRU(page) && page_count(page) == 2)
> __lru_cache_del(page);
> spin_unlock(&pagemap_lru_lock);
> }
> put_page(page);
> }

Just saw that this can still race e.g. with lru_cache_add (not
hard to fix though):

| void lru_cache_add(struct page * page)
| {
| if (!TestSetPageLRU(page)) {

Window is here: Once we set the PageLRU bit page_cache_release
assumes that there is a reference held by the lru cache.

| spin_lock(&pagemap_lru_lock);
| add_page_to_inactive_list(page);
| get_page(page);

But only starting at this point the reference actually exists.

| spin_unlock(&pagemap_lru_lock);
| }

Solution: Change the PageLRU bit inside the lock. Looks like
lru_cache_add is the only place that doesn't hold the lru lock to
change the LRU flag and it's probably not a good idea even without
the patch.

Two more comments: I don't think it is a good idea to use
put_page_nofree in __lru_cache_del. This is probably safe now but
it adds an additional rule that lru_cache_del can't be called without
holding a second reference to the page.
Also there may be lru only pages on the active list, i.e. refill
inactive should have this hunk as well:

> +#if LRU_PLUS_CACHE==2
> + BUG_ON(!page_count(page));
> + if (unlikely(page_count(page) == 1)) {
> + mmstat(vmscan_free_page);
> + BUG_ON(!TestClearPageLRU(page)); // side effect abuse!!
> + put_page(page);
> + continue;
> + }
> +#endif

regards Christian Ehrhardt

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