Re: [patch 1/4] mm: page refcount use atomic primitives

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > The VM has an interesting race where a page refcount can drop to zero, but
> > it is still on the LRU lists for a short time. This was solved by testing
> > a 0->1 refcount transition when picking up pages from the LRU, and dropping
> > the refcount in that case.
>
>
> Tell me about it...
>
>
> > Instead, use atomic_inc_not_zero to ensure we never pick up a 0 refcount
> > page from the LRU (ie. we guarantee the page will not be touched).
>
>
> atomic_inc_not_zero() looks rather bloaty, but a single call site is OK.

A little. On generic cmpxchg/cas architectures it isn't too bad, and
LL/SC architectures presently implement it fairly stupidly with cmpxchg
but they can do a much better job using ll/sc directly.

> > This ensures we can test PageLRU without taking the lru_lock,
>
>
> Let me write some changelog for you.
>
> isolate_lru_pages() can remove live pages from the LRU at any time and
> shrink_cache() can put them back at any time.  As we don't hold the
> zone->lock we can race against that.
>
>
> > void fastcall __page_cache_release(struct page *page)
> > {
> > 	if (PageLRU(page)) {
> > 		unsigned long flags;
>
>
> isolate_lru_pages() removes the page here.
>
>
> > 		struct zone *zone = page_zone(page);
> > 		spin_lock_irqsave(&zone->lru_lock, flags);
> > 		if (!TestClearPageLRU(page))
> > 			BUG();
>
>
> blam.
>
>
> > 		del_page_from_lru(zone, page);
> > 		spin_unlock_irqrestore(&zone->lru_lock, flags);
> > 	}
> >
> > 	BUG_ON(page_count(page) != 0);
> > 	free_hot_page(page);
> > }
>
>
> But put_page() wouldn't have entered __page_cache_release() at all, because
> isolate_lru_page() is changed by this patch to elevated the page refcount
> prior to clearing PG_lru:
>
> 		BUG_ON(!PageLRU(page));
> 		list_del(&page->lru);
> 		target = src;
> 		if (get_page_unless_zero(page)) {
> 			ClearPageLRU(page);
>
>
> So no blam.
>
> That's from a two-minute-peek.  I haven't thought about this dreadfully
> hard.  But I'd like to gain some confidence that you have, please.  This
> stuff is tricky.

Right, no blam. This is how I avoid taking the LRU lock.

 "Instead, use atomic_inc_not_zero to ensure we never **pick up a 0 refcount**
  page from the LRU (ie. we guarantee the page will not be touched)."

I don't understand what you're asking?

> > and allows
> > further optimisations (in later patches) -- we end up saving 2 atomic ops
> > including a spin_lock_irqsave in the !PageLRU case, and 2 or 3 atomic ops
> > in the PageLRU case.
>
>
> Well yeah, but you've pretty much eliminated all those nice speedups by
> adding several BUG_ON(atomic_op)s.  Everyone compiles with CONFIG_BUG.  So
> I'd suggest that such new assertions be broken out into a separate -mm-only
> patch.

Not quite eliminated because ClearPageXXX is an atomic RMW, __ClearPageXXX
is not. Also TestClearXXX includes memory barriers.

Anyway I wanted to keep it equivalent (ie. keep bugs there). I have a future
patch to move these assertions under CONFIG_DEBUG_VM.

--
SUSE Labs, Novell Inc.

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