Re: MM patches against 2.5.31

["Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>]

On Tue, Aug 27, 2002 at 06:48:50PM +0200, Daniel Phillips wrote:
> On Monday 26 August 2002 22:58, Christian Ehrhardt wrote:
> > > Nope, still don't see it.  Whoever hits put_page_testzero frees the page,
> > > secure in the knowlege that there are no other references to it.
> > 
> > Well yes, but we cannot remove the page from the lru atomatically
> > at page_cache_release time if we follow your proposal. If you think we can,
> > show me your implementation of page_cache_release and I'll show
> > you where the races are (unless you do everything under the lru_lock
> > of course).
> 
> void page_cache_release(struct page *page)
> {
> 	spin_lock(&pagemap_lru_lock);
> 	if (PageLRU(page) && page_count(page) == 2) {
> 		__lru_cache_del(page);
> 		atomic_dec(&page->count);
> 	}
> 	spin_unlock(&pagemap_lru_lock);
> 	if (put_page_testzero(page))
> 		__free_pages_ok(page, 0);
> }
>
> This allows the following benign race, with initial page count = 3:
> [ ...]
> Neither holder of a page reference sees the count at 2, and so the page
> is left on the lru with count = 1.  This won't happen often and such
> pages will be recovered from the cold end of the list in due course.

Ok, agreed. I think this will work but taking the lru lock each time
is probably not a good idea.

> We could also do this:
> 
> void page_cache_release(struct page *page)
> {
> 	if (page_count(page) == 2) {
> 		spin_lock(&pagemap_lru_lock);
> 		if (PageLRU(page) && page_count(page) == 2) {
> 			__lru_cache_del(page);
> 			atomic_dec(&page->count);
> 		}
> 		spin_unlock(&pagemap_lru_lock);
> 	}
> 	if (put_page_testzero(page))
> 		__free_pages_ok(page, 0);
> }
> 
> Which avoids taking the lru lock sometimes in exchange for widening the
> hole through which pages can end up with count = 1 on the lru list.

This sounds like something that is worth trying. I missed that one.


Side note: The BUG in __pagevec_lru_del seems strange. refill_inactive
or shrink_cache could have removed the page from the lru before
__pagevec_lru_del acquired the lru lock.

     regards   Christian

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