On Wed, Feb 28, 2018 at 02:31:54PM -0800, Matthew Wilcox wrote: > From: Matthew Wilcox <[email protected]> > > s390 borrows the storage used for _mapcount in struct page in order to > account whether the bottom or top half is being used for 2kB page > tables. I want to use that for something else, so use the top byte of > _refcount instead of the bottom byte of _mapcount. _refcount may > temporarily be incremented by other CPUs that see a stale pointer to > this page in the page cache, but each CPU can only increment it by one, > and there are no systems with 2^24 CPUs today, so they will not change > the upper byte of _refcount. We do have to be a little careful not to > lose any of their writes (as they will subsequently decrement the > counter).
Hm. I'm more worried about false-negative put_page_testzero(). Are you sure it won't lead to leaks. I cannot say from the code changes. And for page-table pages should have planty space in other fields. IIRC page->mapping is unused there. -- Kirill A. Shutemov
