On Thu, Mar 01, 2018 at 03:04:20PM +0100, Martin Schwidefsky wrote: > On Thu, 1 Mar 2018 15:53:10 +0300 > "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > > > On Wed, Feb 28, 2018 at 02:31:54PM -0800, Matthew Wilcox wrote: > > > From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> > > > > > > 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. > > 2^^24 put_page_testzero calls for page table pages? I don't think so. No, I mean oposite: we don't free the page when we should. 2^24 is not zero and page won't be freed if the acctual refcount (without the flag in upper bits) drops to zero. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
