Re: [PATCH v3 1/4] s390: Use _refcount for pgtables

[Kirill A. Shutemov]

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