Messages in this thread | | | Date | Thu, 1 Mar 2018 17:28:55 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH v3 1/4] s390: Use _refcount for pgtables |
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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@shutemov.name> wrote: > > > On Wed, Feb 28, 2018 at 02:31:54PM -0800, Matthew Wilcox wrote: > > > From: Matthew Wilcox <mawilcox@microsoft.com> > > > > > > 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
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