On Thu, 1 Mar 2018 15:44:12 +0300 "Kirill A. Shutemov" <kirill@xxxxxxxxxxxxx> wrote: > On Thu, Mar 01, 2018 at 08:17:50AM +0100, Martin Schwidefsky wrote: > > On Wed, 28 Feb 2018 14:31:53 -0800 > > Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > > From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx> > > > > > > I want to use the _mapcount field to record what a page is in use as. > > > This can help with debugging and we can also expose that information to > > > userspace through /proc/kpageflags to help diagnose memory usage (not > > > included as part of this patch set). > > > > > > First, we need s390 to stop using _mapcount for its own purposes; > > > Martin, I hope you have time to look at this patch. I must confess I > > > don't quite understand what the different bits are used for in the upper > > > nybble of the _mapcount, but I tried to replicate what you were doing > > > faithfully. > > > > Yeah, that is a nasty bit of code. On s390 we have 2K page tables (pte) > > but 4K pages. If we use full pages for the pte tables we waste 2K of > > memory for each of the tables. So we allocate 4K and split it into two > > 2K pieces. Now we have to keep track of the pieces to be able to free > > them again. > > Have you considered to use slab for page table allocation instead? > IIRC some architectures practice this already. Well there is a complication with KVM and the page table management for gmaps. If mm_alloc_pgste(mm) == true then a 4K page page table has to be allocated. For the gmap I need a place to store an 8 byte value, currently we use page->index. But the slab/slub code uses page->index for its own purpose. This creates a conflict, but maybe doing a get_free_page for mm_alloc_pgste(mm) == true and using a slab cache for normal page tables might work. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- 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>
