> From: Dave Hansen [mailto:dave@xxxxxxxxxxxxxxxxxx] > Subject: Re: [PATCH 1/5] staging: zsmalloc: zsmalloc memory allocation library > > On 02/06/2012 09:26 AM, Seth Jennings wrote: > > On 01/26/2012 01:12 PM, Dave Hansen wrote: > >> void *kmap_atomic_prot(struct page *page, pgprot_t prot) > >> { > >> ... > >> type = kmap_atomic_idx_push(); > >> idx = type + KM_TYPE_NR*smp_processor_id(); > >> vaddr = __fix_to_virt(FIX_KMAP_BEGIN + idx); > >> > >> I think if you do a get_cpu()/put_cpu() or just a preempt_disable() > >> across the operations you'll be guaranteed to get two contiguous addresses. > > > > I'm not quite following here. kmap_atomic() only does this for highmem pages. > > For normal pages (all pages for 64-bit), it doesn't do any mapping at all. It > > just returns the virtual address of the page since it is in the kernel's address > > space. > > > > For this design, the pages _must_ be mapped, even if the pages are directly > > reachable in the address space, because they must be virtually contiguous. > > I guess you could use vmap() for that. It's just going to be slower > than kmap_atomic(). I'm really not sure it's worth all the trouble to > avoid order-1 allocations, though. Seth, Nitin, please correct me if I am wrong, but... Dave, your comment makes me wonder if maybe you might be missing the key value of the new allocator. The zsmalloc allocator can grab any random* page "A" with X unused bytes at the END of the page, and any random page "B" with Y unused bytes at the BEGINNING of the page and "coalesce" them to store any byte sequence with a length** Z not exceeding X+Y. Presumably this markedly increases the density of compressed-pages-stored-per-physical-page***. I don't see how allowing order-1 allocations helps here but if I am missing something clever, please explain further. (If anyone missed Jonathan Corbet's nice lwn.net article, see: https://lwn.net/Articles/477067/ ) * Not really ANY random page, just any random page that has been previously get_free_page'd by the allocator and hasn't been free'd yet. ** X, Y and Z are all rounded to a multiple of 16 so there is still some internal fragmentation cost. *** Would be interesting to see some random and real workload data comparing density for zsmalloc and xvmalloc. And also zbud too as a goal is to replace zbud with zsmalloc too. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
