On Thu, 8 May 2014 15:41:28 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > remap_file_pages(2) was invented to be able efficiently map parts of > huge file into limited 32-bit virtual address space such as in database > workloads. > > Nonlinear mappings are pain to support and it seems there's no > legitimate use-cases nowadays since 64-bit systems are widely available. > > Let's drop it and get rid of all these special-cased code. > > The patch replaces the syscall with emulation which creates new VMA on > each remap_file_pages(), unless they it can be merged with an adjacent > one. > > I didn't find *any* real code that uses remap_file_pages(2) to test > emulation impact on. I've checked Debian code search and source of all > packages in ALT Linux. No real users: libc wrappers, mentions in strace, > gdb, valgrind and this kind of stuff. > > There are few basic tests in LTP for the syscall. They work just fine > with emulation. > > To test performance impact, I've written small test case which > demonstrate pretty much worst case scenario: map 4G shmfs file, write to > begin of every page pgoff of the page, remap pages in reverse order, > read every page. > > The test creates 1 million of VMAs if emulation is in use, so I had to > set vm.max_map_count to 1100000 to avoid -ENOMEM. > > Before: 23.3 ( +- 4.31% ) seconds > After: 43.9 ( +- 0.85% ) seconds > Slowdown: 1.88x > > I believe we can live with that. > There's still all the special-case goop around the place to be cleaned up - VM_NONLINEAR is a decent search term. As is "grep nonlinear mm/*.c". And although this cleanup is the main reason for the patchset, let's not do it now - we can do all that if/after this patch get merged. I'll queue the patches for some linux-next exposure and shall send [1/2] Linuswards for 3.16 if nothing terrible happens. Once we've sorted out the too-many-vmas issue we'll need to work out when to merge [2/2]. -- 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>