On Mon, May 12, 2014 at 11:11:48AM -0400, Sasha Levin wrote: > On 05/08/2014 05:57 PM, Andrew Morton wrote: > > 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]. > > It seems that since no one is really using it, it's also impossible to > properly test it. I've sent a fix that deals with panics in error paths > that are very easy to trigger, but I'm worried that there are a lot more > of those hiding over there. Sorry for that. > Since we can't find any actual users, testing suites are very incomplete > w.r.t this syscall, and the amount of work required to "remove" it is > non-trivial, can we just kill this syscall off? > > It sounds to me like a better option than to ship a new, buggy and possibly > security dangerous version which we can't even test. Taking into account your employment, is it possible to check how the RDBMS (old but it still supported 32-bit versions) would react on -ENOSYS here? I would like to get rid of it completely, but I thought it's not an option for compatibility reason. -- Kirill A. Shutemov -- 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>