On 06/02/2017 09:18 AM, Christian Borntraeger wrote: > On 06/02/2017 09:16 AM, Martin Schwidefsky wrote: >> On Fri, 2 Jun 2017 09:13:03 +0200 >> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote: >> >>> On 06/02/2017 09:02 AM, Heiko Carstens wrote: >>>> On Thu, Jun 01, 2017 at 12:46:51PM +0200, Martin Schwidefsky wrote: >>>>>> Unfortunately, converting all page tables to 4k pgste page tables is >>>>>> not possible without provoking various race conditions. >>>>> >>>>> That is one approach we tried and was found to be buggy. The point is that >>>>> you are not allowed to reallocate a page table while a VMA exists that is >>>>> in the address range of that page table. >>>>> >>>>> Another approach we tried is to use an ELF flag on the qemu executable. >>>>> That does not work either because fs/exec.c allocates and populates the >>>>> new mm struct for the argument pages before fs/binfmt_elf.c comes into >>>>> play. >>>> >>>> How about if you would fail the system call within arch_check_elf() if you >>>> detect that the binary requires pgstes (as indicated by elf flags) and then >>>> restart the system call? >>>> >>>> That is: arch_check_elf() e.g. would set a thread flag that future mm's >>>> should be allocated with pgstes. Then do_execve() would cleanup everything >>>> and return to entry.S. Upon return to userspace we detect this condition >>>> and simply restart the system call, similar to signals vs -ERESTARTSYS. >>>> >>>> That would make do_execve() cleanup everything and upon reentering it would >>>> allocate an mm with the pgste flag set. >>>> >>>> Maybe this is a bit over-simplified, but might work. >>>> >>>> At least I also don't like the next "hack", that is specifically designed >>>> to only work with how QEMU is currently implemented. It might break with >>>> future QEMU changes or the next user space implementation that drives the >>>> kvm interface, but is doing everything differently. >>>> Let's look for a "clean" solution that will always work. We had too many >>>> hacks for this problem and *all* of them were broken. >>> >>> >>> The more I think about it, dropping 2k page tables and always allocate a full >>> page would simplify pgalloc. As far I can see this would also get rid of >>> the &mm->context.pgtable_lock. >> >> And it would waste twice the amount of memory for page tables. NAK. > > Yes and we spend the same amount of memory TODAY, because every distro on the > planet that uses KVM has sysctl.allocate_pgste set. Maybe todays approach might be still the best. (if qemu is installed, its all 4k, if not its all 2k)
