On Wed, 1 Aug 2018, Kirill A. Shutemov wrote: > On Wed, Aug 01, 2018 at 11:31:52AM -0700, Hugh Dickins wrote: > > On Wed, 1 Aug 2018, Linus Torvalds wrote: > > > > > > Anyway, the upshot of all this is that I think I know what the ia64 > > > problem was, and John sent the patch for the ashmem case, and I'm > > > going to hold off reverting that vma_is_anonymous() false-positives > > > commit after all. > > > > I'd better send deletion of zap_pmd_range()'s VM_BUG_ON_VMA(): below > > (but I've no proprietorial interest, if you prefer to do your own). > > Agreed. > > Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Thanks. > > > John's patch is good, and originally I thought it was safe from that > > VM_BUG_ON_VMA(), because the /dev/ashmem fd exposed to the user is > > disconnected from the vm_file in the vma, and madvise(,,MADV_REMOVE) > > insists on VM_SHARED. But afterwards read John's earlier mail, > > drawing attention to the vfs_fallocate() in there: I may be wrong, > > and I don't know if Android has THP in the config anyway, but it looks > > to me like an unmap_mapping_range() from ashmem's vfs_fallocate() > > could hit precisely the VM_BUG_ON_VMA(), once it's vma_is_anonymous(). > > > > (I'm not familiar with ashmem, and I certainly don't understand the > > role of MAP_PRIVATE ashmem mappings - hole-punch's zap_pte_range() > > should end up leaving any anon pages in place; but the presence of > > the BUG is requiring us all to understand too much too quickly.) > > Hugh, do you see any reason why ashmem shouldn't have vm_ops == > shmem_vm_ops? I cannot immediately think of an absolute reason why not, but I'm not giving it much thought; and that might turn it into a stranger beast than it already is. > > I don't understand ashmem, but I feel uncomfortable that we have this > sneaky way to create an anonymous VMA. It feels wrong to me. I agree it's odd, but in this respect it's no odder than /dev/zero: that has exactly the same pattern of shmem_zero_setup() for VM_SHARED, else vma_set_anonymous(): which made me comfortable with John's patch, restoring the way it worked before. Admittedly, the subsequent vfs_fallocate() might generate surprises; and the business of doing a shmem_file_setup() first, and then undoing it with a shmem_zero_setup(), looks weird - from John's old XXX comment, I think it was a quick hack to piece together some functionality they needed in a hurry, which never got revisited (they wanted a name for the area? maybe memfd would be good for that now). But if what's in there is working now, I do not want to mess with it: I'd be adding bugs faster than removing them. Hugh
