On Thu, Sep 04, 2014 at 09:43:11AM -0700, Peter Feiner wrote: > On Mon, Aug 25, 2014 at 09:45:34PM -0700, Hugh Dickins wrote: > > That sets me wondering: have you placed the VM_SOFTDIRTY check in the > > right place in this series of tests? > > > > I think, once pgprot_modify() is correct on all architectures, > > it should be possible to drop that pgprot_val() check from > > vma_wants_writenotify() - which would be a welcome simplification. > > > > But what about the VM_PFNMAP test below it? If that test was necessary, > > then having your VM_SOFTDIRTY check before it seems dangerous. But I'm > > hoping we can persuade ourselves that the VM_PFNMAP test was unnecessary, > > and simply delete it. > > If VM_PFNMAP is necessary, then I definitely put the VM_SOFTDIRTY check in the > wrong spot :-) I don't know much (i.e., anything) about VM_PFNMAP, so I'll > have to bone up on a lot of code before I have an informed opinion about the > necessity of the check. AFAICT, the VM_PFNMAP check is unnecessary since I can't find any drivers that set VM_PFNMAP and enable dirty accounting on their mappings. If anything, VM_PFNMAP precludes mapping dirty tracking since set_page_dirty takes a struct_page argument! Perhaps the VM_PFNMAP check was originally put in vma_wants_writenotify as a safeguard against bogus calls to set_page_dirty? In any case, it seems harmless to me to put the VM_SOFTDIRTY check before the VM_PFNMAP check since none of the fault handling code in mm/memory.c calls set_page_dirty on a VM_PFNMAP fault because either vm_normal_page() returns NULL or ->fault() / ->page_mkwrite() return VM_FAULT_NOPAGE. Moreover, for the purpose of softdirty tracking, enabling write notifications on VM_PFNMAP VMAs is OK since do_wp_page does the right thing when vm_normal_page() returns NULL. -- 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>