Re: [PATCH v4 3/3] fs, xfs: introduce MAP_DIRECT for creating block-map-sealed file ranges

[Dan Williams]

On Tue, Aug 15, 2017 at 9:29 AM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> On Tue, Aug 15, 2017 at 5:42 AM, Jan Kara <jack@xxxxxxx> wrote:
>> On Mon 14-08-17 23:12:22, Dan Williams wrote:
>>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
>>> index ff151814a02d..73fdc0ada9ee 100644
>>> --- a/include/linux/mm_types.h
>>> +++ b/include/linux/mm_types.h
>>> @@ -306,6 +306,7 @@ struct vm_area_struct {
>>>       struct mm_struct *vm_mm;        /* The address space we belong to. */
>>>       pgprot_t vm_page_prot;          /* Access permissions of this VMA. */
>>>       unsigned long vm_flags;         /* Flags, see mm.h. */
>>> +     unsigned long fs_flags;         /* fs flags, see MAP_DIRECT etc */
>>>
>>>       /*
>>>        * For areas with an address space and backing store,
>>
>> Ah, OK, here are VMA flags I was missing in the previous patch :) But why
>> did you create separate fs_flags field for this? on 64-bit archs there's
>> still space in vm_flags and frankly I don't see why we should separate
>> MAP_DIRECT or MAP_SYNC from other flags?
>
> Where would MAP_DIRECT go in the 32-bit case?
>
>> After all a difference in these
>> flags must also prevent VMA merging (which you forgot to handle I think)
>> and they need to be copied on split (which happens by chance even now).
>
> Ah, yes I did miss blocking the merge of a vma with MAP_DIRECT and one
> without. However, the vma split path looks ok.

The merge path already blocks merging vmas that have the ->close()
operation defined in is_mergeable_vma().