Re: [PATCH 6/7] mm, fs: introduce file_operations->post_mmap()

[Dan Williams]

On Wed, Sep 27, 2017 at 4:35 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Tue 26-09-17 14:41:53, Dan Williams wrote:
>> On Tue, Sep 26, 2017 at 2:06 PM, Ross Zwisler
>> <ross.zwisler@xxxxxxxxxxxxxxx> wrote:
>> > On Tue, Sep 26, 2017 at 12:19:21PM -0700, Dan Williams wrote:
>> >> On Tue, Sep 26, 2017 at 11:57 AM, Ross Zwisler
>> > <>
>> >> > This decision can only be made (in this
>> >> > proposed scheme) *after* the inode->i_mapping->i_mmap  tree has been
>> >> > populated, which means we need another call into the filesystem after this
>> >> > insertion has happened.
>> >>
>> >> I get that, but it seems over-engineered and something that can also
>> >> be safely cleaned up after the fact by the code path that is disabling
>> >> DAX.
>> >
>> > I don't think you can safely clean it up after the fact because some thread
>> > might have already called ->mmap() to set up the vma->vm_flags for their new
>> > mapping, but they haven't added it to inode->i_mapping->i_mmap.
>>
>> If madvise(MADV_NOHUGEPAGE) can dynamically change vm_flags, then the
>> DAX disable path can as well. VM_MIXEDMAP looks to be a nop for normal
>> memory mappings.
>>
>> > The inode->i_mapping->i_mmap tree is the only way (that I know of at least)
>> > that the filesystem has any idea about about the mapping.  This is the method
>> > by which we would try and clean up mapping flags, if we were to do so, and
>> > it's the only way that the filesystem can know whether or not mappings exist.
>> >
>> > The only way that I could think of to make this safely work is to have the
>> > insertion into the inode->i_mapping->i_mmap tree be our sync point.  After
>> > that the filesystem and the mapping code can communicate on the state of DAX,
>> > but before that I think it's basically indeterminate.
>>
>> If we lose the race and leak VM_HUGEPAGE to a non-DAX mapping what
>> breaks? I'd rather be in favor of not setting VM_HUGEPAGE at all in
>> the ->mmap() handler and let the default THP policy take over. In
>> fact, see transparent_hugepage_enabled() we already auto-enable huge
>> page support for dax mappings regardless of VM_HUGEPAGE.
>
> Hum, this is an interesting option. So do you suggest that filesystems
> supporting DAX would always setup mappings with VM_MIXEDMAP and without
> VM_HUGEPAGE and thus we'd get rid of dependency on S_DAX flag in ->mmap?
> That could actually work. The only possible issue I can see is that
> VM_MIXEDMAP is still slightly different from normal page mappings and it
> could have some performance implications - e.g. copy_page_range() does more
> work on VM_MIXEDMAP mappings but not on normal page mappings.

We can also get rid of VM_MIXEDMAP if we disable DAX in the
!pfn_t_has_page() case.