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

[Jan Kara <jack@suse.cz>]

On Wed 27-09-17 07:00:53, Dan Williams wrote:
> On Wed, Sep 27, 2017 at 4:35 AM, Jan Kara <jack@suse.cz> 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@linux.intel.com> 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.

Yeah, although it would be a pity to require struct page just to avoid
having to set VM_MIXEDMAP flag...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR