Re: [PATCH 05/11] filesystem-dax: set page->index

[Dan Williams <dan.j.williams@intel.com>]

On Wed, May 30, 2018 at 1:13 AM, Jan Kara <jack@suse.cz> wrote:
> On Tue 29-05-18 18:38:41, Dan Williams wrote:
>> On Wed, May 23, 2018 at 1:40 AM, Jan Kara <jack@suse.cz> wrote:
>> > On Tue 22-05-18 07:39:57, Dan Williams wrote:
>> >> In support of enabling memory_failure() handling for filesystem-dax
>> >> mappings, set ->index to the pgoff of the page. The rmap implementation
>> >> requires ->index to bound the search through the vma interval tree. The
>> >> index is set and cleared at dax_associate_entry() and
>> >> dax_disassociate_entry() time respectively.
>> >>
>> >> Cc: Jan Kara <jack@suse.cz>
>> >> Cc: Christoph Hellwig <hch@lst.de>
>> >> Cc: Matthew Wilcox <mawilcox@microsoft.com>
>> >> Cc: Ross Zwisler <ross.zwisler@linux.intel.com>
>> >> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
>> >> ---
>> >>  fs/dax.c |   11 ++++++++---
>> >>  1 file changed, 8 insertions(+), 3 deletions(-)
>> >>
>> >> diff --git a/fs/dax.c b/fs/dax.c
>> >> index aaec72ded1b6..2e4682cd7c69 100644
>> >> --- a/fs/dax.c
>> >> +++ b/fs/dax.c
>> >> @@ -319,18 +319,22 @@ static unsigned long dax_radix_end_pfn(void *entry)
>> >>       for (pfn = dax_radix_pfn(entry); \
>> >>                       pfn < dax_radix_end_pfn(entry); pfn++)
>> >>
>> >> -static void dax_associate_entry(void *entry, struct address_space *mapping)
>> >> +static void dax_associate_entry(void *entry, struct address_space *mapping,
>> >> +             struct vm_area_struct *vma, unsigned long address)
>> >>  {
>> >> -     unsigned long pfn;
>> >> +     unsigned long size = dax_entry_size(entry), pfn, index;
>> >> +     int i = 0;
>> >>
>> >>       if (IS_ENABLED(CONFIG_FS_DAX_LIMITED))
>> >>               return;
>> >>
>> >> +     index = linear_page_index(vma, address & ~(size - 1));
>> >>       for_each_mapped_pfn(entry, pfn) {
>> >>               struct page *page = pfn_to_page(pfn);
>> >>
>> >>               WARN_ON_ONCE(page->mapping);
>> >>               page->mapping = mapping;
>> >> +             page->index = index + i++;
>> >>       }
>> >>  }
>> >
>> > Hum, this just made me think: How is this going to work with XFS reflink?
>> > In fact is not the page->mapping association already broken by XFS reflink?
>> > Because with reflink we can have two or more mappings pointing to the same
>> > physical blocks (i.e., pages in DAX case)...
>>
>> Good question. I assume we are ok in the non-DAX reflink case because
>> rmap of failing / poison pages is only relative to the specific page
>> cache page for a given inode in the reflink. However, DAX would seem
>> to break this because we only get one shared 'struct page' for all
>> possible mappings of the physical file block. I think this means for
>> iterating over the rmap of "where is this page mapped" would require
>> iterating over the other "sibling" inodes that know about the given
>> physical file block.
>>
>> As far as I can see reflink+dax would require teaching kernel code
>> paths that ->mapping may not be a singular relationship. Something
>> along the line's of what Jerome was presenting at LSF to create a
>> special value to indicate, "call back into the filesystem (or the page
>> owner)" to perform this operation.
>>
>> In the meantime the kernel crashes when userspace accesses poisoned
>> pmem via DAX. I assume that reworking rmap for the dax+reflink case
>> should not block dax poison handling? Yell if you disagree.
>
> The thing is, up until get_user_pages() vs truncate series ("fs, dax: use
> page->mapping to warn if truncate collides with a busy page" in
> particular), DAX was perfectly fine with reflinks since we never needed
> page->mapping.

Sure, but if this rmap series had come first I still would have needed
to implement ->mapping. So unless we invent a general ->mapping
replacement and switch all mapping users, it was always going to
collide with DAX eventually.

> Now this series adds even page->index dependency which makes
> life for rmap with reflinks even harder. So if nothing else we should at
> least make sure reflinked filesystems cannot be mounted with dax mount
> option for now and seriously start looking into how to implement rmap with
> reflinked files for DAX because this noticeably reduces its usefulness.

This restriction is already in place. In xfs_reflink_remap_range() we have:

        /* Don't share DAX file data for now. */
        if (IS_DAX(inode_in) || IS_DAX(inode_out))
                goto out_unlock;

All this said, perhaps we don't need to set ->link, it would just mean
a wider search through the rmap tree to find if the given page is
mapped. So, I think I can forgo setting ->link if I teach the rmap
code to search the entire ->mapping.