On Wed 30-05-18 16:21:33, Dan Williams wrote:
> On Wed, May 30, 2018 at 1:13 AM, Jan Kara <jack@xxxxxxx> wrote:
> > On Tue 29-05-18 18:38:41, Dan Williams wrote:
> >> On Wed, May 23, 2018 at 1:40 AM, Jan Kara <jack@xxxxxxx> 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@xxxxxxx>
> >> >> Cc: Christoph Hellwig <hch@xxxxxx>
> >> >> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> >> >> Cc: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> >> >> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> >> ---
> >> >> 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.
Yes, my comment was more in direction that life would be easier if we could
keep DAX without rmap support but I guess that's just too cumbersome.
> > 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;
OK, good.
> 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.
I guess you mean ->index in the above. And now when thinking about it I don't
think it is worth the complications to avoid using ->index.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
