Re: [PATCH] filesystem-dax: Fix dax_layout_busy_page() livelock

On Sat, Oct 6, 2018 at 11:14 AM Dan Williams dan.j.williams@intel.com wrote:

In the presence of multi-order entries the typical pagevec_lookup_entries() pattern may loop forever:

    while (index < end && pagevec_lookup_entries(&pvec, mapping, index,
                            min(end - index, (pgoff_t)PAGEVEC_SIZE),
                            indices)) {
            ...
            for (i = 0; i < pagevec_count(&pvec); i++) {
                    index = indices[i];
                    ...
            }
            index++; /* BUG */
    }

The loop updates 'index' for each index found and then increments to the next possible page to continue the lookup. However, if the last entry in the pagevec is multi-order then the next possible page index is more than 1 page away. Fix this locally for the filesystem-dax case by checking for dax-multi-order entries. Going forward new users of multi-order entries need to be similarly careful, or we need a generic way to report the page increment in the radix iterator.

Fixes: 5fac7408d828 ("mm, fs, dax: handle layout changes to pinned dax...") Cc: stable@vger.kernel.org Cc: Jan Kara jack@suse.cz Cc: Ross Zwisler zwisler@kernel.org Cc: Matthew Wilcox willy@infradead.org Signed-off-by: Dan Williams dan.j.williams@intel.com

---

fs/dax.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c index 4becbf168b7f..c1472eede1f7 100644 --- a/fs/dax.c +++ b/fs/dax.c @@ -666,6 +666,8 @@ struct page *dax_layout_busy_page(struct address_space *mapping) while (index < end && pagevec_lookup_entries(&pvec, mapping, index, min(end - index, (pgoff_t)PAGEVEC_SIZE), indices)) {

•           pgoff_t nr_pages = 1;
  

•           for (i = 0; i < pagevec_count(&pvec); i++) {
                    struct page *pvec_ent = pvec.pages[i];
                    void *entry;
  

@@ -680,8 +682,11 @@ struct page *dax_layout_busy_page(struct address_space *mapping)

                    xa_lock_irq(&mapping->i_pages);
                    entry = get_unlocked_mapping_entry(mapping, index, NULL);

•                   if (entry)
  

•                   if (entry) {
                            page = dax_busy_page(entry);
  

•                           /* account for multi-order entries */
  

•                           nr_pages = 1UL << dax_radix_order(entry);
  

•                   }
  

Thinking about this a bit further the next index will be at least nr_pages away, but we don't want to accidentally skip over entries as this patch might do. So nr_pages should only be adjusted by the entry size if it is the last entry.