Re: [PATCH] mm: fix livelock caused by iterating multi order entry

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

[ add Sasha for -stable advice ]

On Thu, Jul 18, 2019 at 5:13 PM Liu Bo <bo.liu@linux.alibaba.com> wrote:
>
> The livelock can be triggerred in the following pattern,
>
>         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 */
>         }
>
> multi order exceptional entry is not specially considered in
> invalidate_inode_pages2_range() and it ended up with a livelock because
> both index 0 and index 1 finds the same pmd, but this pmd is binded to
> index 0, so index is set to 0 again.
>
> This introduces a helper to take the pmd entry's length into account when
> deciding the next index.
>
> Note that there're other users of the above pattern which doesn't need to
> fix,
>
> - dax_layout_busy_page
> It's been fixed in commit d7782145e1ad
> ("filesystem-dax: Fix dax_layout_busy_page() livelock")
>
> - truncate_inode_pages_range
> This won't loop forever since the exceptional entries are immediately
> removed from radix tree after the search.
>
> Fixes: 642261a ("dax: add struct iomap based DAX PMD support")
> Cc: <stable@vger.kernel.org> since 4.9 to 4.19
> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
> ---
>
> The problem is gone after commit f280bf092d48 ("page cache: Convert
> find_get_entries to XArray"), but since xarray seems too new to backport
> to 4.19, I made this fix based on radix tree implementation.

I think in this situation, since mainline does not need this change
and the bug has been buried under a major refactoring, is to send a
backport directly against the v4.19 kernel. Include notes about how it
replaces the fix that was inadvertently contained in f280bf092d48
("page cache: Convert find_get_entries to XArray"). Do you have a test
case that you can include in the changelog?