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SubjectRe: [PATCH v2] filesystem-dax: Fix dax_layout_busy_page() livelock
On Sat 06-10-18 22:59:56, Dan Williams 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>
> ---
> Changes in v2:
> * Only update nr_pages if the last entry in the pagevec is multi-order.
>
> fs/dax.c | 13 +++++++++++--
> 1 file changed, 11 insertions(+), 2 deletions(-)

Thanks for fixing this up! It is somewhat ugly but nicer fixes would be
more intrusive so I agree with this approach for the ease of backporting
and let's clean up the iteration code later. Feel free to add:

Reviewed-by: Jan Kara <jack@suse.cz>

Honza

>
> diff --git a/fs/dax.c b/fs/dax.c
> index 4becbf168b7f..0fb270f0a0ef 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 +682,15 @@ 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 at
> + * the end of the pagevec.
> + */
> + if (i + 1 >= pagevec_count(&pvec))
> + nr_pages = 1UL << dax_radix_order(entry);
> + }
> put_unlocked_mapping_entry(mapping, index, entry);
> xa_unlock_irq(&mapping->i_pages);
> if (page)
> @@ -696,7 +705,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
> */
> pagevec_remove_exceptionals(&pvec);
> pagevec_release(&pvec);
> - index++;
> + index += nr_pages;
>
> if (page)
> break;
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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