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

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

[Dan Williams]

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);
+			}
 			put_unlocked_mapping_entry(mapping, index, entry);
 			xa_unlock_irq(&mapping->i_pages);
 			if (page)
@@ -696,7 +701,7 @@ struct page *dax_layout_busy_page(struct address_space *mapping)
 		 */
 		pagevec_remove_exceptionals(&pvec);
 		pagevec_release(&pvec);
-		index++;
+		index += nr_pages;
 
 		if (page)
 			break;

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

[Dan Williams]

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.