Re: [PATCH] fat: Avoid oops when bdi->io_pages==0

[Jens Axboe]

On Sat, Aug 29, 2020 at 7:08 PM OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx> wrote:
>
> On one system, there was bdi->io_pages==0. This seems to be the bug of
> a driver somewhere, and should fix it though. Anyway, it is better to
> avoid the divide-by-zero Oops.
>
> So this check it.
>
> Signed-off-by: OGAWA Hirofumi <hirofumi@xxxxxxxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> ---
>  fs/fat/fatent.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/fat/fatent.c b/fs/fat/fatent.c
> index f7e3304..98a1c4f 100644
> --- a/fs/fat/fatent.c   2020-08-30 06:52:47.251564566 +0900
> +++ b/fs/fat/fatent.c   2020-08-30 06:54:05.838319213 +0900
> @@ -660,7 +660,7 @@ static void fat_ra_init(struct super_blo
>         if (fatent->entry >= ent_limit)
>                 return;
>
> -       if (ra_pages > sb->s_bdi->io_pages)
> +       if (sb->s_bdi->io_pages && ra_pages > sb->s_bdi->io_pages)
>                 ra_pages = rounddown(ra_pages, sb->s_bdi->io_pages);
>         reada_blocks = ra_pages << (PAGE_SHIFT - sb->s_blocksize_bits + 1);

I don't think we should work-around this here. What device is this on?
Something like the below may help.

diff --git a/block/blk-core.c b/block/blk-core.c
index d9d632639bd1..10c08ac50697 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -539,6 +539,7 @@ struct request_queue *blk_alloc_queue(int node_id)
 		goto fail_stats;
 
 	q->backing_dev_info->ra_pages = VM_READAHEAD_PAGES;
+	q->backing_dev_info->io_pages = VM_READAHEAD_PAGES;
 	q->backing_dev_info->capabilities = BDI_CAP_CGROUP_WRITEBACK;
 	q->node = node_id;

-- 
Jens Axboe