linux-kernel - Re: [PATCH] fat: Avoid oops when bdi->io

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Message-ID: <20200830012151.GW14765@casper.infradead.org>
Date:   Sun, 30 Aug 2020 02:21:51 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fat: Avoid oops when bdi->io_pages==0

On Sun, Aug 30, 2020 at 09:59:41AM +0900, OGAWA Hirofumi 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@...l.parknet.co.jp>
> Cc: <stable@...r.kernel.org>
> ---
>  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);

Wait, rounddown?  ->io_pages is supposed to be the maximum number of
pages to readahead.  Shouldn't this be max() instead of rounddown()?