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

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Date:   Sun, 30 Aug 2020 10:54:35 +0900
From:   OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:     Matthew Wilcox <willy@...radead.org>
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

Matthew Wilcox <willy@...radead.org> writes:

> 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()?

Hm, io_pages is limited by driver setting too, and io_pages can be lower
than ra_pages, e.g. usb storage.

Assuming ra_pages is user intent of readahead window. So if io_pages is
lower than ra_pages, this try ra_pages to align of io_pages chunk, but
not bigger than ra_pages. Because if block layer splits I/O requests to
hard limit, then I/O is not optimal.

So it is intent, I can be misunderstanding though.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>