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SubjectRe: [PATCH] fat: Avoid oops when bdi->io_pages==0
Date
Matthew Wilcox <willy@infradead.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@mail.parknet.co.jp>
>> Cc: <stable@vger.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@mail.parknet.co.jp>

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