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SubjectRe: [PATCH] fat: Avoid oops when bdi->io_pages==0
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Jens Axboe <axboe@kernel.dk> writes:

> On 8/31/20 10:56 AM, Matthew Wilcox wrote:
>> On Mon, Aug 31, 2020 at 10:39:26AM -0600, Jens Axboe wrote:
>>> We really should ensure that ->io_pages is always set, imho, instead of
>>> having to work-around it in other spots.
>>
>> Interestingly, there are only three places in the entire kernel which
>> _use_ bdi->io_pages. FAT, Verity and the pagecache readahead code.
>>
>> Verity:
>> unsigned long num_ra_pages =
>> min_t(unsigned long, num_blocks_to_hash - i,
>> inode->i_sb->s_bdi->io_pages);
>>
>> FAT:
>> if (ra_pages > sb->s_bdi->io_pages)
>> ra_pages = rounddown(ra_pages, sb->s_bdi->io_pages);
>>
>> Pagecache:
>> max_pages = max_t(unsigned long, bdi->io_pages, ra->ra_pages);
>> and
>> if (req_size > max_pages && bdi->io_pages > max_pages)
>> max_pages = min(req_size, bdi->io_pages);
>>
>> The funny thing is that all three are using it differently. Verity is
>> taking io_pages to be the maximum amount to readahead. FAT is using
>> it as the unit of readahead (round down to the previous multiple) and
>> the pagecache uses it to limit reads that exceed the current per-file
>> readahead limit (but allows per-file readahead to exceed io_pages,
>> in which case it has no effect).
>>
>> So how should it be used? My inclination is to say that the pagecache
>> is right, by virtue of being the most-used.
>
> When I added ->io_pages, it was for the page cache use case. The others
> grew after that...

FAT and pagecache usage would be similar or same purpose. The both is
using io_pages as optimal IO size.

In pagecache case, it uses io_pages if one request size is exceeding
io_pages. In FAT case, there is perfect knowledge about future/total
request size. So FAT divides request by io_pages, and adjust ra_pages
with knowledge.

I don't know about verity.

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

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