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

--
Jens Axboe

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