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

[Jens Axboe]

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