Re: [RFC PATCH v2] implement orangefs_readahead

[Matthew Wilcox <willy@infradead.org>]

On Fri, Mar 26, 2021 at 10:55:44PM -0400, Mike Marshall wrote:
> Hi David.
> 
> I implemented a version with iov_iter_xarray (below).
> It appears to be doing "the right thing" when it
> gets called, but then I get a backtrace in the kernel
> ring buffer "RIP: 0010:read_pages+0x1a1/0x2c0" which is
> page dumped because: VM_BUG_ON_PAGE(!PageLocked(page))
> ------------[ cut here ]------------
> kernel BUG at include/linux/pagemap.h:892!
> 
> So it seems that in mm/readahead.c/read_pages I end up
> entering the "Clean up the remaining pages" part, and
> never make it through even one iteration... it happens
> whether I use readahead_expand or not.
> 
> I've been looking at it a long time :-), I'll look more
> tomorrow... do you see anything obvious?

Yes; Dave's sample code doesn't consume the pages from the readahead
iterator, so the core code thinks you didn't consume them and unlocks
/ puts the pages for you.  That goes wrong, because you did actually
consume them.  Glad I added the assertions now!

We should probably add something like:

static inline void readahead_consume(struct readahead_control *ractl,
		unsigned int nr)
{
	ractl->_nr_pages -= nr;
	ractl->_index += nr;
}

to indicate that you consumed the pages other than by calling
readahead_page() or readahead_page_batch().  Or maybe Dave can
wrap iov_iter_xarray() in a readahead_iter() macro or something
that takes care of adjusting index & nr_pages for you.