Re: [PATCH] iomap: Handle memory allocation failure in readahead

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, Apr 01, 2020 at 04:23:21AM -0700, Matthew Wilcox wrote:
> On Tue, Mar 31, 2020 at 09:31:25PM -0700, Darrick J. Wong wrote:
> > On Tue, Mar 31, 2020 at 08:04:21PM -0700, Matthew Wilcox wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > > 
> > > bio_alloc() can fail when we use GFP_NORETRY.  If it does, allocate
> > > a bio large enough for a single page like mpage_readpages() does.
> > 
> > Why does mpage_readpages() do that?
> > 
> > Is this a means to guarantee some kind of forward (readahead?) progress?
> > Forgive my ignorance, but if memory is so tight we can't allocate a bio
> > for readahead then why not exit having accomplished nothing?
> 
> As far as I can tell, it's just a general fallback in mpage_readpages().
> 
>  * If anything unusual happens, such as:
>  *
>  * - encountering a page which has buffers
>  * - encountering a page which has a non-hole after a hole
>  * - encountering a page with non-contiguous blocks
>  *
>  * then this code just gives up and calls the buffer_head-based read function.
> 
> The actual code for that is:
> 
>                 args->bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
>                                         min_t(int, args->nr_pages,
>                                               BIO_MAX_PAGES),
>                                         gfp);
>                 if (args->bio == NULL)
>                         goto confused;
> ...
> confused:
>         if (args->bio)
>                 args->bio = mpage_bio_submit(REQ_OP_READ, op_flags, args->bio);
>         if (!PageUptodate(page))
>                 block_read_full_page(page, args->get_block);
>         else
>                 unlock_page(page);
> 
> As the comment implies, there are a lot of 'goto confused' cases in
> do_mpage_readpage().
> 
> Ideally, yes, we'd just give up on reading this page because it's
> only readahead, and we shouldn't stall actual work in order to reclaim
> memory so we can finish doing readahead.  However, handling a partial
> page read is painful.  Allocating a bio big enough for a single page is
> much easier on the mm than allocating a larger bio (for a start, it's a
> single allocation, not a pair of allocations), so this is a reasonable
> compromise between simplicity of code and quality of implementation.

Hmm, ok.  I'll add a comment about that:

		/*
		 * If the bio_alloc fails, try it again for a single page to
		 * avoid having to deal with partial page reads.  This emulates
		 * what do_mpage_readpage does.
		 */
		if (!ctx->bio)
			ctx->bio = bio_alloc(orig_gfp, 1);

...in the hopes that if anyone ever makes partial page reads less
painful, they'll hopefully find this breadcrumb and clean up iomap too.

If that's ok,
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>

--D