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

[Matthew Wilcox <willy@infradead.org>]

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.