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

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 01, 2020 at 09:48:25AM -0700, Darrick J. Wong wrote:
> 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>

That makes perfect sense; thank you.  Assuming you'll just apply it with
that change.