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@xxxxxxxxxxxxx> > > > > > > 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@xxxxxxxxxx> --D
