| Message ID | 20200401030421.17195-1-willy@infradead.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | iomap: Handle memory allocation failure in readahead | expand |
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? --D > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> > --- > fs/iomap/buffered-io.c | 3 +++ > 1 file changed, 3 insertions(+) > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > index 417115bfaf6b..c258801f18d4 100644 > --- a/fs/iomap/buffered-io.c > +++ b/fs/iomap/buffered-io.c > @@ -302,6 +302,7 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data, > > if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) { > gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL); > + gfp_t orig_gfp = gfp; > int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT; > > if (ctx->bio) > @@ -310,6 +311,8 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data, > if (ctx->is_readahead) /* same as readahead_gfp_mask */ > gfp |= __GFP_NORETRY | __GFP_NOWARN; > ctx->bio = bio_alloc(gfp, min(BIO_MAX_PAGES, nr_vecs)); > + if (!ctx->bio) > + ctx->bio = bio_alloc(orig_gfp, 1); > ctx->bio->bi_opf = REQ_OP_READ; > if (ctx->is_readahead) > ctx->bio->bi_opf |= REQ_RAHEAD; > -- > 2.25.1 >
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.
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. > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org> Looks ok - not because I'm a fan of the pattern, but because we have a real bug and this seems to be the quickest fix and similar to the mpage codebase.. Reviewed-by: Christoph Hellwig <hch@lst.de>
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
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.
diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c index 417115bfaf6b..c258801f18d4 100644 --- a/fs/iomap/buffered-io.c +++ b/fs/iomap/buffered-io.c @@ -302,6 +302,7 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data, if (!ctx->bio || !is_contig || bio_full(ctx->bio, plen)) { gfp_t gfp = mapping_gfp_constraint(page->mapping, GFP_KERNEL); + gfp_t orig_gfp = gfp; int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT; if (ctx->bio) @@ -310,6 +311,8 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data, if (ctx->is_readahead) /* same as readahead_gfp_mask */ gfp |= __GFP_NORETRY | __GFP_NOWARN; ctx->bio = bio_alloc(gfp, min(BIO_MAX_PAGES, nr_vecs)); + if (!ctx->bio) + ctx->bio = bio_alloc(orig_gfp, 1); ctx->bio->bi_opf = REQ_OP_READ; if (ctx->is_readahead) ctx->bio->bi_opf |= REQ_RAHEAD;