[PATCH 2/2] direct IO: respect backing device max IO size

[stockhausen]

blkdev_direct_IO() currently uses BIO_MAX_PAGES for maximum IO transfer size.
For more than a decade its value has been fixed to 256. With 4.14+ and commit
225311a4 ("mm: test code to write THP to swap device as a whole") this has
increased to 512. Tests with older FusionIO cards show that the new setting
totally breaks direct IO. The driver advertises max_hw_blocks_kb=1024 and
max_segments=257 and expects the upper layers to cut data correspondingly.
Other propretary drivers might face similar problems. Fix that by respecting
sane values in bdi->io_pages. Patch could qualify for stable.

Signed-off-by: Markus Stockhausen stockhau...@collogia.de


diff --git a/fs/block_dev.c b/fs/block_dev.c
index 2f777f9..6263f75 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -420,15 +420,21 @@ static void blkdev_bio_end_io(struct bio *bio)
 static ssize_t
 blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 {
+       struct file *file = iocb->ki_filp;
+       struct inode *inode = bdev_file_inode(file);
+       struct block_device *bdev = I_BDEV(inode);
+       struct backing_dev_info *bdi = bdev->bd_bdi;
+       int max_io_pages;
        int nr_pages;
 
-       nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
+       max_io_pages = min_not_zero(bdi->io_pages, (unsigned 
long)BIO_MAX_PAGES);
+       nr_pages = iov_iter_npages(iter, max_io_pages + 1);
        if (!nr_pages)
                return 0;
-       if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
+       if (is_sync_kiocb(iocb) && nr_pages <= max_io_pages)
                return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
 
-       return __blkdev_direct_IO(iocb, iter, min(nr_pages, BIO_MAX_PAGES), 
BIO_MAX_PAGES);
+       return __blkdev_direct_IO(iocb, iter, min(nr_pages, max_io_pages), 
max_io_pages);
 }
 
 static __init int blkdev_init(void)
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