Re: [PATCH] xfs: allocate sector sized IO buffer via page_frag_alloc

[Matthew Wilcox <willy@infradead.org>]

On Tue, Feb 26, 2019 at 09:42:48PM +0800, Ming Lei wrote:
> On Tue, Feb 26, 2019 at 05:02:30AM -0800, Matthew Wilcox wrote:
> > Wait, we're imposing a ridiculous amount of complexity on XFS for no
> > reason at all?  We should just change this to 512-byte alignment.  Tying
> > it to the blocksize of the device never made any sense.
> 
> OK, that is fine since we can fallback to buffered IO for loop in case of
> unaligned dio.
> 
> Then something like the following patch should work for all fs, could
> anyone comment on this approach?

That's not even close to what I meant.

diff --git a/fs/direct-io.c b/fs/direct-io.c
index ec2fb6fe6d37..dee1fc47a7fc 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -1185,18 +1185,20 @@ do_blockdev_direct_IO(struct kiocb *iocb, struct inode *inode,
 	struct dio_submit sdio = { 0, };
 	struct buffer_head map_bh = { 0, };
 	struct blk_plug plug;
-	unsigned long align = offset | iov_iter_alignment(iter);
 
 	/*
 	 * Avoid references to bdev if not absolutely needed to give
 	 * the early prefetch in the caller enough time.
 	 */
 
-	if (align & blocksize_mask) {
+	if (iov_iter_alignment(iter) & 511)
+		goto out;
+
+	if (offset & blocksize_mask) {
 		if (bdev)
 			blkbits = blksize_bits(bdev_logical_block_size(bdev));
 		blocksize_mask = (1 << blkbits) - 1;
-		if (align & blocksize_mask)
+		if (offset & blocksize_mask)
 			goto out;
 	}