On Wed, 2019-08-21 at 07:44 +1000, Dave Chinner wrote:

However, the case here is that:

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i.e. page		offset	len	sector
00000000a77f0146	768	3328	0x7d0048
000000006ceca91e	0	768	0x7d004e

The second page added to the bvec is actually offset alignedr. Hence
the check would do nothing on the first page because the bvec array
is empty (so goes into a new bvec anyway), and the check on the
second page would do nothing an it would merge with first because
the offset is aligned correctly. In both cases, the length of the
segment is not aligned, so that needs to be checked, too.

IOWs, I think the check needs to be in bio_add_page, it needs to
check both the offset and length for alignment, and it needs to grab
the alignment from queue_dma_alignment(), not use a hard coded value
of 511.

So something like this?

diff --git a/block/bio.c b/block/bio.c
index 299a0e7651ec..80f449d23e5a 100644
--- a/block/bio.c
+++ b/block/bio.c

@@ -822,8 +822,12 @@ EXPORT_SYMBOL_GPL(__bio_add_page);
 int bio_add_page(struct bio *bio, struct page *page,
                 unsigned int len, unsigned int offset)
 {
+       struct request_queue *q = bio->bi_disk->queue;
        bool same_page = false;
 
+       if (offset & queue_dma_alignment(q) || len & queue_dma_alignment(q))
+               return 0;
+
        if (!__bio_try_merge_page(bio, page, len, offset, &same_page)) {
                if (bio_full(bio, len))
                        return 0;

I tried this, but the 'mount' just hangs - which looks like it might be
due to xfs_rw_bdev() doing:

  while (bio_add_page(bio, page, len, off) != len) {
  	...