Re: [PATCH] bcache: Revert "bcache: use bvec_virt"

[Christoph Hellwig <hch@lst.de>]

On Thu, Nov 04, 2021 at 12:11:45AM +0800, Coly Li wrote:
>> fresh page for each vec, and bio_for_each_segment_all iterates page
>> by page.  IFF there is an offset there is proble in the surrounding
>> code as bch_bio_alloc_pages assumes that it is called on a freshly
>> allocate and initialized bio.
>
> Yes, the offset is modified in bch_bio_alloc_pages().

Where?   In my upstream copy of bch_bio_alloc_pages there is no bv_offset
manipulation, and I could not see how such a manipulation would make
sense.

> Normally the bcache 
> defined block size is 4KB so the issue was not triggered frequently. I 
> found it during testing my nvdimm enabling code for bcache, where I happen 
> to make the bcache defined block size to non-4KB. The offset is from the 
> previous written bkey set, which the minimized unit size is 1 
> bcache-defined-block-size.

So you have some out of tree changes here?  Copying a PAGE_SIZE into
a 'segment' bvec just does not make any sense if there is an offset,
as segments are defined as bvecs that do not span page boundaries.

I suspect the best thing to do in do_btree_node_write would be something
like the patch below instead of poking into the internals here, but I'd
also really like to understand the root cause as it does point to a bug
somewhere else.


diff --git a/drivers/md/bcache/btree.c b/drivers/md/bcache/btree.c
index 93b67b8d31c3d..f69914848f32f 100644
--- a/drivers/md/bcache/btree.c
+++ b/drivers/md/bcache/btree.c
@@ -378,8 +378,8 @@ static void do_btree_node_write(struct btree *b)
 		struct bvec_iter_all iter_all;
 
 		bio_for_each_segment_all(bv, b->bio, iter_all) {
-			memcpy(bvec_virt(bv), addr, PAGE_SIZE);
-			addr += PAGE_SIZE;
+			memcpy_to_bvec(bvec_virt(bv), addr);
+			addr += bv->bv_len;
 		}
 
 		bch_submit_bbio(b->bio, b->c, &k.key, 0);