On Tue, May 30 2023 at 11:13P -0400,
Mikulas Patocka [off-list ref] wrote:

On Tue, 2 May 2023, Johannes Thumshirn wrote:

quoted

Check if adding pages to clone bio fails and if it does retry with
reclaim. This mirrors the behaviour of page allocation in
crypt_alloc_buffer().

This way we can mark bio_add_pages as __must_check.

Reviewed-by: Damien Le Moal <redacted>
Signed-off-by: Johannes Thumshirn <redacted>
---
 drivers/md/dm-crypt.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
index 8b47b913ee83..b234dc089cee 100644
--- a/drivers/md/dm-crypt.c
+++ b/drivers/md/dm-crypt.c

@@ -1693,7 +1693,14 @@ static struct bio *crypt_alloc_buffer(struct dm_crypt_io *io, unsigned int size)
 
 		len = (remaining_size > PAGE_SIZE) ? PAGE_SIZE : remaining_size;
 
-		bio_add_page(clone, page, len, 0);
+		if (!bio_add_page(clone, page, len, 0)) {
+			mempool_free(page, &cc->page_pool);
+			crypt_free_buffer_pages(cc, clone);
+			bio_put(clone);
+			gfp_mask |= __GFP_DIRECT_RECLAIM;
+			goto retry;
+
+		}
 
 		remaining_size -= len;
 	}

Hi

I nack this. This just adds code that can't ever be executed.

dm-crypt already allocates enough entries in the vector (see "unsigned int 
nr_iovecs = (size + PAGE_SIZE - 1) >> PAGE_SHIFT;"), so bio_add_page can't 
fail.

If you want to add __must_check to bio_add_page, you should change the 
dm-crypt code to:
if (!bio_add_page(clone, page, len, 0)) {
	WARN(1, "this can't happen");
	return NULL;
}
and not write recovery code for a can't-happen case.

Thanks for the review Mikulas. But the proper way forward, in the
context of this patchset, is to simply change bio_add_page() to
__bio_add_page()

Subject becomes: "dm crypt: use __bio_add_page to add single page to clone bio"

And header can explain that "crypt_alloc_buffer() already allocates
enough entries in the clone bio's vector, so bio_add_page can't fail".

Mike