Re: [PATCH v5 16/20] dm-crypt: check if adding pages to clone bio fails

On Tue, 30 May 2023, Mike Snitzer wrote:

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

quoted

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

Yes, __bio_add_page would look nicer. But bio_add_page can merge adjacent 
pages into a single bvec entry and __bio_add_page can't (I don't know how 
often the merging happens or what is the performance implication of 
non-merging).

I think that for the next merge window, we can apply this patch: 
https://listman.redhat.com/archives/dm-devel/2023-May/054046.html
which makes this discussion irrelevant. (you can change bio_add_page to 
__bio_add_page in it)

Mikukas