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

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 30 May 2023, Mike Snitzer wrote:

> On Tue, May 30 2023 at 11:13P -0400,
> Mikulas Patocka <mpatocka@redhat.com> wrote:
> 
> > 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