Re: [PATCH v5 8/9] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

Christoph Hellwig
2023-01-12T14:26:59Z

On Thu, Jan 12, 2023 at 06:09:16AM -0800, Christoph Hellwig wrote:
> On Thu, Jan 12, 2023 at 10:28:41AM +0000, David Howells wrote:
> > Christoph Hellwig <hch@infradead.org> wrote:
> >
> > > if (cleanup_mode & FOLL_GET) {
> > > WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_PINNED));
> > > bio_set_flag(bio, BIO_PAGE_REFFED);
> > > }
> > > if (cleanup_mode & FOLL_PIN) {
> > > WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_REFFED));
> > > bio_set_flag(bio, BIO_PAGE_PINNED);
> > > }
> >
> > That won't necessarily work as you might get back cleanup_mode == 0, in which
> > case both flags are cleared - and neither warning will trip on the next
> > addition.
>
> Well, it will work for the intended use case even with
> cleanup_mode == 0, we just won't get the debug check. Or am I missing
> something fundamental?

In fact looking at the code we can debug check that case too by doing:

if (cleanup_mode & FOLL_GET) {
WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_PINNED));
bio_set_flag(bio, BIO_PAGE_REFFED);
} else if (cleanup_mode & FOLL_PIN) {
WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_REFFED));
bio_set_flag(bio, BIO_PAGE_PINNED);
} else {
WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_PINNED) ||
bio_test_flag(bio, BIO_PAGE_REFFED));
}

But given that all calls for the same iter type return the same
cleanup_mode by defintion I'm not even sure we need any of this
debug checking, and might as well just do:

if (cleanup_mode & FOLL_GET)
bio_set_flag(bio, BIO_PAGE_REFFED);
else if (cleanup_mode & FOLL_PIN)
bio_set_flag(bio, BIO_PAGE_PINNED);