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

[David Howells]

Christoph Hellwig <hch@xxxxxxxxxxxxx> 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.

I could change it so that rather than using a pair of flags, it uses a
four-state variable (which can be stored in bi_flags): BIO_PAGE_DEFAULT,
BIO_PAGE_REFFED, BIO_PAGE_PINNED, BIO_PAGE_NO_CLEANUP, say.

Or I could add an extra flag to say that the setting is locked.  Or we could
just live with the scenario I outlined possibly happening.

David