Re: [PATCH] block: Add bio_limit

On Mon, Jan 18, 2021 at 07:31:13PM +0100, Christoph Hellwig wrote:

On Mon, Jan 18, 2021 at 06:17:12PM +0000, Matthew Wilcox wrote:

quoted

On Mon, Jan 18, 2021 at 07:13:38PM +0100, Christoph Hellwig wrote:

quoted

On Thu, Jan 14, 2021 at 07:47:06PM +0000, Matthew Wilcox (Oracle) wrote:

quoted

It's often inconvenient to use BIO_MAX_PAGES due to min() requiring the
sign to be the same.  Introduce bio_limit() and change BIO_MAX_PAGES to
be unsigned to make it easier for the users.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

I like the helper, but I'm not too happy with the naming.  Why not
something like bio_guess_nr_segs() or similar?

This feels like it's a comment on an entirely different patch, like this one:

https://git.infradead.org/users/willy/pagecache.git/commitdiff/fe9841debe24e15100359acadd0b561bbb2dceb1

bio_limit() doesn't guess anything, it just clamps the argument to
BIO_MAX_PAGES (which is itself misnamed; it's BIO_MAX_SEGS now)

No, it was for thi patch.  Yes, it divides and clamps.  Which is sort of
a guess as often we might need less of them.  That being said I'm not
very fond of my suggestion either, but limit sounds wrong as well.

bio_limit() doesn't divide.  Some of the callers divide.

+static inline unsigned int bio_limit(unsigned int nr_segs)
+{
+       return min(nr_segs, BIO_MAX_PAGES);
+}

I'd rather the callers didn't have to worry about this at all (just pass
in a number and then deal with however many bvecs you were given), but
there are callers which depend on the current if-too-big-return-NULL
behaviour, and I don't want to track all of those down and fix them.

I chose limit because it's imposing the bio's limit.  Could be called
bio_clamp(), but the bio also doesn't impose a minimum, so that seemed
wrong.