On Wed, May 22, 2019 at 01:03:18PM -0700, Andrew Morton wrote:
> On Mon, 13 May 2019 15:43:08 +0300 Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> > > +/*
> > > + * Returns the number of bytes in this potentially compound page.
> > > + * Must be called with the head page, not a tail page.
> > > + */
> > > +static inline unsigned long page_size(struct page *page)
> > > +{
> >
> > Maybe we should underline commented head page limitation with VM_BUG_ON()?
>
> VM_WARN_ONCE() if poss, please.
>
> The code bloatage from that is likely to be distressing. Perhaps
> adding an out-of-line compound_order_head_only() for this reason would
> help. In which case, just uninline the whole thing...
I think this is unnecessary. Nobody's currently calling the code it
replaces on a tail page, and the plan is to reduce or eliminate the
amount of places that parts of the system see tail pages. I strongly
oppose adding any kind of check here.
> > + return (unsigned long)PAGE_SIZE << compound_order(page);
> > + }
>
> Also, I suspect the cast here is unneeded. Architectures used to
> differe in the type of PAGE_SIZE but please tell me that's been fixed
> for a lomng time...
It's an unsigned int for most, if not all architectures. For, eg,
PowerPC, a PUD page is larger than 4GB. So let's just include the cast
and not have to worry about undefined semantics screwing us over.
