Re: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero support

[Mike Kravetz <mike.kravetz@oracle.com>]

On 12/08/22 19:33, Matthew Wilcox wrote:
> On Thu, Dec 08, 2022 at 10:06:07AM -0800, Sidhartha Kumar wrote:
> > On 12/7/22 6:27 PM, John Hubbard wrote:
> > > On 12/7/22 17:42, Sidhartha Kumar wrote:
> > This works for me, I will take this approach along with Muchun's feedback
> > about a wrapper function so as not to touch _folio_order directly and send
> > out a new version.
> > 
> > One question I have is if I should then get rid of
> > folio_set_compound_order() as hugetlb is the only compound page user I've
> > converted to folios so far and its use can be replaced by the suggested
> > folio_set_nr_pages() and folio_set_order().
> > 
> > Hugetlb also has one has one call to folio_set_compound_order() with a
> > non-zero order, should I replace this with a call to folio_set_order() and
> > folio_set_nr_pages() as well, or keep folio_set_compound_order() and remove
> > zero order support and the comment. Please let me know which approach you
> > would prefer.
> 
> None of the above!
> 
> Whatever we're calling this function *it does not belong* in mm.h.
> Anything outside the MM calling it is going to be a disaster -- can you
> imagine what will happen if a filesystem or device driver is handed a
> folio and decides "Oh, I'll just change the size of this folio"?  It is
> an attractive nuisance and should be confined to mm/internal.h *at best*.

I suspect it was placed in mm.h as it is the 'folio version' of
set_compound_order which resides in mm.h.  But, no need to repeat that
unfortunate placement.

> 
> Equally, we *must not have* separate folio_set_order() and
> folio_set_nr_pages().  These are the same thing!  They must be kept
> in sync!  If we are to have a folio_set_order() instead of open-coding
> it, then it should also update nr_pages.

Ok.  Agree.

> So, given that this is now an internal-to-mm, if not internal-to-hugetlb
> function, I see no reason that it should not handle the case of 0.
> I haven't studied what hugetlb_dissolve does, or why it can't use the
> standard split_folio(), but I'm sure there's a good reason.

The hugetlb code is changing the compound page/folio it created from a set of
individual pages back to individual pages so they can be returned to the
low level allocator.  Somewhat like what page_alloc/page_free do.  split_folio
is overkill.  split_page would be a closer match.

It makes perfect sense to put the function in mm internal.h.

Thanks,
-- 
Mike Kravetz