Re: [PATCH 2/7] mm: add private field of first tail to struct page and struct folio

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

On 09/01/22 19:32, Matthew Wilcox wrote:
> On Thu, Sep 01, 2022 at 10:32:43AM -0700, Mike Kravetz wrote:
> > Not really an issue with this patch, but it made me read more of this
> > comment about folios.  It goes on to say ...
> > 
> >  * same power-of-two.  It is at least as large as %PAGE_SIZE.  If it is
> >  * in the page cache, it is at a file offset which is a multiple of that
> >  * power-of-two.  It may be mapped into userspace at an address which is
> >  * at an arbitrary page offset, but its kernel virtual address is aligned
> >  * to its size.
> >  */
> > 
> > This series is to begin converting hugetlb code to folios.  Just want to
> > note that 'hugetlb folios' have specific user space alignment restrictions.
> > So, I do not think the comment about arbitrary page offset would apply to
> > hugetlb.
> > 
> > Matthew, should we note that hugetlb is special in the comment?  Or, is it
> > not worth updating?
> 
> I'm open to updating it if we can find good wording.  What I'm trying
> to get across there is that when dealing with folios, you can assume
> that they're naturally aligned physically, logically (in the file) and
> virtually (kernel address), but not necessarily virtually (user
> address).  Hugetlb folios are special in that they are guaranteed to
> be virtually aligned in user space, but I don't know if here is the
> right place to document that.  It's an additional restriction, so code
> which handles generic folios doesn't need to know it.

Fair enough.  No need to change.  It just caught my eye.

> > Also, folio_get_private_1 will be used for the hugetlb subpool pointer
> > which resides in page[1].private.  This is used in the next patch of
> > this series.  I'm sure you are aware that hugetlb also uses page private
> > in sub pages 2 and 3.  Can/will/should this method of accessing private
> > in sub pages be expanded to cover these as well?  Expansion can happen
> > later, but if this can not be expanded perhaps we should come up with
> > another scheme.
> 
> There's a few ways of tackling this.  What I'm currently thinking is
> that we change how hugetlbfs uses struct page to store its extra data.
> It would end up looking something like this (in struct page):
> 
> +++ b/include/linux/mm_types.h
> @@ -147,9 +147,10 @@ struct page {
>                 };
>                 struct {        /* Second tail page of compound page */
>                         unsigned long _compound_pad_1;  /* compound_head */
> -                       unsigned long _compound_pad_2;
>                         /* For both global and memcg */
>                         struct list_head deferred_list;
> +                       unsigned long hugetlbfs_private_2;
> +                       unsigned long hugetlbfs_private_3;
>                 };
>                 struct {        /* Page table pages */
>                         unsigned long _pt_pad_1;        /* compound_head */
> 
> although we could use better names and/or types?  I haven't looked to
> see what you're storing here yet.  And then we can make the
> corresponding change to struct folio to add these elements at the
> right place.

I am terrible at names.  hugetlb is storing pointers in the private fields.
FWICT, something like this would work.

> 
> Does that sound sensible?

-- 
Mike Kravetz