Re: [PATCH v2] mm: Split page_type out from _map_count

[Christopher Lameter <cl@linux.com>]

On Fri, 9 Feb 2018, Dave Hansen wrote:

> Are there any straightforward rules that we can enforce here?  For
> instance, if you are using "page_type", you can never have PG_lru set.
>
> Not that we have done this at all for 'struct page' historically, it
> would be really convenient to have a clear definition for when
> "page_type" is valid vs. "_mapcount".

Well in general we would like to be able to enforce uses depending on
the contents of other fields in struct page. That would require compiler
support I guess?

What we could do is write a struct page validator that checks contents
using some macros? Could be added to the usual places where we check
consistency and could also be used for a global sweep over struct pages
for validation.

SLUB can do that for metadata. If we could express consistency rules for
objects in general then it may even have a wider applicability.

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