Re: [PATCH 0/2] Fix parameter passed to page_mapcount_is_type()

[David Hildenbrand]

On 21.03.25 10:23, Vlastimil Babka wrote:
> On 3/21/25 06:31, Gavin Shan wrote:
> > Found by code inspection. There are two places where the parameter
> > passed to page_mapcount_is_type() is (page->__mapcount), which is
> > correct since it should be one more than the value, as explained in
> > the comments to page_mapcount_is_type(): (a) page_has_type() in
> > page-flags.h (b) __dump_folio() in mm/debug.c
>
> IIUC you are right. Luckily thanks to the the PGTY_mapcount_underflow limit,
> this off-by-one error doesn't currently cause visible issues i.e.
> misclassifications legitimate mapcount as page type and vice versa, right?
> We'd have to have a mapcount underflown severely right to the limit to make
> that off-by-one error cross it?

Agreed. Likely not stable material because it isn't actually fixing 
anything (because of the safety gaps).

> I wonder if a more future-proof solution would be to redefine
> page_mapcount_is_type() instead to not subtract. But I'll leave that to willy.

With upcoming changes around that, likely best to leave that alone. I 
expect page_mapcount_is_type() to completely vanish.

--
Cheers,

David / dhildenb