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
