On 3/21/25 12:25, Gavin Shan wrote: > On 3/21/25 8:11 PM, David Hildenbrand wrote: >> 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). >> > > Yes, it shouldn't cause any visible impacts so far due to the gap. Thanks for confirming, please state that in the commit log/cover letter too. > I just found the issue by code inspection. Lets drop the fix tags > in v2. Fixes: tag is fine and correct, just Cc: stable is unnecessary. Thanks. >>> >>> 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. >> > > +1 to remove page_mapcount_is_type(). After Willy confirms, I can post > an extra series to do it if needed. > > Thanks, > Gavin >
