Re: [syzbot] [mm?] WARNING in __folio_rmap_sanity_checks (2)

[David Hildenbrand]

On 11.01.25 02:00, Hillf Danton wrote:
> On Fri, 10 Jan 2025 17:35:25 +0100 David Hildenbrand <david@xxxxxxxxxx>
> > On 31.12.24 09:41, Hillf Danton wrote:
> > > On Fri, 27 Dec 2024 20:56:21 -0800
> > > > syzbot has found a reproducer for the following issue on:
> > > >
> > > > HEAD commit:    8155b4ef3466 Add linux-next specific files for 20241220
> > > > git tree:       linux-next
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1652fadf980000
> > >
> > > #syz test
> > >
> > > --- x/mm/filemap.c
> > > +++ y/mm/filemap.c
> > > @@ -3636,6 +3636,10 @@ static vm_fault_t filemap_map_folio_rang
> > >    		continue;
> > >    skip:
> > >    		if (count) {
> > > +			for (unsigned int i = 0; i < count; i++) {
> > > +				if (page_folio(page + i) != folio)
> > > +					goto out;
> > > +			}
> >
> > IIRC, count <= nr_pages. Wouldn't that mean that we somehow pass in
> > nr_pages that already exceeds the given folio+start?
> >
> > When I last looked at this, I was not able to spot the error in the
> > caller :(
> This is a debug patch at the first place, and this hunk overlaps with the
> next one.

Yeah, I was rather wondering if you had any clue why that hunk might 
help on its own.

--
Cheers,

David / dhildenb