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Howlett wrote: > * Vlastimil Babka [241031 13:07]: > > On 10/31/24 18:01, Liam R. Howlett wrote: > > > From: "Liam R. Howlett" > > > > > > There have been no reported infinite loops in the tree, but checking the > > > detection of an infinite loop during validation is simple enough. Add > > > the detection to the validate_mm() function so that error reports are > > > clear and don't just report stalls. > > > > > > This does not protect against internal maple tree issues, but it does > > > detect too many vmas being returned from the tree. > > > > > > Signed-off-by: Liam R. Howlett > > > Cc: Lorenzo Stoakes > > > Cc: Vlastimil Babka > > > Cc: Jann Horn > > > --- > > > mm/vma.c | 3 ++- > > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > > > diff --git a/mm/vma.c b/mm/vma.c > > > index 68138e8c153e..60ed8cc187ad 100644 > > > --- a/mm/vma.c > > > +++ b/mm/vma.c > > > @@ -615,7 +615,8 @@ void validate_mm(struct mm_struct *mm) > > > anon_vma_unlock_read(anon_vma); > > > } > > > #endif > > > - i++; > > > + if (++i > mm->map_count) > > > + break; > > > > Would it make sense to allow some slack so that the error below can > > distinguish better between off-by-one/few error from a complete corruption? > > > > And in that case assign some special value to "i" (-1?) to make it clear > > this was triggered? > > Yes, probably. 10 would be plenty. In recent memory I cannot think of > an example that we exceeded 7 munmap()'s in a single operation - > although it is easily possible to do. > > I like the idea of -1 too, at least someone would come to inspect where > it came from at that point. Hm this feels a little arbitrary though... I mean can we race with map_count at this point or is everything locked down such that no munmaps() can happen? Otherwise it feels a bit whack-a-mole. I agree with Vlastimil though it'd be nice to sort of differentiate, but if we _absolutely can only iterate mm->map_count times_ here, it might be worth letting a few more go, then in the next bit of code... > > > > > > } > > > if (i != mm->map_count) { > > > pr_emerg("map_count %d vma iterator %d\n", mm->map_count, i); > > ...here which does indeed imply that i literally cannot be anything but mm->map_count, I mean I guess we already get to see here how far off we are. So yeah something like letting it go 10 more times (maybe like #define UNUSUALLY_BAD_CORRUPTION_COUNT 10 or something I don't know naming is hard) just so we can pick that out would be nice. But I do like the general idea here though!