On Tue, 14 Oct 2025, Kalesh Singh wrote:
On Mon, Oct 13, 2025 at 11:28 PM Hugh Dickins hughd@google.com wrote:
Sorry for letting you go so far before speaking up (I had to test what I believed to be true, and had hoped that meanwhile one of your many illustrious reviewers would say so first, but no): it's a NAK from me.
These are not off-by-ones: at the point of these checks, it is not known whether an additional map/vma will have to be added, or the addition will be merged into an existing map/vma. So the checks err on the lenient side, letting you get perhaps one more than the sysctl said, but not allowing any more than that.
Which is all that matters, isn't it? Limiting unrestrained growth.
In this patch you're proposing to change it from erring on the lenient side to erring on the strict side - prohibiting merges at the limit which have been allowed for many years.
Whatever one thinks about the merits of erring on the lenient versus erring on the strict side, I see no reason to make this change now, and most certainly not with a Fixes Cc: stable. There is no danger in the current behaviour; there is danger in prohibiting what was allowed before.
As to the remainder of your series: I have to commend you for doing a thorough and well-presented job, but I cannot myself see the point in changing 21 files for what almost amounts to a max_map_count subsystem. I call it misdirected effort, not at all to my taste, which prefers the straightforward checks already there; but accept that my taste may be out of fashion, so won't stand in the way if others think it worthwhile.
Hi Hugh,
Thanks for the detailed review and for taking the time to test the behavior.
You've raised a valid point. I wasn't aware of the history behind the lenient check for merges. The lack of a comment, like the one that exists for exceeding the limit in munmap(), led me to misinterpret this as an off-by-one bug. The convention makes sense if we consider potential merges.
Yes, a comment there would be helpful (and I doubt it's worth more than adding a comment); but I did not understand at all, Liam's suggestion for the comment "to state that the count may not change".
If it was in-fact the intended behavior, then I agree we should keep it lenient. It would mean though, that munmap() being able to free a VMA if a split is required (by permitting exceeding the limit by 1) would not work in the case where we have already exceeded the limit. I find this to be inconsistent but this is also the current behavior ...
You're saying that once we go one over the limit, say with a new mmap, an munmap check makes it impossible to munmap that or any other vma?
If that's so, I do agree with you, that's nasty, and I would hate any new code to behave that way. In code that's survived as long as this without troubling anyone, I'm not so sure: but if it's easily fixed (a more lenient check at the munmap end?) that would seem worthwhile.
Ah, but reading again, you say "if a split is required": I guess munmapping the whole vma has no problem; and it's fine for a middle munmap, splitting into three before munmapping the middle, to fail. I suppose it would be nicer if munmaping start or end succeeeded, but I don't think that matters very much in this case.
I will drop this patch and the patch that introduces the vma_count_remaining() helper, as I see your point about it potentially being unnecessary overhead.
Regarding your feedback on the rest of the series, I believe the 3 remaining patches are still valuable on their own.
- The selftest adds a comprehensive tests for VMA operations at the
sysctl_max_map_count limit. This will self-document the exact behavior expected, including the leniency for potential merges that you highlighted, preventing the kind of misunderstanding that led to my initial patch.
- The rename of mm_struct->map_count to vma_count, is a
straightforward cleanup for code clarity that makes the purpose of the field more explicit.
- The tracepoint adds needed observability for telemetry, allowing us
to see when processes are failing in the field due to VMA count limit.
The selftest, is what makes up a large portion of the diff you sited, and with vma_count_remaining() gone the series will not touch nearly as many files.
Would this be an acceptable path forward?
Possibly, if others like it: my concern was to end a misunderstanding (I'm generally much too slow to get involved in cleanups).
Though given that the sysctl is named "max_map_count", I'm not very keen on renaming everything else from map_count to vma_count (and of course I'm not suggesting to rename the sysctl).
Hugh