On Wed, Feb 19, 2025 at 12:56:31PM -0800, Kalesh Singh wrote:
We also can't change smaps in the way you want, it _has_ to still give output per VMA information.
Sorry I wasn't suggesting to change the entries in smaps, rather agreeing to your marker suggestion. Maybe a set of ranges for each smaps entry that has guards? It doesn't solve the use case, but does make these regions visible to userspace.
No, you are not providing a usecase for this. /proc/$pid/pagemaps does not contaminate the smaps output, mess with efforts to make it RCU readable, require updating the ioctl interface, etc. so it is clearly the better choice.
The proposed change that would be there would be a flag or something indicating that the VMA has guard regions _SOMEWHERE_ in it.
Since this doesn't solve your problem, adds complexity, and nobody else seems to need it, I would suggest this is not worthwhile and I'd rather not do this.
Therefore for your needs there are literally only two choices here:
- Add a bit to /proc/$pid/pagemap OR
- a new interface.
I am not in favour of a new interface here, if we can just extend pagemap.
What you'd have to do is:
- Find virtual ranges via /proc/$pid/maps
- iterate through /proc/$pid/pagemaps to retrieve state for all ranges.
Could we also consider an smaps field like:
VmGuards: [AAA, BBB), [CCC, DDD), ...
or something of that sort?
No, absolutely, categorically not. You realise these could be thousands of characters long right?
/proc/$pid/pagemaps resolves this without contaminating this output.
Well I'm glad that you guys find it useful for _something_ ;)
Again this wasn't written only for you (it is broadly a good feature for upstream), but I did have your use case in mind, so I'm a little disappointed that it doesn't help, as I like to solve problems.
But I'm glad it solves at least some for you...
I recall Liam had a proposal to store the guard ranges in the maple tree?
I wonder if that can be used in combination with this approach to have a better representation of this?
This was an alternative proposal made prior to the feature being implemented (and you and others at Google were welcome to comment and many were cc'd, etc.).
There is no 'in combination with'. This feature would take weeks/months to implement, fundamentally impact the maple tree VMA implementation and... not actually achieve anything + immediately be redundant.
Plus it'd likely be slower, have locking implications, would have kernel memory allocation implications, a lot more complexity and probably other problems besides (we discussed this at length at the time and a number of issues came up, I can't recall all of them).
To be crystal clear - we are empathically NOT changing /proc/$pid/maps to lie about VMAs regardless of underlying implementation, nor adding thousands of characters to /proc/$pid/smaps entries.
This is independent of implementation and would have been the case had we gone with a maple node version.
So in no world is your problem solved here, unfortunately you have inextricably tied yourself to a VMA representation here.
I still wonder if you could find some means of abstracting this, but /proc/$pid/pagemaps is where I am likely to expose this information for anybody who needs it, and will likely send a series for this relatively soon.
If you _can_ abstract this in some way, then if we provide this information _anywhere_ you can get it.
As I said to you earlier, the _best_ we could do in smaps would be to add a flag like 'Grd' or something to indicate some part of the VMA is guarded. But I won't do that unless somebody has an -actual use case- for it.
David's /proc/$pid/pagemaps suggestion is excellent, avoids all the pitfalls, exposes guard regions to anybody who really really wants to know and doesn't interfere with anything else, so this is what we'll go with.
Regards, Lorenzo