On Thu, Feb 20, 2025 at 01:44:20PM +0100, David Hildenbrand wrote:
On 20.02.25 11:15, Lorenzo Stoakes wrote:
On Thu, Feb 20, 2025 at 11:03:02AM +0100, David Hildenbrand wrote:
Your conclusion is 'did not participate with upstream'; I don't agree with that. But maybe you and Kalesh have a history on that that let's you react on his questions IMHO more emotionally than it should have been.
This is wholly unfair, I have been very reasonable in response to this thread. I have offered to find solutions, I have tried to understand the problem in spite of having gone to great lengths to try to discuss the limitations of the proposed approach in every venue I possibly could.
I go out of my way to deal professionally and objectively with what is presented. Nothing here is emotional. So I'd ask that you please abstain from making commentary like this which has no basis.
I appreciate everything you write below. But this request is just impossible. I will keep raising my opinion and misunderstandings will happen.
Well I wouldn't ask you not to express your opinion David, you know I respect and like you, and by all means push back hard or call out what you think is bad behaviour :)
I just meant to say, in my view, that there was no basis, but I appreciate miscommunications happen.
So apologies if I came off as being difficult or rude, it actually
wasn't
intended. And to re-emphasise - I have zero personal issue with anybody in this thread whatsoever!
It sounded to me like you were trying to defend your work (again, IMHO too emotionally, and I might have completely misinterpreted that) and slowly switching to "friendly fire" (towards me). Apologies from my side if I completely misunderstood/misinterpreted that.
Right this was not at all my intent, sorry if it seemed that way. I may well have communicated terribly, so apologies on my side too.
To recap: what we have upstream is great; you did a great job. Yes, the mechanism has its drawbacks, but that's just part of the design.
Thanks :)
Some people maybe have wrong expectations, maybe there were misunderstandings, or maybe there are requirements that only now pop up; it's sometimes unavoidable, and that's ok.
We can try to document it better (and I was trying to find clues why people might be mislead), and see if/how we could sort out these requirements. But we can likely not make it perfect in any possible way (I'm sure there are plenty of use cases where what we currently have is more than sufficient).
Sure and I"m very open to adding a documentation page for guard regions, in fact was considering this very thing recently. I already added man pages but be good to be able to go into more depth.
I just want to find the best way forward, technically and am willing to
do
whatever work is required to make the guard region implementation as good as it possibly can be.
Note that the whole "Honestly David you and the naming. .." thing could have been written as "I don't think it's a naming problem."
I feel like I _always_ get in trouble when I try to write in a 'tongue-in-cheek' style, which is what this was meant to be... so I think herein lies the basis of the miscommunication :)
I apologise, the household is ill, which maybe affects my judgment in how I write these, but in general text is a very poor medium. It was meant to be said in a jolly tone with a wink...
I think maybe I should learn my lesson with these things, I thought the ':p' would make this clear but yeah, text, poor medium.
Anyway apologies if this seemed disrespectful.
No worries, it's hard to really make me angry, and I appreciate your openness and your apology (well, and you and your work, obviously).
I'll note, though, if my memory serves me right, that nobody so far ever criticized the way I communicate upstream, or even told me to abstain from certain communication.
I wish I could say the same haha, so perhaps this was a problem on my side honestly. I do have a habit of being 'tongue in cheek' and failing to communicate that which I did say the last time I wouldn't repeat. It is not intended, I promise.
As the abstain, was more a British turn of phrase, meaning to say - I dispute the claim that this is an emotional thing and please don't say this if it isn't so.
But I understand that of course, you may have interpreted it as so, due to my having failed to communicate it well.
Again, I must say, text remains replete with possibilities for miscommunication, misunderstanding and it can so often be difficult to communicate one's intent.
But again of course, I apologise if I overstepped the line in any way!
That probably hurt most, now that a couple of hours passed. Nothing that a couple of beers and a bit of self-reflection on my communication style can't fix ;)
Ugh sorry, man. Not my intent. And it seems - I literally OWE YOU pints now. :) we will fix this at lsf...
Perhaps owe Kalesh some too should he be there... will budget accordingly... :P
[...]
Yeah that's a good point, but honestly if you're reading smaps that reads the page tables, then reading /proc/$pid/pagemaps and reading page tables TWICE that seems inefficient vs. just reading /proc/$pid/maps, then reading /proc/$pid/pagemaps and reading page tables once.
Right; I recently wished that we would have an interface to obtain more VMA flags without having to go through smaps
Well maybe that lends itself to the idea of adding a whole new interface in general...
An extended "maps" interface might be reasonable, that allows for exposing more things without walking the page tables. (e.g., flags)
Maybe one could have an indicator that says "ever had guard regions in this mapping" without actually walking the page tables.
Yeah this is something we've discussed before, but it's a little fraught. Let's say it was a VMA flag, in this case we'd have to make this flag 'sticky' and not impact merging (easy enough) to account for splits/merges.
The problem comes in that we would then need to acquire the VMA write
lock to do
it, something we don't currently require on application of guard regions.
Right, and we shouldn't write-lock the mmap. We'd need some way to just atomically set such an indicator on a VMA.
Hm yeah, could be tricky, we definitely can't manage a new field in vm_area_struct, this is a very sensitive subject at the moment really with Suren's work with VMAs allocated via SLAB_TYPESAFE_BY_RCU, putting the lock into the VMA and the alignment requirements.
Not sure what precedent we'd have with atomic setting of a VMA flag for this... could be tricky.
I'll also note that it might be helpful for smallish region, but especially for large ones (including when they are split and the indicator is wrong), it's less helpful. I don't have to tell you about the VMA merging implications, probably it would be like VM_SOFTDIRTY handling :)
Yeah indeed now we've simplified merging a lot of possibilities emerge, this is one!
We'd also have to make sure nothing else makes any assumptions about VMA flags implying differences in VMAs in this one instance (though we do already do this for VM_SOFTDIRTY).
I saw this as possibly something like VM_MAYBE_GUARD_REGIONS or something.
Yes.
-- Cheers,
David / dhildenb
Best, Lorenzo