On 20.02.25 09:51, Lorenzo Stoakes wrote:
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/$pid/pagemaps does not
contaminate the smaps output, mess with efforts to make it RCU readable,
require updating the ioctl interface, etc. so it is clearly the better
choice.
The proposed change that would be there would be a flag or something
indicating that the VMA has guard regions _SOMEWHERE_ in it.
Since this doesn't solve your problem, adds complexity, and nobody else
seems to need it, I would suggest this is not worthwhile and I'd rather not
do this.
Therefore for your needs there are literally only two choices here:
1. Add a bit to /proc/$pid/pagemap OR
2. 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:
1. Find virtual ranges via /proc/$pid/maps
2. iterate through /proc/$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.
Yes. Calling it a "guard region" might be part of the problem
(/"misunderstanding"), because it reminds people of "virtual memory
regions".
"Guard markers" or similar might have been clearer that these operate on
individual PTEs, require page table scanning etc ... which makes them a
lot more scalable and fine-grained and provides all these benfits, with
the downside being that we don't end up with that many "virtual memory
regions" that maps/smaps operate on.
[...]
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.
Right, and that would limit where you have to manually scan. Something
similar is being done with uffd-wp markers IIRC.
--
Cheers,
David / dhildenb
