On 20.02.24 12:26, Alexandru Elisei wrote:
Hello,
Hi!
This is a request to discuss alternatives to the current approach for
reusing the MTE tag storage memory for data allocations [1]. Each iteration
of the series uncovered new issues, the latest being that memory allocation
is being performed in atomic contexts [2]; I would like to start a
discussion regarding possible alternative, which would integrate better
with the memory management code.
This is a high level overview of the current approach:
* Tag storage pages are put on the MIGRATE_CMA lists, meaning they can be
used for data allocations like (almost) any other page in the system.
* When a page is allocated as tagged, the corresponding tag storage is
also allocated.
* There's a static relationship between a page and the location in memory
where its tags are stored. Because of this, if the corresponding tag
storage is used for data, the tag storage page is migrated.
Although this is the most generic approach because tag storage pages are
treated like normal pages, it has some disadvantages:
* HW KASAN (MTE in the kernel) cannot be used. The kernel allocates memory
in atomic context, where migration is not possible.
* Tag storage pages cannot be themselves tagged, and this means that all
CMA pages, even those which aren't tag storage, cannot be used for
tagged allocations.
* Page migration is costly, and a process that uses MTE can experience
measurable slowdowns if the tag storage it requires is in use for data.
There might be ways to reduce this cost (by reducing the likelihood that
tag storage pages are allocated), but it cannot be completely
eliminated.
* Worse yet, a userspace process can use a tag storage page in such a way
that migration is effectively impossible [3],[4]. A malicious process
can make use of this to prevent the allocation of tag storage for other
processes in the system, leading to a degraded experience for the
affected processes. Worst case scenario, progress becomes impossible for
those processes.
One alternative approach I'm looking at right now is cleancache. Cleancache
was removed in v5.17 (commit 0a4ee518185e) because the only backend, the
tmem driver, had been removed earlier (in v5.3, commit 814bbf49dcd0).
With this approach, MTE tag storage would be implemented as a driver
backend for cleancache. When a tag storage page is needed for storing tags,
the page would simply be dropped from the cache (cleancache_get_page()
returns -1).
With large folios in place, we'd likely want to investigate not working
on individual pages, but on (possibly large) folios instead.
I believe this is a very good fit for tag storage reuse, because it allows
tag storage to be allocated even in atomic contexts, which enables MTE in
the kernel. As a bonus, all of the changes to MM from the current approach
wouldn't be needed, as tag storage allocation can be handled entirely in
set_ptes_at(), copy_*highpage() or arch_swap_restore().
Is this a viable approach that would be upstreamable? Are there other
solutions that I haven't considered? I'm very much open to any alternatives
that would make tag storage reuse viable.
As raised recently, I had similar ideas with something like virtio-mem
in the past (wanted to call it virtio-tmem back then), but didn't have
time to look into it yet.
I considered both, using special device memory as "cleancache" backend,
and using it as backend storage for something similar to zswap. We would
not need a memmap/"struct page" for that special device memory, which
reduces memory overhead and makes "adding more memory" a more reliable
operation.
Using it as "cleancache" backend does make some things a lot easier.
The idea would be to provide a variable amount of additional memory to a
VM, that can be reclaimed easily and reliably on demand.
The details are a bit more involved, but in essence, imagine a special
physical memory region that is provided by a the hypervisor via a device
to the VM. A virtio device "owns" that region and the driver manages it,
based on requests from the hypervisor.
Similar to virtio-mem, there are ways for the hypervisor to request
changes to the memory consumption of a device (setting the requested
size). So when requested to consume less, clean pagecache pages can be
dropped and the memory can be handed back to the hypervisor.
Of course, likely we would want to consider using "slower" memory in the
hypervisor to back such a device.
I also thought about better integrating memory reclaim in the
hypervisor, similar to "MADV_FREE" semantic way. One idea I had was that
the memory provided by the device might have "special" semantics (as if
the memory is always marked MADV_FREE), whereby the hypervisor could
reclaim+discard any memory in that region any time, and the driver would
have ways to get notified about that, or detect that reclaim happened.
I learned that there are cases where data that is significantly larger
than main memory might be read repeatedly. As long as there is free
memory in the hypervisor, it could be used as a cache for clean
pagecache pages. In contrast to memory ballonning + virtio-mem, that
memory can be easily and reliably reclaimed. And reclaiming that memory
cannot really hurt the VM, it would only affect performance.
Long story short: what I had in mind would require similar hooks (again).
In contrast to tmem, with arm64 MTE we could get an actual supported
cleancache backend fairly easily. I recall that tmem was abandoned in
XEN and never really reached production quality.
--
Cheers,
David / dhildenb
