Re: [RFC 00/14] Dynamic Kernel Stacks

[H. Peter Anvin]

On March 14, 2024 8:13:56 PM PDT, Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:
>On Thu, Mar 14, 2024 at 3:57 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>>
>> On Thu, Mar 14, 2024 at 03:53:39PM -0400, Kent Overstreet wrote:
>> > On Thu, Mar 14, 2024 at 07:43:06PM +0000, Matthew Wilcox wrote:
>> > > On Tue, Mar 12, 2024 at 10:18:10AM -0700, H. Peter Anvin wrote:
>> > > > Second, non-dynamic kernel memory is one of the core design decisions in
>> > > > Linux from early on. This means there are lot of deeply embedded assumptions
>> > > > which would have to be untangled.
>> > >
>> > > I think there are other ways of getting the benefit that Pasha is seeking
>> > > without moving to dynamically allocated kernel memory.  One icky thing
>> > > that XFS does is punt work over to a kernel thread in order to use more
>> > > stack!  That breaks a number of things including lockdep (because the
>> > > kernel thread doesn't own the lock, the thread waiting for the kernel
>> > > thread owns the lock).
>> > >
>> > > If we had segmented stacks, XFS could say "I need at least 6kB of stack",
>> > > and if less than that was available, we could allocate a temporary
>> > > stack and switch to it.  I suspect Google would also be able to use this
>> > > API for their rare cases when they need more than 8kB of kernel stack.
>> > > Who knows, we might all be able to use such a thing.
>> > >
>> > > I'd been thinking about this from the point of view of allocating more
>> > > stack elsewhere in kernel space, but combining what Pasha has done here
>> > > with this idea might lead to a hybrid approach that works better; allocate
>> > > 32kB of vmap space per kernel thread, put 12kB of memory at the top of it,
>> > > rely on people using this "I need more stack" API correctly, and free the
>> > > excess pages on return to userspace.  No complicated "switch stacks" API
>> > > needed, just an "ensure we have at least N bytes of stack remaining" API.
>
>I like this approach! I think we could also consider having permanent
>big stacks for some kernel only threads like kvm-vcpu. A cooperative
>stack increase framework could work well and wouldn't negatively
>impact the performance of context switching. However, thorough
>analysis would be necessary to proactively identify potential stack
>overflow situations.
>
>> > Why would we need an "I need more stack" API? Pasha's approach seems
>> > like everything we need for what you're talking about.
>>
>> Because double faults are hard, possibly impossible, and the FRED approach
>> Peter described has extra overhead?  This was all described up-thread.
>
>Handling faults in #DF is possible. It requires code inspection to
>handle race conditions such as what was shown by tglx. However, as
>Andy pointed out, this is not supported by SDM as it is an abort
>context (yet we return from it because of ESPFIX64, so return is
>possible).
>
>My question, however, if we ignore memory savings and only consider
>reliability aspect of this feature.  What is better unconditionally
>crashing the machine because a guard page was reached, or printing a
>huge warning with a backtracing information about the offending stack,
>handling the fault, and survive? I know that historically Linus
>preferred WARN() to BUG() [1]. But, this is a somewhat different
>scenario compared to simple BUG vs WARN.
>
>Pasha
>
>[1] https://lore.kernel.org/all/Pine.LNX.4.44.0209091832160.1714-100000@xxxxxxxxxxxxxxxxxx
>

The real issue with using #DF is that if the event that caused it was asynchronous, you could lose the event.