Re: [PATCH] percpu: Sync vmalloc mappings in pcpu_alloc() and free_percpu()

[Steven Rostedt]

On Mon, 4 May 2020 20:38:32 +0200
Joerg Roedel <jroedel@xxxxxxx> wrote:

> Fair point, but this just shows how problematic it is to call something
> like vmalloc_sync_mappings() from a lower-level kernel API function.
> The obvious fix for this would be to make pgd_lock irq-safe, but this is
> getting more and more ridiculous.

Yeah, it's just getting more deeper into the rabbit hole.

> 
> I know you don't like to have a vmalloc_sync_mappings() call in the
> tracing code, but can you live with it until we get rid of this broken
> interface?

I'm fine with adding it to the tracing code (with that ridiculous
comment! ;-)

I'll even tag is as stable, but again, it's uncertain what commit that it
"fixes".

> 
> My plan for this is to use a small bitmap to track in the vmalloc and
> the (x86-)ioremap code at which levels of the page-tables the code made
> changes and combine that with an architecture-dependend mask to decide
> whether anything needs to be synced.
> 
> On x86-64 the sync would be necessary at most 64 times after boot, so I
> think this will only have a very small performance impact, even with
> VMAP_STACKS. And as a bonus it would also get rid of vmalloc faulting on
> x86, fixing the issue with tracing too.

Just because tracing get's a workaround, let's hopefully not get distracted
and forgot to add this. It looks like the best solution is to not need the
vmalloc_sync_mappings() call in the future. But, as we are about to release
trace-cmd with a feature that reliably triggers the bug, I can't wait for
the proper fix.

-- Steve