linux-kernel - Re: [PATCH] percpu: Sync vmalloc mappings in pcpu_alloc() and free

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Date:   Mon, 4 May 2020 15:10:06 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Shile Zhang <shile.zhang@...ux.alibaba.com>,
        Andy Lutomirski <luto@...capital.net>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tzvetomir Stoyanov <tz.stoyanov@...il.com>
Subject: Re: [PATCH] percpu: Sync vmalloc mappings in pcpu_alloc() and
 free_percpu()

On Mon, 4 May 2020 20:38:32 +0200
Joerg Roedel <jroedel@...e.de> 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