linux-kernel - Re: [RFC PATCH resend 3/6] mm: Add refcount for preserving mm

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Date:   Fri, 16 Oct 2020 20:21:53 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Eric Biederman <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH resend 3/6] mm: Add refcount for preserving mm_struct
 without pgd

On Sat, Oct 17, 2020 at 01:09:12AM +0200, Jann Horn wrote:
> Currently, mm_struct has two refcounts:
> 
>  - mm_users: preserves everything - the mm_struct, the page tables, the
>    memory mappings, and so on
>  - mm_count: preserves the mm_struct and pgd
> 
> However, there are three types of users of mm_struct:
> 
> 1. users that want page tables, memory mappings and so on
> 2. users that want to preserve the pgd (for lazy TLB)
> 3. users that just want to keep the mm_struct itself around (e.g. for
>    mmget_not_zero() or __ptrace_may_access())
> 
> Dropping mm_count references can be messy because dropping mm_count to
> zero deletes the pgd, which takes the pgd_lock on x86, meaning it doesn't
> work from RCU callbacks (which run in IRQ context). In those cases,
> mmdrop_async() must be used to punt the invocation of __mmdrop() to
> workqueue context.
> 
> That's fine when mmdrop_async() is a rare case, but the preceding patch
> "ptrace: Keep mm around after exit_mm() for __ptrace_may_access()" makes it
> the common case; we should probably avoid punting freeing to workqueue
> context all the time if we can avoid it?
> 
> To resolve this, add a third refcount that just protects the mm_struct and
> the user_ns it points to, and which can be dropped with synchronous freeing
> from (almost) any context.
> 
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
>  arch/x86/kernel/tboot.c    |  2 ++
>  drivers/firmware/efi/efi.c |  2 ++
>  include/linux/mm_types.h   | 13 +++++++++++--
>  include/linux/sched/mm.h   | 13 +++++++++++++
>  kernel/fork.c              | 14 ++++++++++----
>  mm/init-mm.c               |  2 ++
>  6 files changed, 40 insertions(+), 6 deletions(-)

I think mmu notifiers and the stuff in drivers/infiniband/core/ can be
converted to this as well..

Actually I kind of wonder if you should go the reverse and find the
few callers that care about the pgd and give them a new api with a
better name (mmget_pgd?).

Jason