Re: [RFC PATCH resend 3/6] mm: Add refcount for preserving mm_struct without pgd

[Jason Gunthorpe <jgg@ziepe.ca>]

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@google.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