Subject: Re: [PATCH v4 04/16] powerpc/vas: Create take/drop pid and mm references

From: Nicholas Piggin <[email-masked]>
Date: Sat, 05 Jun 2021 13:03:24 +1000

Excerpts from Nicholas Piggin's message of June 5, 2021 10:31 am:
> Excerpts from Haren Myneni's message of June 4, 2021 2:08 pm:
>> On Thu, 2021-06-03 at 14:21 +1000, Nicholas Piggin wrote:
>>> Excerpts from Haren Myneni's message of May 21, 2021 7:31 pm:
>>> > Take pid and mm references when each window opens and drops during
>>> > close. This functionality is needed for powerNV and pseries. So
>>> > this patch defines the existing code as functions in common book3s
>>> > platform vas-api.c
>>> > 
>>> > Signed-off-by: Haren Myneni <[email-masked]>
>>> 
>>> Seems like a good idea to put these into their own helper functions.
>>> 
>>> > ---
>>> >  arch/powerpc/include/asm/vas.h              | 25 +++++++++
>>> >  arch/powerpc/platforms/book3s/vas-api.c     | 51
>>> > ++++++++++++++++++
>>> >  arch/powerpc/platforms/powernv/vas-fault.c  | 10 ++--
>>> >  arch/powerpc/platforms/powernv/vas-window.c | 57 ++---------------
>>> > ----
>>> >  arch/powerpc/platforms/powernv/vas.h        |  6 +--
>>> >  5 files changed, 88 insertions(+), 61 deletions(-)
>>> > 
>>> > diff --git a/arch/powerpc/include/asm/vas.h
>>> > b/arch/powerpc/include/asm/vas.h
>>> > index 668303198772..3f2b02461a76 100644
>>> > --- a/arch/powerpc/include/asm/vas.h
>>> > +++ b/arch/powerpc/include/asm/vas.h
>>> > @@ -5,6 +5,9 @@
>>> >  
>>> >  #ifndef _ASM_POWERPC_VAS_H
>>> >  #define _ASM_POWERPC_VAS_H
>>> > +#include <linux/sched/mm.h>
>>> > +#include <linux/mmu_context.h>
>>> > +#include <asm/icswx.h>
>>> >  #include <uapi/asm/vas-api.h>
>>> >  
>>> >  struct vas_window;
>>> > @@ -49,6 +52,17 @@ enum vas_cop_type {
>>> >  	VAS_COP_TYPE_MAX,
>>> >  };
>>> >  
>>> > +/*
>>> > + * User space VAS windows are opened by tasks and take references
>>> > + * to pid and mm until windows are closed.
>>> > + * Stores pid, mm, and tgid for each window.
>>> > + */
>>> > +struct vas_user_win_ref {
>>> > +	struct pid *pid;	/* PID of owner */
>>> > +	struct pid *tgid;	/* Thread group ID of owner */
>>> > +	struct mm_struct *mm;	/* Linux process mm_struct */
>>> > +};
>>> > +
>>> >  /*
>>> >   * User space window operations used for powernv and powerVM
>>> >   */
>>> > @@ -59,6 +73,16 @@ struct vas_user_win_ops {
>>> >  	int (*close_win)(void *);
>>> >  };
>>> >  
>>> > +static inline void vas_drop_reference_pid_mm(struct
>>> > vas_user_win_ref *ref)
>>> > +{
>>> > +	/* Drop references to pid and mm */
>>> > +	put_pid(ref->pid);
>>> > +	if (ref->mm) {
>>> > +		mm_context_remove_vas_window(ref->mm);
>>> > +		mmdrop(ref->mm);
>>> > +	}
>>> > +}
>>> 
>>> You don't have to make up a new name for such a thing because you 
>>> already have one
>>> 
>>> put_vas_user_win_ref(struct vas_user_win_ref *ref)
>>> 
>>> 
>>> > +
>>> >  /*
>>> >   * Receive window attributes specified by the (in-kernel) owner of
>>> > window.
>>> >   */
>>> > @@ -192,4 +216,5 @@ int vas_register_coproc_api(struct module *mod,
>>> > enum vas_cop_type cop_type,
>>> >  			    struct vas_user_win_ops *vops);
>>> >  void vas_unregister_coproc_api(void);
>>> >  
>>> > +int vas_reference_pid_mm(struct vas_user_win_ref *task_ref);
>>> >  #endif /* __ASM_POWERPC_VAS_H */
>>> > diff --git a/arch/powerpc/platforms/book3s/vas-api.c
>>> > b/arch/powerpc/platforms/book3s/vas-api.c
>>> > index 6c39320bfb9b..a0141bfb2e4b 100644
>>> > --- a/arch/powerpc/platforms/book3s/vas-api.c
>>> > +++ b/arch/powerpc/platforms/book3s/vas-api.c
>>> > @@ -55,6 +55,57 @@ static char *coproc_devnode(struct device *dev,
>>> > umode_t *mode)
>>> >  	return kasprintf(GFP_KERNEL, "crypto/%s", dev_name(dev));
>>> >  }
>>> >  
>>> > +/*
>>> > + * Take reference to pid and mm
>>> > + */
>>> > +int vas_reference_pid_mm(struct vas_user_win_ref *task_ref)
>>> > +{
>>> 
>>> So this is quite different from a typical refcount object in that
>>> it's 
>>> opening it for access as well. I would split it in two functions, one
>>> matching put_vas_user_win_ref() and appearing in the same place in
>>> code,
>>> which is up to about mmput and another function that adds the window
>>> and
>>> does the CP_ABORT etc... hmm, where do you release tgid?
>> 
>> Basically copied the existing code in to these functions
>> (vas_reference_pid_mm/vas_drop_reference_pid_mm) so that useful for
>> both platforms. 
>> 
>> mm_context_add/remove_vas_window() is also like taking reference. So
>> instead of adding 2 seperate functions, how about naming
>> get/put_vas_user_win_ref() 
> 
> It's actually different though. What I'm asking is the parts where you 
> interact with core kernel data structure refcounts go into their own 
> get/put functions.
> 
> Someone who understands that refcounting and looks at the code will care 
> about those bits, so having them all together I think is helpful. They 
> don't know about adding vas windows or CP_ABORT.
> 
>> Regarding tgid, the reference is taking only with pid, but not tgid.
>> pid reuse can happen only in the case of multithread applications when
>> the child that opened VAS window exits. But these windows will be
>> closed when tgid exists. So do not need tgid reference.
> 
> I don't understand you.  The code you added does take a reference to 
> tgid...
> 
>>> > +	/*
>>> > +	 * Window opened by a child thread may not be closed when
>>> > +	 * it exits. So take reference to its pid and release it
>>> > +	 * when the window is free by parent thread.
>>> > +	 * Acquire a reference to the task's pid to make sure
>>> > +	 * pid will not be re-used - needed only for multithread
>>> > +	 * applications.
>>> > +	 */
>>> > +	task_ref->pid = get_task_pid(current, PIDTYPE_PID);

Quoted the wrong bit obviously:

+       /*
+        * Process closes window during exit. In the case of
+        * multithread application, the child thread can open
+        * window and can exit without closing it. Expects parent
+        * thread to use and close the window. So do not need
+        * to take pid reference for parent thread.
+        */
+       task_ref->tgid = find_get_pid(task_tgid_vnr(current));

aka vas_tx_win_open() in upstream code.

It's not a comment about this patch specificlaly, I just noticed it and 
I wanted to make sure I'm not missing somehting or the existing code 
isn't buggy before the patch goes in.

Thanks,
Nick