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 <haren@xxxxxxxxxxxxx>
>>>
>>> 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
