linux-kernel - Re: [PATCH] misc: fastrpc: copy to user only for non-DMA-BUF heap buffers

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Date:   Thu, 23 Sep 2021 19:43:26 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     jeyr@...eaurora.org
Cc:     linux-arm-msm@...r.kernel.org, srinivas.kandagatla@...aro.org,
        linux-kernel@...r.kernel.org, fastrpc.upstream@....qualcomm.com
Subject: Re: [PATCH] misc: fastrpc: copy to user only for non-DMA-BUF heap
 buffers

On Thu, Sep 23, 2021 at 03:54:15PM +0530, jeyr@...eaurora.org wrote:
> On 2021-09-23 14:19, Greg KH wrote:
> > On Thu, Sep 23, 2021 at 02:07:52PM +0530, Jeya R wrote:
> > > fastrpc_put_args is copying all the output buffers to user. For large
> > > number of output context buffers, this might cause performance
> > > degradation. Copying is not needed for DMA-BUF heap buffers.
> > 
> > What does "performance degradation" really mean?
> 
> Unnecessary copying for large number of buffers would cause some
> additional time which would get added to overall fastrpc call cost.
> 
> > 
> > > 
> > > Signed-off-by: Jeya R <jeyr@...eaurora.org>
> > > ---
> > >  drivers/misc/fastrpc.c | 18 ++++++++++--------
> > >  1 file changed, 10 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> > > index beda610..536eabf 100644
> > > --- a/drivers/misc/fastrpc.c
> > > +++ b/drivers/misc/fastrpc.c
> > > @@ -890,15 +890,17 @@ static int fastrpc_put_args(struct
> > > fastrpc_invoke_ctx *ctx,
> > >  	inbufs = REMOTE_SCALARS_INBUFS(ctx->sc);
> > > 
> > >  	for (i = inbufs; i < ctx->nbufs; ++i) {
> > > -		void *src = (void *)(uintptr_t)rpra[i].pv;
> > > -		void *dst = (void *)(uintptr_t)ctx->args[i].ptr;
> > > -		u64 len = rpra[i].len;
> > > +		if (!ctx->maps[i]) {
> > > +			void *src = (void *)(uintptr_t)rpra[i].pv;
> > > +			void *dst = (void *)(uintptr_t)ctx->args[i].ptr;
> > 
> > uintptr_t is not a kernel variable type.  Please use the real kernel
> > type for this as you are touching these lines.
> > 
> 
> Sure, thanks for pointing this. Will update this in the next patch.
> 
> > > +			u64 len = rpra[i].len;
> > > 
> > > -		if (!kernel) {
> > > -			if (copy_to_user((void __user *)dst, src, len))
> > > -				return -EFAULT;
> > > -		} else {
> > > -			memcpy(dst, src, len);
> > > +			if (!kernel) {
> > > +				if (copy_to_user((void __user *)dst, src, len))
> > > +					return -EFAULT;
> > > +			} else {
> > > +				memcpy(dst, src, len);
> > > +			}
> > 
> > So you were copying buffers that didn't need to be copied?  So you are
> > now doing less work?  Or is this fixing a bug where you were copying
> > things that you should not have been copying?
> > 
> > What commit does this fix?  Does this need to go to the stable kernel
> > trees?
> > 
> 
> Yes, not all buffer needs to be copied. This change would avoid unnecessary
> copying of buffers. Not adding fix tag as it's not exactly fixing any bug.
> This should go to stable kernel trees.

If it's not fixing a bug, why should it go into the stable trees?