Re: [PATCH v2 04/17] IB/core: Add umem function to read data from user-space

[Yann Droneaud <ydroneaud-RlY5vtjFyJ3QT0dZR+AlfA@public.gmane.org>]

Le jeudi 11 décembre 2014 à 13:09 +0200, Haggai Eran a écrit :
> On 10/12/2014 18:22, Yann Droneaud wrote:
> > Hi,
> > 
> > Le mardi 11 novembre 2014 à 18:36 +0200, Haggai Eran a écrit :
> >> In some drivers there's a need to read data from a user space area that
> >> was pinned using ib_umem, when running from a different process context.
> >>
> >> The ib_umem_copy_from function allows reading data from the physical pages
> >> pinned in the ib_umem struct.
> >>
> >> Signed-off-by: Haggai Eran <haggaie-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
> >> ---
> >>  drivers/infiniband/core/umem.c | 26 ++++++++++++++++++++++++++
> >>  include/rdma/ib_umem.h         |  2 ++
> >>  2 files changed, 28 insertions(+)
> >>
> >> diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
> >> index e0f883292374..77bec75963e7 100644
> >> --- a/drivers/infiniband/core/umem.c
> >> +++ b/drivers/infiniband/core/umem.c
> >> @@ -292,3 +292,29 @@ int ib_umem_page_count(struct ib_umem *umem)
> >>  	return n;
> >>  }
> >>  EXPORT_SYMBOL(ib_umem_page_count);
> >> +
> >> +/*
> >> + * Copy from the given ib_umem's pages to the given buffer.
> >> + *
> >> + * umem - the umem to copy from
> >> + * offset - offset to start copying from
> >> + * dst - destination buffer
> >> + * length - buffer length
> >> + *
> >> + * Returns the number of copied bytes, or an error code.
> >> + */
> >> +int ib_umem_copy_from(struct ib_umem *umem, size_t offset, void *dst,
> >> +		      size_t length)
> > 
> > I would prefer the arguments in the same order as ib_copy_from_udata()
> > 
> > int ib_umem_copy_from(void *dst,
> >                       struct ib_umem *umem, size_t umem_offset,
> >                       size_t length);
> 
> No problem.
> 
> >> +{
> >> +	size_t end = offset + length;
> >> +
> >> +	if (offset > umem->length || end > umem->length || end < offset) {
> >> +		pr_err("ib_umem_copy_from not in range. offset: %zd umem length: %zd end: %zd\n",
> >> +		       offset, umem->length, end);
> >> +		return -EINVAL;
> >> +	}
> >> +

I think the test could be rewritten as:

	if (offset > umem->length || length > umem->length - offset)

That's one operation less.


> >> +	return sg_pcopy_to_buffer(umem->sg_head.sgl, umem->nmap, dst, length,
> >> +			offset + ib_umem_offset(umem));
> >> +}
> >> +EXPORT_SYMBOL(ib_umem_copy_from);
> > 
> > As the function return a "int", no more than INT_MAX bytes (likely 2^31
> > - 1) can be copied. Perhaps changing the return type to to ssize_t would
> > be better (and a check to enfore ssize_t maximum value). Or change the
> > function could return 0 in case of success or a error code, just like
> > ib_copy_from_udata().
> > 
> 
> Okay. I'll change it to match ib_copy_from_udata. We're checking the
> umem size in the call site of this function anyway, and the only reason
> I see sg_pcopy_to_buffer would return less than *length* bytes is when
> reaching the end of the scatterlist.
> 

As the length is compared against umem->length (+ offset), this would 
mean umem->length is not "synchronized" with the length of the data 
described by the scatter/gather list ?


BTW, ib_copy_from_udata() is defined as an inline function. Would it be
better to have ib_umem_copy_from() being an inline function too ?
(In such case, I would remove the error message to not duplicate it
 across all modules using the function)

Regards.

-- 
Yann Droneaud
OPTEYA


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