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

Yann Droneaud
2014-12-11T12:39:50Z

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