As per documentation, pin_user_pages_fast() & get_user_pages_fast()
will return 0, if nr_pages <= 0. But this can be figure out only after
going inside the internal_get_user_pages_fast().
This can be handled early. Adding a check for the same.
Signed-off-by: Souptick Joarder <[email protected]>
---
mm/gup.c | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/mm/gup.c b/mm/gup.c
index 50681f0..a13aaa6 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
*/
if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
return -EINVAL;
+ if (nr_pages <= 0)
+ return 0;
/*
* The caller may or may not have explicitly set FOLL_GET; either way is
@@ -2854,6 +2856,8 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
/* FOLL_GET and FOLL_PIN are mutually exclusive. */
if (WARN_ON_ONCE(gup_flags & FOLL_GET))
return -EINVAL;
+ if (nr_pages <= 0)
+ return 0;
gup_flags |= FOLL_PIN;
return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
--
1.9.1
On Fri, May 01, 2020 at 01:41:58AM +0530, Souptick Joarder wrote:
> As per documentation, pin_user_pages_fast() & get_user_pages_fast()
> will return 0, if nr_pages <= 0. But this can be figure out only after
> going inside the internal_get_user_pages_fast().
Why is nr_pages not unsigned? I seem to have convinced myself before that
there was a good reason for it but really what is the point of calling either
of these functions with nr_pages not > 0?
>
> This can be handled early. Adding a check for the same.
>
> Signed-off-by: Souptick Joarder <[email protected]>
> ---
> mm/gup.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 50681f0..a13aaa6 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> */
> if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> return -EINVAL;
> + if (nr_pages <= 0)
> + return 0;
I think the documentation may be wrong here... Is there a caller who expects a
return of 0 for this behavior?
It seems like these should be a warn on and return -EINVAL. I just don't see
the use case here.
Ira
>
> /*
> * The caller may or may not have explicitly set FOLL_GET; either way is
> @@ -2854,6 +2856,8 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
> /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> return -EINVAL;
> + if (nr_pages <= 0)
> + return 0;
>
> gup_flags |= FOLL_PIN;
> return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
> --
> 1.9.1
>
>
On Fri, 1 May 2020 01:41:58 +0530 Souptick Joarder <[email protected]> wrote:
> As per documentation, pin_user_pages_fast() & get_user_pages_fast()
> will return 0, if nr_pages <= 0. But this can be figure out only after
> going inside the internal_get_user_pages_fast().
>
> This can be handled early. Adding a check for the same.
Calling get_user_pages() for (nrpages <= 0) is a nonsensical thing to
do so it isn't a thing we should care to optimize. Adding new checks
will actually slow down the use cases which we *do* care about. And it
adds more code.
On Fri, May 1, 2020 at 2:54 AM Ira Weiny <[email protected]> wrote:
>
> On Fri, May 01, 2020 at 01:41:58AM +0530, Souptick Joarder wrote:
> > As per documentation, pin_user_pages_fast() & get_user_pages_fast()
> > will return 0, if nr_pages <= 0. But this can be figure out only after
> > going inside the internal_get_user_pages_fast().
>
> Why is nr_pages not unsigned?
Not sure of why but it can be unsigned.
>
> >
> > This can be handled early. Adding a check for the same.
> >
> > Signed-off-by: Souptick Joarder <[email protected]>
> > ---
> > mm/gup.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 50681f0..a13aaa6 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> > */
> > if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > return -EINVAL;
> > + if (nr_pages <= 0)
> > + return 0;
>
> I think the documentation may be wrong here... Is there a caller who expects a
> return of 0 for this behavior?
>
My understanding is -
{get/pin}_user_pages_fast() -> internal_get_user_pages_fast()
Inside internal_get_user_pages_fast()
...
start = untagged_addr(start) & PAGE_MASK;
addr = start;
len = (unsigned long) nr_pages << PAGE_SHIFT;
end = start + len;
if (end <= start)
return 0;
...
This is the only place where {get/pin}_user_pages_fast() returns 0 which
indirectly checks for nr_pages <= 0. For each call to
{get/pin}_user_pages_fast()
we end up checking *nr_pages <= 0* anyway.
There are some instances where caller of these APIs handles return
value 0 as well. Example -
arch/ia64/kernel/err_inject.c#L145
arch/powerpc/kvm/book3s_64_mmu_hv.c#L582
arch/powerpc/kvm/book3s_64_mmu_hv.c#L1174
drivers/staging/gasket/gasket_page_table.c#L489
drivers/rapidio/devices/rio_mport_cdev.c#L865
drivers/platform/goldfish/goldfish_pipe.c#L277
drivers/gpu/drm/via/via_dmablit.c#L242
(some of the error handling looks incorrect)
If we end up checking *nr_pages <= 0* inside internal_get_user_pages_fast(),
then why not at first place like this patch does ?
2nd opinion is, inside internal_get_user_pages_fast(), for *nr_pages
<= 0* better
to return -ERRNO rather than 0 ?
On 2020-04-30 13:11, Souptick Joarder wrote:
> As per documentation, pin_user_pages_fast() & get_user_pages_fast()
> will return 0, if nr_pages <= 0. But this can be figure out only after
> going inside the internal_get_user_pages_fast().
>
> This can be handled early. Adding a check for the same.
Please, no. For precisely the reasons that Andrew gave: you are
attempting to optimize for a case that doesn't matter other than
for error handling, and which is already handled adequately. And
as he also pointed out, it very slightly UN-optimizes the path that
we *do* care about. So why are you still advocating for this?
If you want to change the gup/pup API so that asking for zero pages
means -EINVAL, then fine, go for it. That's a large thing, and a
tree-wide audit, but if you feel it's worth pursuing then it's at
least consistent.
But this patch here needs to be abandoned.
thanks,
--
John Hubbard
NVIDIA
>
> Signed-off-by: Souptick Joarder <[email protected]>
> ---
> mm/gup.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 50681f0..a13aaa6 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> */
> if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> return -EINVAL;
> + if (nr_pages <= 0)
> + return 0;
>
> /*
> * The caller may or may not have explicitly set FOLL_GET; either way is
> @@ -2854,6 +2856,8 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
> /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> return -EINVAL;
> + if (nr_pages <= 0)
> + return 0;
>
> gup_flags |= FOLL_PIN;
> return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
>
On Sat, May 2, 2020 at 3:03 AM John Hubbard <[email protected]> wrote:
>
> On 2020-04-30 13:11, Souptick Joarder wrote:
> > As per documentation, pin_user_pages_fast() & get_user_pages_fast()
> > will return 0, if nr_pages <= 0. But this can be figure out only after
> > going inside the internal_get_user_pages_fast().
> >
> > This can be handled early. Adding a check for the same.
>
>
> Please, no. For precisely the reasons that Andrew gave: you are
> attempting to optimize for a case that doesn't matter other than
> for error handling, and which is already handled adequately. And
> as he also pointed out, it very slightly UN-optimizes the path that
> we *do* care about. So why are you still advocating for this?
>
> If you want to change the gup/pup API so that asking for zero pages
> means -EINVAL, then fine, go for it. That's a large thing, and a
> tree-wide audit, but if you feel it's worth pursuing then it's at
> least consistent.
>
> But this patch here needs to be abandoned.
Sure, will drop this patch.
>
>
> thanks,
> --
> John Hubbard
> NVIDIA
>
> >
> > Signed-off-by: Souptick Joarder <[email protected]>
> > ---
> > mm/gup.c | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 50681f0..a13aaa6 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -2817,6 +2817,8 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
> > */
> > if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > return -EINVAL;
> > + if (nr_pages <= 0)
> > + return 0;
> >
> > /*
> > * The caller may or may not have explicitly set FOLL_GET; either way is
> > @@ -2854,6 +2856,8 @@ int pin_user_pages_fast(unsigned long start, int nr_pages,
> > /* FOLL_GET and FOLL_PIN are mutually exclusive. */
> > if (WARN_ON_ONCE(gup_flags & FOLL_GET))
> > return -EINVAL;
> > + if (nr_pages <= 0)
> > + return 0;
> >
> > gup_flags |= FOLL_PIN;
> > return internal_get_user_pages_fast(start, nr_pages, gup_flags, pages);
> >
>