On Thu, Apr 05, 2018 at 11:43:27AM -0700, Linus Torvalds wrote:
> On Thu, Apr 5, 2018 at 11:28 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >
> > to repeat what you are saying IIUC __get_user_pages_fast returns 0 if it can't
> > pin any pages and that is by design. Returning 0 on error isn't usual I think
> > so I guess this behaviour should we well documented.
>
> Arguably it happens elsewhere too, and not just in the kernel.
> "read()" at past the end of a file is not an error, you'll just get 0
> for EOF.
>
> So it's not really "returning 0 on error".
>
> It really is simply returning the number of pages it got. End of
> story. That number of pages can be smaller than the requested number
> of pages, and _that_ is due to some error, but note how it can return
> "5" on error too - you asked for 10 pages, but the error happened in
> the middle!
>
> So the right way to check for error is to bverify that you get the
> number of pages that you asked for. If you don't, something bad
> happened.
>
> Of course, many users don't actually care about "I didn't get
> everything". They only care about "did I get _something_". Then that 0
> ends up being the error case, but note how it depends on the caller.
>
> > What about get_user_pages_fast though?
>
> We do seem to special-case the first page there. I'm not sure it's a
> good idea. But like the __get_user_pages_fast(), we seem to have users
> that know about the particular semantics and depend on it.
>
> It's all ugly, I agree.
>
> End result: we can't just change semantics of either of them.
>
> At least not without going through every single user and checking that
> they are ok with it.
>
> Which I guess I could be ok with. Maybe changing the semantics of
> __get_user_pages_fast() is acceptable, if you just change it
> *everywhere* (which includes not just he users, but also the couple of
> architecture-specific versions of that same function that we have.
>
> Linus
OK I hope I understood what you are saying here.
At least drivers/gpu/drm/i915/i915_gem_userptr.c seems to
get it wrong:
pinned = __get_user_pages_fast(obj->userptr.ptr,
if (pinned < 0) {
pages = ERR_PTR(pinned);
pinned = 0;
} else if (pinned < num_pages) {
pages = __i915_gem_userptr_get_pages_schedule(obj);
active = pages == ERR_PTR(-EAGAIN);
} else {
pages = __i915_gem_userptr_alloc_pages(obj, pvec, num_pages);
active = !IS_ERR(pages);
}
The <0 path is never taken.
Cc maintainers - should that driver be changed to use
get_user_pages_fast?