On 10/27/20 6:15 AM, Jason Gunthorpe wrote:
On Tue, Oct 27, 2020 at 10:33:01AM +0100, Jan Kara wrote:
On Fri 23-10-20 21:44:17, John Hubbard wrote:
On 10/23/20 5:19 PM, Jason Gunthorpe wrote:
+ start += (unsigned long)nr_pinned << PAGE_SHIFT;
+ pages += nr_pinned;
+ ret = __gup_longterm_unlocked(start, nr_pages - nr_pinned, gup_flags,
+ pages);
+ if (ret < 0) {
/* Have to be a bit careful with return values */
...and can we move that comment up one level, so that it reads:
/* Have to be a bit careful with return values */
if (ret < 0) {
if (nr_pinned)
return nr_pinned;
return ret;
}
return ret + nr_pinned;
Thinking about this longer term, it would be nice if the whole gup/pup API
set just stopped pretending that anyone cares about partial success, because
they *don't*. If we had return values of "0 or -ERRNO" throughout, and an
additional set of API wrappers that did some sort of limited retry just like
some of the callers do, that would be a happier story.
Actually there are callers that care about partial success. See e.g.
iov_iter_get_pages() usage in fs/direct_io.c:dio_refill_pages() or
bio_iov_iter_get_pages(). These places handle partial success just fine and
not allowing partial success from GUP could regress things...
I looked through a bunch of call sites, and there are a wack that
So did I! :)
actually do only want a complete return and are carrying a bunch of
code to fix it:
pvec = kvmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);
if (!pvec)
return -ENOMEM;
do {
unsigned num_pages = npages - pinned;
uint64_t ptr = userptr->ptr + pinned * PAGE_SIZE;
struct page **pages = pvec + pinned;
ret = pin_user_pages_fast(ptr, num_pages,
!userptr->ro ? FOLL_WRITE : 0, pages);
if (ret < 0) {
unpin_user_pages(pvec, pinned);
kvfree(pvec);
return ret;
}
pinned += ret;
} while (pinned < npages);
Is really a lot better if written as:
pvec = kvmalloc_array(npages, sizeof(struct page *), GFP_KERNEL);
if (!pvec)
return -ENOMEM;
ret = pin_user_pages_fast(userptr->ptr, npages, FOLL_COMPLETE |
(!userptr->ro ? FOLL_WRITE : 0),
pvec);
if (ret) {
kvfree(pvec);
return ret;
}
(eg FOLL_COMPLETE says to return exactly npages or fail)
Yes, exactly. And if I reverse the polarity (to Christoph's FOLL_PARTIAL, instead
of FOLL_COMPLETE) it's even smaller, slightly. Which is where I am leaning now.
Some code assumes things work that way already anyhow:
/* Pin user pages for DMA Xfer */
err = pin_user_pages_unlocked(user_dma.uaddr, user_dma.page_count,
dma->map, FOLL_FORCE);
if (user_dma.page_count != err) {
IVTV_DEBUG_WARN("failed to map user pages, returned %d instead of %d\n",
err, user_dma.page_count);
if (err >= 0) {
unpin_user_pages(dma->map, err);
return -EINVAL;
}
return err;
}
Actually I'm quite surprised I didn't find too many missing the tricky
unpin_user_pages() on the error path - eg
videobuf_dma_init_user_locked() is wrong.
Well. That's not accidental. "Some People" (much thanks to Souptick Joarder, btw) have
been fixing up many of those sites, during the pin_user_pages() conversions. Otherwise
you would have found about 10 or 15 more.
I'll fix up that one above (using your Reported-by, I assume), unless someone else is
already taking care of it.
thanks,
--
John Hubbard
NVIDIA
