On Thu, Dec 19, 2019 at 01:13:54PM -0800, John Hubbard wrote:
On 12/19/19 1:07 PM, Jason Gunthorpe wrote:
On Thu, Dec 19, 2019 at 12:30:31PM -0800, John Hubbard wrote:
On 12/19/19 5:26 AM, Leon Romanovsky wrote:
On Mon, Dec 16, 2019 at 02:25:12PM -0800, John Hubbard wrote:
Hi,
This implements an API naming change (put_user_page*() --> unpin_user_page*()), and also implements tracking of FOLL_PIN pages. It extends that tracking to a few select subsystems. More subsystems will be added in follow up work.
Hi John,
The patchset generates kernel panics in our IB testing. In our tests, we allocated single memory block and registered multiple MRs using the single block.
The possible bad flow is: ib_umem_geti() -> pin_user_pages_fast(FOLL_WRITE) -> internal_get_user_pages_fast(FOLL_WRITE) -> gup_pgd_range() -> gup_huge_pd() -> gup_hugepte() -> try_grab_compound_head() ->
Hi Leon,
Thanks very much for the detailed report! So we're overflowing...
At first look, this seems likely to be hitting a weak point in the GUP_PIN_COUNTING_BIAS-based design, one that I believed could be deferred (there's a writeup in Documentation/core-api/pin_user_page.rst, lines 99-121). Basically it's pretty easy to overflow the page->_refcount with huge pages if the pages have a *lot* of subpages.
We can only do about 7 pins on 1GB huge pages that use 4KB subpages.
Considering that establishing these pins is entirely under user control, we can't have a limit here.
There's already a limit, it's just a much larger one. :) What does "no limit" really mean, numerically, to you in this case?
I guess I mean 'hidden limit' - hitting the limit and failing would be managable.
I think 7 is probably too low though, but we are not using 1GB huge pages, only 2M..
If the number of allowed pins are exhausted then the pin_user_pages_fast() must fail back to the user.
I'll poke around the IB call stack and see how much of that return path is in place, if any. Because it's the same situation for get_user_pages_fast(). This code just added a warning on overflow so we could spot it early.
All GUP callers must be prepared for failure, IB should be fine...
Jason