On 2020-08-21 15:01, Andrew Morton wrote:
On Tue, 11 Aug 2020 15:20:47 -0700 cgold...@codeaurora.org wrote:
One
thing to stress is that there are other instances of CMA page pinning,
that
this patch isn't attempting to address.
Oh. How severe are these?
Hey Andrew,
- get_user_pages() will pin pages (I think that's a 100% guarantee but
I'd need to double check that). There isn't a workaround for this as far
as I know.
- One issue we've encountered is when the pages for buffer heads are
stuck on an LRU list (see the call to buffer_busy() in drop_buffers()
https://elixir.bootlin.com/linux/v5.8.8/source/fs/buffer.c#L3225). We
deal with this by allowing the buffers to be dropped, if the reason
buffer_busy() returns true is because the page is on an LRU list.
Well. Why not wait infinitely? Because there are other sources of CMA
page pinning, I guess.
Could we take a sleeping lock on the exit_mmap() path and on the
migration path? So that the migration path automatically waits for
the exact amount of time?
Retrying indefinitely whilst alloc_contig_range() returns -EBUSY is
actually a viable approach. From the perspective of how long it takes
to perform a CMA allocation, it is preferable compared to the lock-based
method, in terms of how long it would take to do a CMA allocation. With
our current method, we attempt allocations across an entire CMA region
before sleeping and retrying. With the lock-based method, we'd be
sleeping on a per-page basis - this could lead to a situation where we
spend a great deal of time waiting for a particular page A to be freed,
that lies in the subset of the CMA region we're allocating form. We
could have instead given up on allocating that subset of the CMA region
(because page A is pinned), and have moved on to a different subset of
the CMA region, and have successfully allocated that subset, whilst page
A is still pinned.
I have a patch ready that does this indefinite-retrying, that will be
sent in reply to this e-mail.
Regards,
Chris.
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