Re: [PATCH v2] mm: cma: indefinitely retry allocations in cma_alloc

Chris Goldsworthy
2020-09-24T05:13:23Z

On 2020-09-17 10:54, Chris Goldsworthy wrote:
> On 2020-09-15 00:53, David Hildenbrand wrote:
>> On 14.09.20 20:33, Chris Goldsworthy wrote:
>>> On 2020-09-14 02:31, David Hildenbrand wrote:
>>>> On 11.09.20 21:17, Chris Goldsworthy wrote:
>>>>>
>>>>> So, inside of cma_alloc(), instead of giving up when
>>>>> alloc_contig_range()
>>>>> returns -EBUSY after having scanned a whole CMA-region bitmap,
>>>>> perform
>>>>> retries indefinitely, with sleeps, to give the system an
>>>>> opportunity
>>>>> to
>>>>> unpin any pinned pages.
>>>>>
>>>>> Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>
>>>>> Co-developed-by: Vinayak Menon <vinmenon@codeaurora.org>
>>>>> Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>
>>>>> ---
>>>>> mm/cma.c | 25 +++++++++++++++++++++++--
>>>>> 1 file changed, 23 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/mm/cma.c b/mm/cma.c
>>>>> index 7f415d7..90bb505 100644
>>>>> --- a/mm/cma.c
>>>>> +++ b/mm/cma.c
>>>>> @@ -442,8 +443,28 @@ struct page *cma_alloc(struct cma *cma, size_t
>>>>> count, unsigned int align,
>>>>> bitmap_maxno, start, bitmap_count, mask,
>>>>> offset);
>>>>> if (bitmap_no >= bitmap_maxno) {
>>>>> - mutex_unlock(&cma->lock);
>>>>> - break;
>>>>> + if (ret == -EBUSY) {
>>>>> + mutex_unlock(&cma->lock);
>>>>> +
>>>>> + /*
>>>>> + * Page may be momentarily pinned by some other
>>>>> + * process which has been scheduled out, e.g.
>>>>> + * in exit path, during unmap call, or process
>>>>> + * fork and so cannot be freed there. Sleep
>>>>> + * for 100ms and retry the allocation.
>>>>> + */
>>>>> + start = 0;
>>>>> + ret = -ENOMEM;
>>>>> + msleep(100);
>>>>> + continue;
>>>>> + } else {
>>>>> + /*
>>>>> + * ret == -ENOMEM - all bits in cma->bitmap are
>>>>> + * set, so we break accordingly.
>>>>> + */
>>>>> + mutex_unlock(&cma->lock);
>>>>> + break;
>>>>> + }
>>>>> }
>>>>> bitmap_set(cma->bitmap, bitmap_no, bitmap_count);
>>>>> /*
>>>>>
>>>>
>>>> What about long-term pinnings? IIRC, that can happen easily e.g.,
>>>> with
>>>> vfio (and I remember there is a way via vmsplice).
>>>>
>>>> Not convinced trying forever is a sane approach in the general case
>>>> ...
>>>
>>> V1:
>>> [1] https://lkml.org/lkml/2020/8/5/1097
>>> [2] https://lkml.org/lkml/2020/8/6/1040
>>> [3] https://lkml.org/lkml/2020/8/11/893
>>> [4] https://lkml.org/lkml/2020/8/21/1490
>>> [5] https://lkml.org/lkml/2020/9/11/1072
>>>
>>> We're fine with doing indefinite retries, on the grounds that if
>>> there
>>> is some long-term pinning that occurs when alloc_contig_range returns
>>> -EBUSY, that it should be debugged and fixed. Would it be possible
>>> to
>>> make this infinite-retrying something that could be enabled or
>>> disabled
>>> by a defconfig option?
>>
>> Two thoughts:
>>
>> This means I strongly prefer something like [3] if feasible.
>
> _Resending so that this ends up on LKML_
>
> I can give [3] some further thought then. Also, I realized [3] will
> not
> completely solve the problem, it just reduces the window in which
> _refcount > _mapcount (as mentioned in earlier threads, we encountered
> the pinning when a task in copy_one_pte() or in the exit_mmap() path
> gets context switched out). If we were to try a sleeping-lock based
> solution, do you think it would be permissible to add another lock to
> struct page?

I have not been able to think of a clean way of introducing calls to
preempt_disable() in exit_mmap(), which is the more problematic case.
We would need to track state across multiple invocations of
zap_pte_range() (which is called for each entry in a PMD when a
process's memory is being unmapped), and would also need to extend this
to tlb_finish_mmu(), which is called after all the process's memory has
been unmapped:
https://elixir.bootlin.com/linux/v5.8.10/source/mm/mmap.c#L3164. As a
follow-up to this patch, I'm submitting a patch that re-introduces the
GFP mask for cma_alloc, that will perform indefinite retires if
__GFP_NOFAIL is passed to the function.

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