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

Chris Goldsworthy
2020-09-17T17:54:58Z

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?

> 2. The issue that I am having is that long-term pinnings are
> (unfortunately) a real thing. It's not something to debug and fix as
> you
> suggest. Like, run a VM with VFIO (e.g., PCI passthrough). While that
> VM
> is running, all VM memory will be pinned. If memory falls onto a CMA
> region your cma_alloc() will be stuck in an (endless, meaning until the
> VM ended) loop. I am not sure if all cma users are fine with that -
> especially, think about CMA being used for gigantic pages now.
>
> Assume you want to start a new VM while the other one is running and
> use
> some (new) gigantic pages for it. Suddenly you're trapped in an endless
> loop in the kernel. That's nasty.

Thanks for providing this example.

>
> If we want to stick to retrying forever, can't we use flags like
> __GFP_NOFAIL to explicitly enable this new behavior for selected
> cma_alloc() users that really can't fail/retry manually again?

This would work, we would just have to undo the work done by this patch
/ re-introduce the GFP parameter for cma_alloc():
http://lkml.kernel.org/r/20180709122019eucas1p2340da484acfcc932537e6014f4fd2c29~-sqTPJKij2939229392eucas1p2j@eucas1p2.samsung.com
, and add the support __GFP_NOFAIL (and ignore any flag that is not one
of __GFP_NOFAIL or __GFP_NOWARN).

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