Messages in this thread | | | Date | Thu, 17 Sep 2020 10:54:04 -0700 | From | Chris Goldsworthy <> | Subject | Re: [PATCH v2] mm: cma: indefinitely retry allocations in cma_alloc |
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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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