Messages in this thread | | | Date | Wed, 23 Sep 2020 22:13:07 -0700 | From | Chris Goldsworthy <> | Subject | Re: [PATCH v2] mm: cma: indefinitely retry allocations in cma_alloc |
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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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