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

[Chris Goldsworthy]

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@xxxxxxxxxxxxxx>
> > > > Co-developed-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
> > > > Signed-off-by: Vinayak Menon <vinmenon@xxxxxxxxxxxxxx>
> > > > ---
> > > >  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.

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@xxxxxxxxxxxxxxxxxxxx 
, and add the support __GFP_NOFAIL (and ignore any flag that is not one 
of __GFP_NOFAIL or __GFP_NOWARN).

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