Messages in this thread | | | Date | Mon, 14 Sep 2020 14:52:37 -0700 | From | Chris Goldsworthy <> | Subject | Re: [PATCH v2] mm: cma: indefinitely retry allocations in cma_alloc |
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On 2020-09-14 11:33, Chris Goldsworthy wrote: > On 2020-09-14 02:31, David Hildenbrand wrote: >> 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 >> ... > > Hi David, > > I've botched the threading, so there are discussions with respect to > the previous patch-set that is missing on this thread, which I will > summarize below: > > 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 > > [1] features version of the patch featured a finite number of retries, > which has been stable for our kernels. In [2], Andrew questioned > whether we could actually find a way of solving the problem on the > grounds that doing a finite number of retries doesn't actually fix the > problem (more importantly, in [4] Andrew indicated that he would > prefer not to merge the patch as it doesn't solve the issue). In [3], > I suggest one actual fix for this, which is to use > preempt_disable/enable() to prevent context switches from occurring > during the periods in copy_one_pte() and exit_mmap() (I forgot to > mention this case in the commit text) in which _refcount > _mapcount > for a page - you would also need to prevent interrupts from occurring > to if we were to fully prevent the issue from occurring. I think this > would be acceptable for the copy_one_pte() case, since there _refcount > > _mapcount for little time. For the exit_mmap() case, however, _refcount is greater than _mapcount whilst the page-tables are being torn down for a process - that could be too long for disabling preemption / interrupts. > > So, in [4], Andrew asks about two alternatives to see if they're > viable: (1) acquiring locks on the exit_mmap path and migration paths, > (2) retrying indefinitely. In [5], I discuss how using locks could > increase the time it takes to perform a CMA allocation, such that a > retry approach would avoid increased CMA allocation times. I'm also > uncertain about how the locking scheme could be implemented > effectively without introducing a new per-page lock that will be used > specifically to solve this issue, and I'm not sure this would be > accepted. > > 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? > > Thanks, > > Chris.
Actually, if we were willing to have a defconfig option for enabling / disabling indefinite retries on the return of -EBUSY, would it be possibly to re-structure the patch to allow either (1) indefinite retrying, or (2) doing a fixed number of retires (as some people might want to tolerate CMA allocation failures in favor of making progress)?
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