Messages in this thread | | | Subject | Re: [PATCH 03/14] mm/hmm: HMM should have a callback before MM is destroyed v2 | From | John Hubbard <> | Date | Fri, 16 Mar 2018 21:39:00 -0700 |
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On 03/16/2018 08:47 PM, John Hubbard wrote: > On 03/16/2018 07:36 PM, John Hubbard wrote: >> On 03/16/2018 12:14 PM, jglisse@redhat.com wrote: >>> From: Ralph Campbell <rcampbell@nvidia.com> >>> >> >> <snip> >> >>> +static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm) >>> +{ >>> + struct hmm *hmm = mm->hmm; >>> + struct hmm_mirror *mirror; >>> + struct hmm_mirror *mirror_next; >>> + >>> + down_write(&hmm->mirrors_sem); >>> + list_for_each_entry_safe(mirror, mirror_next, &hmm->mirrors, list) { >>> + list_del_init(&mirror->list); >>> + if (mirror->ops->release) >>> + mirror->ops->release(mirror); >>> + } >>> + up_write(&hmm->mirrors_sem); >>> +} >>> + >> >> OK, as for actual code review: >> >> This part of the locking looks good. However, I think it can race against >> hmm_mirror_register(), because hmm_mirror_register() will just add a new >> mirror regardless. >> >> So: >> >> thread 1 thread 2 >> -------------- ----------------- >> hmm_release hmm_mirror_register >> down_write(&hmm->mirrors_sem); <blocked: waiting for sem> >> // deletes all list items >> up_write >> unblocked: adds new mirror >> >>
Mark Hairgrove just pointed out some more fun facts:
1. Because hmm_mirror_register() needs to be called with an mm that has a non-zero refcount, you generally cannot get an hmm_release callback, so the above race should not happen.
2. We looked around, and the code is missing a call to mmu_notifier_unregister(). That means that it is going to leak memory and not let the mm get released either.
Maybe having each mirror have its own mmu notifier callback is a possible way to solve this.
thanks, -- John Hubbard NVIDIA
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