On Tue, 31 May 2022 15:30:59 -0700 Suren Baghdasaryan surenb@google.com wrote:
The primary reason to invoke the oom reaper from the exit_mmap path used to be a prevention of an excessive oom killing if the oom victim exit races with the oom reaper (see [1] for more details). The invocation has moved around since then because of the interaction with the munlock logic but the underlying reason has remained the same (see [2]).
Munlock code is no longer a problem since [3] and there shouldn't be any blocking operation before the memory is unmapped by exit_mmap so the oom reaper invocation can be dropped. The unmapping part can be done with the non-exclusive mmap_sem and the exclusive one is only required when page tables are freed.
Remove the oom_reaper from exit_mmap which will make the code easier to read. This is really unlikely to make any observable difference although some microbenchmarks could benefit from one less branch that needs to be evaluated even though it almost never is true.
[1] 212925802454 ("mm: oom: let oom_reap_task and exit_mmap run concurrently") [2] 27ae357fa82b ("mm, oom: fix concurrent munlock and oom reaper unmap, v3") [3] a213e5cf71cb ("mm/munlock: delete munlock_vma_pages_all(), allow oomreap")
I've just reinstated the mapletree patchset so there are some conflicting changes.
--- a/include/linux/oom.h +++ b/include/linux/oom.h @@ -106,8 +106,6 @@ static inline vm_fault_t check_stable_address_space(struct mm_struct *mm) return 0; } -bool __oom_reap_task_mm(struct mm_struct *mm);
long oom_badness(struct task_struct *p, unsigned long totalpages); diff --git a/mm/mmap.c b/mm/mmap.c index 2b9305ed0dda..b7918e6bb0db 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -3110,30 +3110,13 @@ void exit_mmap(struct mm_struct *mm) /* mm's last user has gone, and its about to be pulled down */ mmu_notifier_release(mm);
- if (unlikely(mm_is_oom_victim(mm))) {
/** Manually reap the mm to free as much memory as possible.* Then, as the oom reaper does, set MMF_OOM_SKIP to disregard* this mm from further consideration. Taking mm->mmap_lock for* write after setting MMF_OOM_SKIP will guarantee that the oom* reaper will not run on this mm again after mmap_lock is* dropped.** Nothing can be holding mm->mmap_lock here and the above call* to mmu_notifier_release(mm) ensures mmu notifier callbacks in* __oom_reap_task_mm() will not block.*/(void)__oom_reap_task_mm(mm);set_bit(MMF_OOM_SKIP, &mm->flags);- }
- mmap_write_lock(mm);
- mmap_read_lock(mm);
Unclear why this patch fiddles with the mm_struct locking in this fashion - changelogging that would have been helpful.
But iirc mapletree wants to retain a write_lock here, so I ended up with
void exit_mmap(struct mm_struct *mm) { struct mmu_gather tlb; struct vm_area_struct *vma; unsigned long nr_accounted = 0; MA_STATE(mas, &mm->mm_mt, 0, 0); int count = 0;
/* mm's last user has gone, and its about to be pulled down */ mmu_notifier_release(mm);
mmap_write_lock(mm); arch_exit_mmap(mm);
vma = mas_find(&mas, ULONG_MAX); if (!vma) { /* Can happen if dup_mmap() received an OOM */ mmap_write_unlock(mm); return; }
lru_add_drain(); flush_cache_mm(mm); tlb_gather_mmu_fullmm(&tlb, mm); /* update_hiwater_rss(mm) here? but nobody should be looking */ /* Use ULONG_MAX here to ensure all VMAs in the mm are unmapped */ unmap_vmas(&tlb, &mm->mm_mt, vma, 0, ULONG_MAX);
/* * Set MMF_OOM_SKIP to hide this task from the oom killer/reaper * because the memory has been already freed. Do not bother checking * mm_is_oom_victim because setting a bit unconditionally is cheaper. */ set_bit(MMF_OOM_SKIP, &mm->flags); free_pgtables(&tlb, &mm->mm_mt, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING); tlb_finish_mmu(&tlb);
/* * Walk the list again, actually closing and freeing it, with preemption * enabled, without holding any MM locks besides the unreachable * mmap_write_lock. */ do { if (vma->vm_flags & VM_ACCOUNT) nr_accounted += vma_pages(vma); remove_vma(vma); count++; cond_resched(); } while ((vma = mas_find(&mas, ULONG_MAX)) != NULL);
BUG_ON(count != mm->map_count);
trace_exit_mmap(mm); __mt_destroy(&mm->mm_mt); mm->mmap = NULL; mmap_write_unlock(mm); vm_unacct_memory(nr_accounted); }