Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap
From: Suren Baghdasaryan <surenb@google.com>
Date: 2021-12-09 16:24:20
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On Thu, Dec 9, 2021 at 1:12 AM Michal Hocko [off-list ref] wrote:
Do we want this on top?
As we discussed in this thread https://lore.kernel.org/all/YY4snVzZZZYhbigV@dhcp22.suse.cz (local), __oom_reap_task_mm in exit_mmap allows oom-reaper/process_mrelease to unmap pages in parallel with exit_mmap without blocking each other. Removal of __oom_reap_task_mm from exit_mmap prevents this parallelism and has a negative impact on performance. So the conclusion of that thread I thought was to keep that part. My understanding is that we also wanted to remove MMF_OOM_SKIP as a follow-up patch but __oom_reap_task_mm would stay.
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---- From 58b04ae6dc97b0105ea2651daca55cf2386f69b4 Mon Sep 17 00:00:00 2001 From: Michal Hocko <mhocko@suse.com> Date: Thu, 9 Dec 2021 10:07:51 +0100 Subject: [PATCH] mm: drop MMF_OOM_SKIP from exit_mmap MMF_OOM_SKIP used to play a synchronization role between exit_mmap and oom repear in the past. Since the exclusive mmap_sem is held in exit_mmap to cover all destructive operations the flag synchronization is not needed anymore and we can safely drop it. Just make sure that mm->mmap is set to NULL so that nobody will access the freed vma list. Signed-off-by: Michal Hocko <mhocko@suse.com> --- mm/mmap.c | 23 +---------------------- 1 file changed, 1 insertion(+), 22 deletions(-)diff --git a/mm/mmap.c b/mm/mmap.c index f4e09d390a07..0d6af9d89aa8 100644 --- a/mm/mmap.c +++ b/mm/mmap.c@@ -3129,28 +3129,6 @@ 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. - * - * This needs to be done before calling unlock_range(), - * which clears VM_LOCKED, otherwise the oom reaper cannot - * reliably test it. - */ - (void)__oom_reap_task_mm(mm); - - set_bit(MMF_OOM_SKIP, &mm->flags); - } - mmap_write_lock(mm); if (mm->locked_vm) unlock_range(mm->mmap, ULONG_MAX);@@ -3180,6 +3158,7 @@ void exit_mmap(struct mm_struct *mm) vma = remove_vma(vma); cond_resched(); } + mm->mmap = NULL; mmap_write_unlock(mm); vm_unacct_memory(nr_accounted); } --2.30.2 -- Michal Hocko SUSE Labs