Re: [PATCH RESEND v2 1/2] mm: drop oom code from exit_mmap

[Michal Hocko]

On Thu 02-06-22 13:31:27, Liam Howlett wrote:
> * Michal Hocko <mhocko@xxxxxxxx> [220602 02:53]:
> > On Wed 01-06-22 14:47:41, Suren Baghdasaryan wrote:
> > > On Wed, Jun 1, 2022 at 2:36 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > [...]
> > > > 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;
> > > 
> > > ^^^ this line above needs to be removed when the patch is applied over
> > > the maple tree patchset.
> > 
> > I am not fully up to date on the maple tree changes. Could you explain
> > why resetting mm->mmap is not needed anymore please?
> 
> The maple tree patch set removes the linked list, including mm->mmap.
> The call to __mt_destroy() means none of the old VMAs can be found in
> the race condition that mm->mmap = NULL was solving.

Thanks for the clarification, Liam.
-- 
Michal Hocko
SUSE Labs