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

[Liam Howlett <liam.howlett@oracle.com>]

* Suren Baghdasaryan <surenb@google.com> [220519 17:44]:
> On Thu, May 19, 2022 at 1:33 PM Liam Howlett <liam.howlett@oracle.com> wrote:
> >
> > * Andrew Morton <akpm@linux-foundation.org> [220519 15:29]:
> > > On Mon, 16 May 2022 00:56:18 -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.
> > > >
> > >
> > > Liam, this mucks "mm: start tracking VMAs with maple tree" somewhat.
> > >
> > > > --- 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 313b57d55a63..ded42150e706 100644
> > > > --- a/mm/mmap.c
> > > > +++ b/mm/mmap.c
> > > > @@ -3105,30 +3105,13 @@ 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);
> > > >     arch_exit_mmap(mm);
> > > >
> > > >     vma = mm->mmap;
> > > >     if (!vma) {
> > > >             /* Can happen if dup_mmap() received an OOM */
> > > > -           mmap_write_unlock(mm);
> > > > +           mmap_read_unlock(mm);
> > > >             return;
> > > >     }
> > > >
> > > > @@ -3138,6 +3121,16 @@ void exit_mmap(struct mm_struct *mm)
> > > >     /* update_hiwater_rss(mm) here? but nobody should be looking */
> > > >     /* Use -1 here to ensure all VMAs in the mm are unmapped */
> > > >     unmap_vmas(&tlb, vma, 0, -1);
> > > > +   mmap_read_unlock(mm);
> > > > +
> > > > +   /*
> > > > +    * 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);
> > > > +
> > > > +   mmap_write_lock(mm);
> > > >     free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
> > > >     tlb_finish_mmu(&tlb);
> > > >
> > >
> > > I ended up with the below rework of "mm: start tracking VMAs with maple
> > > tree".  Please triple check?
> >
> > One small fix in the first one.  Suren found a race with the oom or
> > process_mrelease that needed the linked list to be removed here.  Please
> > correct me if I am mistaken, Suren?
> >
> > >
> > > void exit_mmap(struct mm_struct *mm)
> > > {
> > >       struct mmu_gather tlb;
> > >       struct vm_area_struct *vma;
> > >       unsigned long nr_accounted = 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 = mm->mmap;
> > >       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 -1 here to ensure all VMAs in the mm are unmapped */
> > >       unmap_vmas(&tlb, vma, 0, -1);
> > >
> > >       /*
> > >        * 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, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
> > >       tlb_finish_mmu(&tlb);
> > >
> > >       /* Walk the list again, actually closing and freeing it. */
> > >       while (vma) {
> > >               if (vma->vm_flags & VM_ACCOUNT)
> > >                       nr_accounted += vma_pages(vma);
> > >               vma = remove_vma(vma);
> > >               cond_resched();
> > >       }
> > >
> > >       trace_exit_mmap(mm);
> > >       __mt_destroy(&mm->mm_mt);
> >
> > +       mm->mmap = NULL;
> 
> That's correct. We need to reset mm->mmap so that the loop in
> __oom_reap_task_mm() stops immediately. However with maple trees I
> believe that loop is different and with an empty tree there would be
> no dereferencing. Liam?

That's correct after the "mm: remove the vma linked list".  At this
point we still need it.  The maple tree is tracking the VMAs but it's
not being used - just checked to be the same.

> 
> >
> > >       mmap_write_unlock(mm);
> > >       vm_unacct_memory(nr_accounted);
> > > }
> > >
> > >
> > > And "mm: remove the vma linked list" needed further reworking.  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);
> > >       mmap_write_unlock(mm);
> > >       vm_unacct_memory(nr_accounted);
> > > }
> >
> > It is worth noting that this drops the mmap_read_lock/unlock before the
> > write locking.  I'm not sure why Suren had it in his patches and I've
> > responded just now asking about it.  It may be an important aspect of
> > what he was planning.
> 
> unmap_vmas() does not require a mmap_write_lock and doing in under
> read lock protection allows OOM-killer and process_mrelease to run in
> parallel with exit_mmap. That was the reason I start with
> mmap_read_lock and then switch to mmap_write_lock. If that creates
> issues we should switch to mmap_write_lock for the whole duration of
> this call.
> Thanks,
> Suren.
> 
> >
> > Thanks,
> > Liam
> >
> >
> > >
> > >
> > > The mapletree patches remain hidden from mm.git until, I expect, next week.
> > >
> > > Thanks.
> > >
> > >
> >
> > --
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> >