linux-kernel - Re: [PATCH RESEND v2 1/2] mm: drop oom code from exit

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Date:   Thu, 2 Jun 2022 08:53:10 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Minchan Kim <minchan@...nel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Peter Xu <peterx@...hat.com>,
        John Hubbard <jhubbard@...dia.com>, shuah@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, linux-kselftest@...r.kernel.org,
        kernel-team <kernel-team@...roid.com>,
        Liam Howlett <liam.howlett@...cle.com>
Subject: Re: [PATCH RESEND v2 1/2] mm: drop oom code from exit_mmap

On Wed 01-06-22 14:47:41, Suren Baghdasaryan wrote:
> On Wed, Jun 1, 2022 at 2:36 PM Andrew Morton <akpm@...ux-foundation.org> 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?

-- 
Michal Hocko
SUSE Labs