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

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Message-ID: <20220602133121.p6uytrcde3schym6@revolver>
Date:   Thu, 2 Jun 2022 13:31:27 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Michal Hocko <mhocko@...e.com>
CC:     Suren Baghdasaryan <surenb@...gle.com>,
        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" <shuah@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH RESEND v2 1/2] mm: drop oom code from exit_mmap

* Michal Hocko <mhocko@...e.com> [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@...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?

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,
Liam