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From:   James Houghton <jthoughton@google.com>
Date:   Tue, 17 Jan 2023 13:38:24 -0800
Message-ID: <CADrL8HVDyT6D-O=BoeHkA9oaRPLJR62Sxba8FdTjMaQYW-Ttfw@mail.gmail.com>
Subject: Re: [PATCH 35/46] hugetlb: add MADV_COLLAPSE for hugetlb
To:     Peter Xu <peterx@redhat.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>,
        Muchun Song <songmuchun@bytedance.com>,
        David Hildenbrand <david@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Axel Rasmussen <axelrasmussen@google.com>,
        Mina Almasry <almasrymina@google.com>,
        "Zach O'Keefe" <zokeefe@google.com>,
        Manish Mishra <manish.mishra@nutanix.com>,
        Naoya Horiguchi <naoya.horiguchi@nec.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Baolin Wang <baolin.wang@linux.alibaba.com>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Yang Shi <shy828301@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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> > +             if (curr < end) {
> > +                     /* Don't hold the VMA lock for too long. */
> > +                     hugetlb_vma_unlock_write(vma);
> > +                     cond_resched();
> > +                     hugetlb_vma_lock_write(vma);
>
> The intention is good here but IIUC this will cause vma lock to be taken
> after the i_mmap_rwsem, which can cause circular deadlocks.  If to do this
> properly we'll need to also release the i_mmap_rwsem.

Sorry if you spent a long time debugging this! I sent a reply a week
ago about this too.

>
> However it may make the resched() logic over complicated, meanwhile for 2M
> huge pages I think this will be called for each 2M range which can be too
> fine grained, so it looks like the "cur < end" check is a bit too aggresive.
>
> The other thing is I noticed that the long period of mmu notifier
> invalidate between start -> end will (in reallife VM context) causing vcpu
> threads spinning.
>
> I _think_ it's because is_page_fault_stale() (when during a vmexit
> following a kvm page fault) always reports true during the long procedure
> of MADV_COLLAPSE if to be called upon a large range, so even if we release
> both locks here it may not tremedously on the VM migration use case because
> of the long-standing mmu notifier invalidation procedure.

Oh... indeed. Thanks for pointing that out.

>
> To summarize.. I think a simpler start version of hugetlb MADV_COLLAPSE can
> drop this "if" block, and let the userapp decide the step size of COLLAPSE?

I'll drop this resched logic. Thanks Peter.