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From: Liu Bo <obuil.liubo@gmail.com>
Date: Wed, 31 Oct 2018 11:49:23 -0700
Message-ID: <CANQeFDC6BXEXDRr5UJOCjheoxX05NfdsE98fmDG3zXWbJNcifQ@mail.gmail.com>
Subject: Re: [PATCH RFC] Ext4: fix deadlock on dirty pages between fault and writeback
To: Liu Bo <bo.liu@linux.alibaba.com>,
        "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: linux-ext4@vger.kernel.org
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Sender: linux-ext4-owner@vger.kernel.org
List-ID: <linux-ext4.vger.kernel.org>

Hi Ted,

Could you please take a look at this?

(unfortunately I failed to come up with a reproducer as it mixed
'short of memory, writeback and fault'.)

thanks,
liubo

On Mon, Oct 29, 2018 at 5:26 PM Liu Bo <bo.liu@linux.alibaba.com> wrote:
>
> mpage_prepare_extent_to_map() tries to build up a large bio to stuff down
> the pipe.  But if it needs to wait for a page lock, it needs to make sure
> and send down any pending writes so we don't deadlock with anyone who has
> the page lock and is waiting for writeback of things inside the bio.
>
> The related lock stack is shown as follows,
>
> task1:
> [<ffffffff811aaa52>] wait_on_page_bit+0x82/0xa0
> [<ffffffff811c5777>] shrink_page_list+0x907/0x960
> [<ffffffff811c6027>] shrink_inactive_list+0x2c7/0x680
> [<ffffffff811c6ba4>] shrink_node_memcg+0x404/0x830
> [<ffffffff811c70a8>] shrink_node+0xd8/0x300
> [<ffffffff811c73dd>] do_try_to_free_pages+0x10d/0x330
> [<ffffffff811c7865>] try_to_free_mem_cgroup_pages+0xd5/0x1b0
> [<ffffffff8122df2d>] try_charge+0x14d/0x720
> [<ffffffff812320cc>] memcg_kmem_charge_memcg+0x3c/0xa0
> [<ffffffff812321ae>] memcg_kmem_charge+0x7e/0xd0
> [<ffffffff811b68a8>] __alloc_pages_nodemask+0x178/0x260
> [<ffffffff8120bff5>] alloc_pages_current+0x95/0x140
> [<ffffffff81074247>] pte_alloc_one+0x17/0x40
> [<ffffffff811e34de>] __pte_alloc+0x1e/0x110
> [<ffffffffa06739de>] alloc_set_pte+0x5fe/0xc20
> [<ffffffff811e5d93>] do_fault+0x103/0x970
> [<ffffffff811e6e5e>] handle_mm_fault+0x61e/0xd10
> [<ffffffff8106ea02>] __do_page_fault+0x252/0x4d0
> [<ffffffff8106ecb0>] do_page_fault+0x30/0x80
> [<ffffffff8171bce8>] page_fault+0x28/0x30
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> task2:
> [<ffffffff811aadc6>] __lock_page+0x86/0xa0
> [<ffffffffa02f1e47>] mpage_prepare_extent_to_map+0x2e7/0x310 [ext4]
> [<ffffffffa08a2689>] ext4_writepages+0x479/0xd60
> [<ffffffff811bbede>] do_writepages+0x1e/0x30
> [<ffffffff812725e5>] __writeback_single_inode+0x45/0x320
> [<ffffffff81272de2>] writeback_sb_inodes+0x272/0x600
> [<ffffffff81273202>] __writeback_inodes_wb+0x92/0xc0
> [<ffffffff81273568>] wb_writeback+0x268/0x300
> [<ffffffff81273d24>] wb_workfn+0xb4/0x390
> [<ffffffff810a2f19>] process_one_work+0x189/0x420
> [<ffffffff810a31fe>] worker_thread+0x4e/0x4b0
> [<ffffffff810a9786>] kthread+0xe6/0x100
> [<ffffffff8171a9a1>] ret_from_fork+0x41/0x50
> [<ffffffffffffffff>] 0xffffffffffffffff
>
> task1 is waiting for the PageWriteback bit of the page that task2 has
> collected in mpd->io_submit->io_bio, and tasks2 is waiting for the LOCKED
> bit the page which tasks1 has locked.
>
> It seems that this deadlock only happens when those pages are mapped pages
> so that mpage_prepare_extent_to_map() can have pages queued in io_bio and
> when waiting to lock the subsequent page.
>
> Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>
> ---
>
> Only did build test.
>
>  fs/ext4/inode.c | 21 ++++++++++++++++++++-
>  1 file changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c3d9a42c561e..becbfb292bf0 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2681,7 +2681,26 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
>                         if (mpd->map.m_len > 0 && mpd->next_page != page->index)
>                                 goto out;
>
> -                       lock_page(page);
> +                       if (!trylock_page(page)) {
> +                               /*
> +                                * A rare race may happen between fault and
> +                                * writeback,
> +                                *
> +                                * 1. fault may have raced in and locked this
> +                                * page ahead of us, and if fault needs to
> +                                * reclaim memory via shrink_page_list(), it may
> +                                * also wait on the writeback pages we've
> +                                * collected in our mpd->io_submit.
> +                                *
> +                                * 2. We have to submit mpd->io_submit->io_bio
> +                                * to let memory reclaim make progress in order
> +                                * to avoid the deadlock between fault and
> +                                * ourselves(writeback).
> +                                */
> +                               ext4_io_submit(&mpd->io_submit);
> +                               lock_page(page);
> +                       }
> +
>                         /*
>                          * If the page is no longer dirty, or its mapping no
>                          * longer corresponds to inode we are writing (which
> --
> 1.8.3.1
>