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Subject: Re: [PATCH v2 1/2] fuse: Fix possible deadlock when writing back
 dirty pages
To:     miklos@szeredi.hu
Cc:     tao.peng@linux.alibaba.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <807bb470f90bae5dcd80a29020d38f6b5dd6ef8e.1616826872.git.baolin.wang@linux.alibaba.com>
From:   Baolin Wang <baolin.wang@linux.alibaba.com>
Message-ID: <f72f28cd-06b5-fb84-c7ce-ad1a3d14c016@linux.alibaba.com>
Date:   Mon, 12 Apr 2021 21:23:50 +0800
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Hi Miklos,

?? 2021/3/27 14:36, Baolin Wang д??:
> We can meet below deadlock scenario when writing back dirty pages, and
> writing files at the same time. The deadlock scenario can be reproduced
> by:
> 
> - A writeback worker thread A is trying to write a bunch of dirty pages by
> fuse_writepages(), and the fuse_writepages() will lock one page (named page 1),
> add it into rb_tree with setting writeback flag, and unlock this page 1,
> then try to lock next page (named page 2).
> 
> - But at the same time a file writing can be triggered by another process B,
> to write several pages by fuse_perform_write(), the fuse_perform_write()
> will lock all required pages firstly, then wait for all writeback pages
> are completed by fuse_wait_on_page_writeback().
> 
> - Now the process B can already lock page 1 and page 2, and wait for page 1
> waritehack is completed (page 1 is under writeback set by process A). But
> process A can not complete the writeback of page 1, since it is still
> waiting for locking page 2, which was locked by process B already.
> 
> A deadlock is occurred.
> 
> To fix this issue, we should make sure each page writeback is completed
> after lock the page in fuse_fill_write_pages() separately, and then write
> them together when all pages are stable.
> 
> [1450578.772896] INFO: task kworker/u259:6:119885 blocked for more than 120 seconds.
> [1450578.796179] kworker/u259:6  D    0 119885      2 0x00000028
> [1450578.796185] Workqueue: writeback wb_workfn (flush-0:78)
> [1450578.796188] Call trace:
> [1450578.798804]  __switch_to+0xd8/0x148
> [1450578.802458]  __schedule+0x280/0x6a0
> [1450578.806112]  schedule+0x34/0xe8
> [1450578.809413]  io_schedule+0x20/0x40
> [1450578.812977]  __lock_page+0x164/0x278
> [1450578.816718]  write_cache_pages+0x2b0/0x4a8
> [1450578.820986]  fuse_writepages+0x84/0x100 [fuse]
> [1450578.825592]  do_writepages+0x58/0x108
> [1450578.829412]  __writeback_single_inode+0x48/0x448
> [1450578.834217]  writeback_sb_inodes+0x220/0x520
> [1450578.838647]  __writeback_inodes_wb+0x50/0xe8
> [1450578.843080]  wb_writeback+0x294/0x3b8
> [1450578.846906]  wb_do_writeback+0x2ec/0x388
> [1450578.850992]  wb_workfn+0x80/0x1e0
> [1450578.854472]  process_one_work+0x1bc/0x3f0
> [1450578.858645]  worker_thread+0x164/0x468
> [1450578.862559]  kthread+0x108/0x138
> [1450578.865960] INFO: task doio:207752 blocked for more than 120 seconds.
> [1450578.888321] doio            D    0 207752 207740 0x00000000
> [1450578.888329] Call trace:
> [1450578.890945]  __switch_to+0xd8/0x148
> [1450578.894599]  __schedule+0x280/0x6a0
> [1450578.898255]  schedule+0x34/0xe8
> [1450578.901568]  fuse_wait_on_page_writeback+0x8c/0xc8 [fuse]
> [1450578.907128]  fuse_perform_write+0x240/0x4e0 [fuse]
> [1450578.912082]  fuse_file_write_iter+0x1dc/0x290 [fuse]
> [1450578.917207]  do_iter_readv_writev+0x110/0x188
> [1450578.921724]  do_iter_write+0x90/0x1c8
> [1450578.925598]  vfs_writev+0x84/0xf8
> [1450578.929071]  do_writev+0x70/0x110
> [1450578.932552]  __arm64_sys_writev+0x24/0x30
> [1450578.936727]  el0_svc_common.constprop.0+0x80/0x1f8
> [1450578.941694]  el0_svc_handler+0x30/0x80
> [1450578.945606]  el0_svc+0x10/0x14
> 
> Suggested-by: Peng Tao <tao.peng@linux.alibaba.com>
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>

Do you have any comments for this patch set? Thanks.

> ---
> Changes from v1:
>   - Use fuse_wait_on_page_writeback() instead to wait for page stable.
> ---
>   fs/fuse/file.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 8cccecb..9a30093 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -1101,9 +1101,6 @@ static ssize_t fuse_send_write_pages(struct fuse_io_args *ia,
>   	unsigned int offset, i;
>   	int err;
>   
> -	for (i = 0; i < ap->num_pages; i++)
> -		fuse_wait_on_page_writeback(inode, ap->pages[i]->index);
> -
>   	fuse_write_args_fill(ia, ff, pos, count);
>   	ia->write.in.flags = fuse_write_flags(iocb);
>   	if (fm->fc->handle_killpriv_v2 && !capable(CAP_FSETID))
> @@ -1140,6 +1137,7 @@ static ssize_t fuse_fill_write_pages(struct fuse_args_pages *ap,
>   				     unsigned int max_pages)
>   {
>   	struct fuse_conn *fc = get_fuse_conn(mapping->host);
> +	struct inode *inode = mapping->host;
>   	unsigned offset = pos & (PAGE_SIZE - 1);
>   	size_t count = 0;
>   	int err;
> @@ -1166,6 +1164,8 @@ static ssize_t fuse_fill_write_pages(struct fuse_args_pages *ap,
>   		if (!page)
>   			break;
>   
> +		fuse_wait_on_page_writeback(inode, page->index);
> +
>   		if (mapping_writably_mapped(mapping))
>   			flush_dcache_page(page);
>   
>