Re: [PATCH] f2fs: compress: fix potential deadlock of compress file

[Chao Yu]

On 2021/12/10 12:30, Hyeong-Jun Kim wrote:
> There is a potential deadlock between writeback process and a process
> performing write_begin() or write_cache_pages() while trying to write
> same compress file, but not compressable, as below:
>
> [Process A] - doing checkpoint
> [Process B]                     [Process C]
> f2fs_write_cache_pages()
> - lock_page() [all pages in cluster, 0-31]
> - f2fs_write_multi_pages()
>   - f2fs_write_raw_pages()
>    - f2fs_write_single_data_page()
>     - f2fs_do_write_data_page()
>       - return -EAGAIN [f2fs_trylock_op() failed]
>     - unlock_page(page) [e.g., page 0]
>                                  - generic_perform_write()
>                                   - f2fs_write_begin()
>                                    - f2fs_prepare_compress_overwrite()
>                                     - prepare_compress_overwrite()
>                                      - lock_page() [e.g., page 0]
>                                      - lock_page() [e.g., page 1]
>     - lock_page(page) [e.g., page 0]
>
> Since there is no compress process, it is no longer necessary to hold
> locks on every pages in cluster within f2fs_write_raw_pages().
>
> This patch changes f2fs_write_raw_pages() to release all locks first
> and then perform write same as the non-compress file in
> f2fs_write_cache_pages().
>
> Fixes: 4c8ff7095bef ("f2fs: support data compression")
> Signed-off-by: Hyeong-Jun Kim <hj514.kim@xxxxxxxxxxx>
> Signed-off-by: Sungjong Seo <sj1557.seo@xxxxxxxxxxx>
> Signed-off-by: Youngjin Gil <youngjin.gil@xxxxxxxxxxx>

Looks good to me, thanks for Fengnan and Hyeong-Jun's report and Hyeong-Jun's
fixing. :)

Reviewed-by: Chao Yu <chao@xxxxxxxxxx>

Thanks,