linux-kernel - Re: [PATCH RFC v2] ubi: ubi_wl_get_peb: Increase the number of attempts while getting PEB

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Date:   Tue, 13 Aug 2019 23:38:07 +0200
From:   Richard Weinberger <richard.weinberger@...il.com>
To:     Zhihao Cheng <chengzhihao1@...wei.com>
Cc:     Richard Weinberger <richard@....at>, yi.zhang@...wei.com,
        linux-mtd@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2] ubi: ubi_wl_get_peb: Increase the number of
 attempts while getting PEB

On Sat, Aug 10, 2019 at 11:51 AM Zhihao Cheng <chengzhihao1@...wei.com> wrote:
>
> Running stress test io_paral (A pressure ubi test in mtd-utils) on an
> UBI device with fewer PEBs (fastmap enabled) may cause ENOSPC errors and
> make UBI device read-only, but there are still free PEBs on the UBI
> device. This problem can be easily reproduced by performing the following
> steps on a 2-core machine:
>   $ modprobe nandsim first_id_byte=0x20 second_id_byte=0x33 parts=80
>   $ modprobe ubi mtd="0,0" fm_autoconvert
>   $ ./io_paral /dev/ubi0
>
> We may see the following verbose:
> (output)
>   [io_paral] update_volume():108: failed to write 380 bytes at offset
>   95920 of volume 2
>   [io_paral] update_volume():109: update: 97088 bytes
>   [io_paral] write_thread():227: function pwrite() failed with error 28
>   (No space left on device)
>   [io_paral] write_thread():229: cannot write 15872 bytes to offs 31744,
>   wrote -1
> (dmesg)
>   ubi0 error: ubi_wl_get_peb [ubi]: Unable to get a free PEB from user WL
>   pool
>   ubi0 warning: ubi_eba_write_leb [ubi]: switch to read-only mode
>   CPU: 0 PID: 2027 Comm: io_paral Not tainted 5.3.0-rc2-00001-g5986cd0 #9
>   ubi0 warning: try_write_vid_and_data [ubi]: failed to write VID header
>   to LEB 2:5, PEB 18
>   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0
>   -0-ga698c8995f-prebuilt.qemu.org 04/01/2014
>   Call Trace:
>     dump_stack+0x85/0xba
>     ubi_eba_write_leb+0xa1e/0xa40 [ubi]
>     vol_cdev_write+0x307/0x520 [ubi]
>     vfs_write+0xfa/0x280
>     ksys_pwrite64+0xc5/0xe0
>     __x64_sys_pwrite64+0x22/0x30
>     do_syscall_64+0xbf/0x440
>
> In function ubi_wl_get_peb, the operation of filling the pool
> (ubi_update_fastmap) with free PEBs and fetching a free PEB from the pool
> is not atomic. After thread A filling the pool with free PEB, free PEB may
> be taken away by thread B. When thread A checks the expression again, the
> condition is still unsatisfactory. At this time, there may still be free
> PEBs on UBI that can be filled into the pool.
>
> This patch increases the number of attempts to obtain PEB. An extreme
> case (No free PEBs left after creating test volumes) has been tested on
> different type of machines for 100 times. The biggest number of attempts
> are shown below:
>
>              x86_64     arm64
>   2-core        4         4
>   4-core        8         4
>   8-core        4         4
>
> Signed-off-by: Zhihao Cheng <chengzhihao1@...wei.com>

Thanks for addressing this!
I'll take this version. :-)

-- 
Thanks,
//richard