[RFC] ubi: ubi_wl_get_peb: Replace a limited number of attempts with polling while getting PEB

From: Zhihao Cheng
Date: Thu Aug 01 2019 - 05:02:19 EST

Running pressure 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:
[io_paral] update_volume():105: function write() failed with error 30
(Read-only file system)
[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
ubi0 error: ubi_wl_get_peb [ubi]: Unable to get a free PEB from user WL
ubi0 warning: ubi_eba_write_leb [ubi]: switch to read-only mode
ubi0 error: ubi_io_write [ubi]: 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:
ubi_eba_write_leb+0xa1e/0xa40 [ubi]
vol_cdev_write+0x307/0x520 [ubi]
ubi0 error: vol_cdev_write [ubi]: cannot accept more 380 bytes of data,
error -30

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.
So, ubi_wl_get_peb (in fastmap-wil.c) should be implemented to obtain a
free PEB by polling method. The polling exit condition is that there is
no free PEBs on UBI, no free PEBs in pool, and ubi->works_count is 0.

Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
drivers/mtd/ubi/fastmap-wl.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/mtd/ubi/fastmap-wl.c b/drivers/mtd/ubi/fastmap-wl.c
index d9e2e3a..c5512cf 100644
--- a/drivers/mtd/ubi/fastmap-wl.c
+++ b/drivers/mtd/ubi/fastmap-wl.c
@@ -196,7 +196,7 @@ static int produce_free_peb(struct ubi_device *ubi)
int ubi_wl_get_peb(struct ubi_device *ubi)
- int ret, retried = 0;
+ int ret;
struct ubi_fm_pool *pool = &ubi->fm_pool;
struct ubi_fm_pool *wl_pool = &ubi->fm_wl_pool;

@@ -220,13 +220,14 @@ int ubi_wl_get_peb(struct ubi_device *ubi)

if (pool->used == pool->size) {
- spin_unlock(&ubi->wl_lock);
- if (retried) {
+ if (!ubi->free.rb_node && ubi->works_count == 0) {
ubi_err(ubi, "Unable to get a free PEB from user WL pool");
+ ubi_assert(list_empty(&ubi->works));
+ spin_unlock(&ubi->wl_lock);
ret = -ENOSPC;
goto out;
- retried = 1;
+ spin_unlock(&ubi->wl_lock);
ret = produce_free_peb(ubi);
if (ret < 0) {