[LU-12335] mb_prealloc_table table read/write code is racy

• Type: Bug
• Resolution: Fixed
• Priority: Critical
• Fix Version/s: Lustre 2.13.0
• Severity: 3
• Assignee: Artem Blagodarenko (Inactive)
• Reporter: Artem Blagodarenko (Inactive)

Description

Preallocation table read/write code is racy. There is a  possibility of accessing memory outside of allocated table.
This issue can be easy reproduced. I am not sure, I have to upload test that lead to test system to be crashed. So I put it here.
 
dd if=/dev/zero of=<path_to_ldiskfs_partition> bs=1048576 count=1024 conv=fsync
cat "32 64 128 256" > /proc/fs/ldiskfs/<dev>/prealloc_table

Issue Links

• is related to: LU-12511 Prepare lustre for adoption into the linux kernel
• is related to: LU-12103 Improve block allocation for large partitions

Comments

[Artem Blagodarenko (Inactive)]

No need send this patch to ext4 upstream because no such bug there. Bug was introduced in our ldiskfs patches.

[James A Simmons]

Please make sure this is push to the ext4 maintainers.

[Peter Jones]

As per recent LWG discussion this ticket should be marked as RESOLVED and anyone wanting to keep SLES/Ubuntu servers in sync should do that under a separate ticket

[James A Simmons]

This fix was only every applied to RHEL platforms. SLES and Ubuntu lack this fix.

[Peter Jones]

Landed for 2.13

[Gerrit Updater]

Oleg Drokin ([e-mail]) merged in patch https://review.whamcloud.com/34950/
Subject: LU-12335 ldiskfs: fixed size preallocation table
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: f15995b8e52bafabe55506ad2e12c8a64a373948

[Artem Blagodarenko (Inactive)]

The algorithm is not difficult, as you can see in script. So, can be added to kernel. The most difficult diction - moment then we need to reconfigure preallocation table. With script, administrator decide, then change configuration.

 

> (hard for most users to configure, can die if there are problems (e.g. OOM),

My suggestion, add to cluster scripts and adjust automatically.

>become CPU starved if the server is busy

Preallocation table changing is quite fast operation, and with patch https://review.whamcloud.com/34950, safe and lockless.

>needs extra scanning to learn current filesystem state and may become out of sync with the kernel).

Scanning is made by kernel. Script use "/proc/fs/ldiskfs/loop1/mb_groups" output. This statistic is perfect data for such decision. Anyway, even in kernel we need use this statistic.