New: data loss using fallocate and mmap
https://bugzilla.kernel.org/show_bug.cgi?id=194071
Bug ID: 194071
Summary: data loss using fallocate and mmap
Product: File System
Version: 2.5
Kernel Version: 4.4.0+
Hardware: x86-64
OS: Linux
Tree: Mainline
Status: NEW
Severity: high
Priority: P1
Component: ext4
Assignee: [email protected]
Reporter: [email protected]
Regression: NoCreated attachment 254231
--> https://bugzilla.kernel.org/attachment.cgi?id=254231&action=edit
Example C program
After calling fallocate() on a shared mmap'ed file and writing data into the newly allocated region, occasionally (first observed after running for ~1 week) some data is replaced by 0s. The address and size of corrupted data is also not reproducible.
The initial failure was debugged and reduced to a C++ program that failed with both gcc and clang, and later to the attached C program. The amount allocated every iteration was reduced to 1 byte because that caused faster failures, and wasn't reproducible with higher power of 2 sizes.
Is this a bug or user error?
OS: Ubuntu 16.04.1 LTS
kernel versions: 4.4.0-38-generic, 4.9.7-040907-generic block device: Observed on both /dev/ram0 and local SSD ext4 mount options: (rw, relatime,data=ordered)
Unable to reproduce when using the "FALLOC_FL_ZERO_RANGE" flag, and on a tmpfs ram disk.
Reproduction steps:
sudo mkdir /mnt/ram0
sudo mkfs.ext4 /dev/ram0
sudo mount /dev/ram0 /mnt/ram0/
gcc -O2 tests_mmap_fallocate.c -o tests_mmap_fallocate_gcc while sudo rm -f /mnt/ram0/tests_mmap_fallocate && sudo ./tests_mmap_fallocate_gcc; do date && sleep 1; done
...
...
...
Value has been modified
(Also nothing found in /var/log/kern.log)
On a development machine the failure only occurs after several days of running in a loop, but fails within minutes on a virtualized Linux machine on a server.
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[Bug 194071] data loss using fallocate and mmap
https://bugzilla.kernel.org/show_bug.cgi?id=194071
--- Comment #1 from Michael Zimmer ([email protected]) --- Has anyone investigated or been able to reproduce this failure?
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[Bug 194071] data loss using fallocate and mmap
In reply to [email protected]https://bugzilla.kernel.org/show_bug.cgi?id=194071
Jan Kara ([email protected]) changed:
What |Removed |Added
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CC| |[email protected] Attachment #254231|text/x-csrc |text/plain mime type| |
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[Bug 194071] data loss using fallocate and mmap
In reply to [email protected]https://bugzilla.kernel.org/show_bug.cgi?id=194071
--- Comment #2 from Jan Kara ([email protected]) ---
Looks like a bug in ext4... I'm investigating...
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[Bug 194071] data loss using fallocate and mmap
In reply to [email protected]https://bugzilla.kernel.org/show_bug.cgi?id=194071
--- Comment #3 from Jan Kara ([email protected]) ---
Created attachment 256719
--> https://bugzilla.kernel.org/attachment.cgi?id=256719&action=edit
[PATCH] ext4: Fix data corruption for mmap writes
This patch fixes the issue for me.
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[Bug 194071] data loss using fallocate and mmap
In reply to [email protected]https://bugzilla.kernel.org/show_bug.cgi?id=194071
--- Comment #4 from Jan Kara ([email protected]) ---
BTW, can I base a testcase for fstests on your example program?
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[Bug 194071] data loss using fallocate and mmap
In reply to [email protected]https://bugzilla.kernel.org/show_bug.cgi?id=194071
--- Comment #5 from Michael Zimmer ([email protected]) --- Thanks for investigating and making the patch. Sorry that I missed your last comment, feel free to base a testcase on the example program.
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