linux-kernel - miniroot ramdisk corruption during intense memory usage

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Date:	Wed, 28 Mar 2007 14:05:11 +0200
From:	Peter Bier <peter_bier@....de>
To:	linux-kernel@...r.kernel.org
Subject: miniroot ramdisk corruption during intense memory usage

The following sympthom occured in a variant of the Knoppix-like linux appliance.

I get a corrupted miniroot ramdisk filesystem under kernel version 2.6.19.1 under intense memory usage during early startup of the system. 

In the course of a lengthy investigation of this behavior, I found out that the memory pages containing the miniroot ramdisk are freed during the initial installation 

process due to a memory shortage. It turned out, that shrink_active_list() calls pagevec_strip(), which in turn calls try_to_release_page() which finally calls try_to_free_buffers() which drops the corresponding buffers; if this succeeds, it seems to cause the page to become freed later. 

This causes pointers of indirect blocks of the filesystem of the miniroot ramdisk to be replaced by zero-filled pages later in the the system's history.

I fixed this by adding a new flag PG_ramdisk in the include file page-flags.h, setting the flag during ramdisk loading in ramdisk_commit_write() and checking for the flag in try_to_free_buffers() and retruning 0 as result immediately without freeing the buffers. 

I am not aware if that is the adequate way to solve the problem or if there is a solution that requires fewer and more localized changes of the system, avoiding to give the ramdisk pages a specific treatment

I did not observe this problem prior to kernel version 2.6.19.1, but I'm not sure whether ths was simply caused by coincidental circumstances during startup. 

Peter Bier

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