After more detail examination of this problem, what I see is that the
storage for the shared memory appears to be allocated via RAMFS in the
pagecache in tiny-shmem.c (i.e. a non-mmu platform) and the storage is
mmapped when the shmat is performed. When the pagecache hits its threshold
of low RAM and starts to cleanup, the mmapped storage for the shared
memory segment gets cleaned up as well old file cache entries. Eventually
this storage gets re-used as pagecache and makes it appear as though the
JFFS2 file system has overwritten the shared memory block when in reality
the storage was freed and re-used. From looking at the code, either the
page is not getting properly flagged as being mmapped on shmat or I am
missing something in the pagecache cleanup code since it appears that any
mmapped storage in the pagecache will always be ignored during cleanup.
Any ideas?
John Moore
John B Moore <jmo...@moreycorp.com>
Sent by: uclinux-dev-boun...@uclinux.org
01/20/2010 03:49 PM
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Subject
[uClinux-dev] Disk Cache overlaying Shared Memory?
I am running a 2.6.17 uClinux kernel on a Coldfire 5233 and am having an
issue where it appears as though the disk cache is overlaying shared
memory. In the application, I have allocated a 4K block of shared memory
and am writing files to a JFFS2 file system. The Cached data size
continues to grow as I continue to write files until about 1M of RAM is
still available at which point the shared memory block gets overwritten
with part of the data that was just written to JFFS2. This happens
consistently and is easily reproducible. If I am not using shared memory
but just malloc a block of storage instead, no overlay happens and the
cache grows only slightly larger before it levels off as the cache cleans
itself up.
Any ideas why this overlay may be happening?
John Moore
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