After looking even deeper, it appears as though the shared memory page is
getting properly marked as being mmapped when the shmat is called , but
when shrink_active_list is called to clean up the pagecache in vmscan.c,
the mmapped page still ends up getting freed. In shrink_active_list, it
gets all the way to calling remove_mapping on that block of storage even
though page_mapping_inuse shows the page is in use. However,
page_referenced shows that there are 0 or -1 references even though there
is actually 1 reference in the running application. Since the reference
count on the nommu platforms is derived from a test_and_clear_bit on
page->flags bit 2 (via page_referenced macro in rmap) this would assume
that something is changing that referenced bit when the page is still
being referenced.
I'll keep looking, but any ideas why the referenced bit in the page->flags
could changing are appreciated.
John Moore
Jamie Lokier <ja...@shareable.org>
Sent by: uclinux-dev-boun...@uclinux.org
01/22/2010 05:21 AM
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Subject
Re: [uClinux-dev] Disk Cache overlaying Shared Memory?
John B Moore wrote:
> 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.
If that's what's happening it'd be quite a serious bug, although SYSV
SHM isn't used much on NOMMU so it could have been missed.
Do you know if it does the same thing when using POSIX shared memory
(if that even works)?
-- Jamie
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