cvs commit: src/sys/vm vm_contig.c

[Greg 'groggy' Lehey]

On Wednesday, 24 November 2004 at 18:56:13 +0000, Xin LI wrote:
> delphij     2004-11-24 18:56:13 UTC
>
>   FreeBSD src repository
>
>   Modified files:
>     sys/vm               vm_contig.c
>   Log:
>   Try to close a potential, but serious race in our VM subsystem.
>
>   Historically, our contigmalloc1() and contigmalloc2() assumes
>   that a page in PQ_CACHE can be unconditionally reused by busying
>   and freeing it.  Unfortunatelly, when object happens to be not
>   NULL, the code will set m->object to NULL and disregard the fact
>   that the page is actually in the VM page bucket, resulting in
>   page bucket hash table corruption and finally, a filesystem
>   corruption, or a 'page not in hash' panic.
>
>   This commit has borrowed the idea taken from DragonFlyBSD's fix
>   to the VM fix by Matthew Dillon[1].  This version of patch will
>   do the following checks:
>
>           - When scanning pages in PQ_CACHE, check hold_count and
>             skip over pages that are held temporarily.
>           - For pages in PQ_CACHE and selected as candidate of being
>             freed, check if it is busy at that time.
>
>   Note:  It seems that this is might be unrelated to kern/72539.

Nice log message!  Thanks.

Greg
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