8.2-PRERELEASE: live deadlock, almost all processes in "pfault" state

Kostik Belousov kostikbel at gmail.com
Sat Jan 8 20:20:33 UTC 2011

On Sat, Jan 08, 2011 at 11:10:21PM +0300, Lev Serebryakov wrote:
> Hello, Kostik.
> You wrote 8 января 2011 г., 22:56:13:
> >>   And, if it is "classic deadlock" is here any "classical" solution to
> >> it?
> > Do not allocate during bio processing.
>  So, if GEOM need some cache, it needs pre-allocate it and implements
> custom allocator over allocated chunk? :(
>  And what is "bio processing" in this context? geom_raid5 puts all
bio processing == whole time needed to finish pageout. Pageout is
often performed to clean the page to lower the page shortage.
If pageout requires more free pages to finish during the shortage,
then we get the deadlock.

Also, it seems that you allocate not only bios (small objects, not
every request cause page allocation), but also the huge buffers, that
require free pages each time.

> bios into the (private, internal) queue and geom_start() exits
> immediately, and bio could spend rather long time in queue (if it is
> write request) before it will be sent to underlying provider. And,
> yes, it could be combined with other bios to form new one (why
> allocation of new bio is needed).
>  So, is "bio processing" a whole time before bio is complete, or only
> geom_start() call or what?
>  Also, RAID5 needs to read data (other stripes) and write data (new
> checksum) when "write" bio is processed. BTW, "system" geom_raid3 and
> geom_vinum (with raid5 volume) need to do the same to maintain
> checksums, so they could deadlock (in theory) too, if problem is
> "allocate memory during bio processing". And geom_mirror needs
> allocate bio for second (third, ...) component on every write...
> -- 
> // Black Lion AKA Lev Serebryakov <lev at FreeBSD.org>
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