Re: [PATCH][0/8] (Targeting 2.6.17) Posix memory locking and balanced mlock-LRU semantic

["Stone Wang" <pwstone@gmail.com>]

2006/3/21, Nick Piggin <nickpiggin@yahoo.com.au>:
> Stone Wang wrote:
> > Both one of my friends(who is working on a DBMS oriented from
> > PostgreSQL) and i had encountered unexpected OOMs with mlock/mlockall.
> >
>
> I'm not sure this is a great idea. There are more conditions than just
> mlock that prevent pages being reclaimed. Running out of swap, for
> example, no swap, page temporarily pinned (in other words -- any duration
> from fleeting to permanent). I think something _much_ simpler could be
> done for a more general approach just to teach the VM to tolerate these
> pages a bit better.
>
> Also, supposing we do want this, I think there is a fairly significant
> queue of mm stuff you need to line up behind... it is probably asking
> too much to target 2.6.17 for such a significant change in any case.
>
> But despite all that I looked though and have a few comments ;)
> Kudos for jumping in and getting your hands dirty! It can be tricky code.
>
> > The patch brings Linux with:
> > 1. Posix mlock/munlock/mlockall/munlockall.
> >    Get mlock/munlock/mlockall/munlockall to Posix definiton: transaction-like,
> >    just as described in the manpage(2) of mlock/munlock/mlockall/munlockall.
> >    Thus users of mlock system call series will always have an clear map of
> >    mlocked areas.
>
> In what way are we not now posix compliant now?

Currently, Linux's mlock for example, may fail with  only part of its
task finished.

While accroding to POSIX definition:

man mlock(2)

"
RETURN VALUE
       On success, mlock returns zero.  On error, -1 is returned, errno is set
       appropriately, and no changes are made to  any  locks  in  the  address
       space of the process.
"

Shaoping Wang

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