[PATCH] block layer: ioprio_best function fix
Currently ioprio_best function first checks wethere aioprio or bioprio equals IOPRIO_CLASS_NONE (ioprio_valid() macros does that) and if it is so it returns bioprio/aioprio appropriately. Thus the next four lines, that set aclass/bclass to IOPRIO_CLASS_BE, if aclass/bclass == IOPRIO_CLASS_NONE, are never executed. The second problem: if aioprio from class IOPRIO_CLASS_NONE and bioprio from class IOPRIO_CLASS_IDLE are passed to ioprio_best function, it will return IOPRIO_CLASS_IDLE. It means that during __make_request we can merge two requests and set the priority of merged request to IDLE, while one of the initial requests originates from a process with NONE (default) priority. So we can get a situation when a process with default ioprio will experience IO starvation, while there is no process from real-time class in the system. Just removing ioprio_valid check should correct situation. Signed-off-by: Vasily Tarasov <vtaras@openvz.org> -- --- linux-2.6.18/fs/ioprio.c.orig 2006-09-20 07:42:06.000000000 +0400 +++ linux-2.6.18/fs/ioprio.c 2006-10-12 14:52:36.000000000 +0400 @@ -145,11 +145,6 @@ int ioprio_best(unsigned short aprio, un unsigned short aclass = IOPRIO_PRIO_CLASS(aprio); unsigned short bclass = IOPRIO_PRIO_CLASS(bprio); - if (!ioprio_valid(aprio)) - return bprio; - if (!ioprio_valid(bprio)) - return aprio; - if (aclass == IOPRIO_CLASS_NONE) aclass = IOPRIO_CLASS_BE; if (bclass == IOPRIO_CLASS_NONE)
Re: [PATCH] block layer: ioprio_best function fix
On Thu, Oct 12 2006, Vasily Tarasov wrote: > Currently ioprio_best function first checks wethere aioprio or bioprio equals > IOPRIO_CLASS_NONE (ioprio_valid() macros does that) and if it is so it returns > bioprio/aioprio appropriately. Thus the next four lines, that set aclass/bclass > to IOPRIO_CLASS_BE, if aclass/bclass == IOPRIO_CLASS_NONE, are never executed. > > The second problem: if aioprio from class IOPRIO_CLASS_NONE and bioprio from > class IOPRIO_CLASS_IDLE are passed to ioprio_best function, it will return > IOPRIO_CLASS_IDLE. It means that during __make_request we can merge two > requests and set the priority of merged request to IDLE, while one of > the initial requests originates from a process with NONE (default) priority. > So we can get a situation when a process with default ioprio will experience > IO starvation, while there is no process from real-time class in the system. > > Just removing ioprio_valid check should correct situation. Analysis looks correct, thanks. -- Jens Axboe