On Mon, Jan 21 2008 at 11:31 +0200, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: > On Mon, Jan 21 2008, Boaz Harrosh wrote: >> On Sun, Jan 20 2008 at 22:59 +0200, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: >>> On Sun, 2008-01-20 at 21:01 +0100, Jens Axboe wrote: >>>> On Sun, Jan 20 2008, Jens Axboe wrote: >>>>> On Sun, Jan 20 2008, Boaz Harrosh wrote: >>>>>> On Sun, Jan 20 2008 at 21:29 +0200, Jens Axboe <jens.axboe@xxxxxxxxxx> wrote: >>>>>>> On Sun, Jan 20 2008, James Bottomley wrote: >>>>>>>> On Sun, 2008-01-20 at 21:18 +0200, Boaz Harrosh wrote: >>>>>>>>> On Tue, Jan 15 2008 at 19:52 +0200, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: >>>>>>>>>> this patch depends on the sg branch of the block tree >>>>>>>>>> >>>>>>>>>> James >>>>>>>>>> >>>>>>>>>> --- >>>>>>>>>> From: James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> >>>>>>>>>> Date: Tue, 15 Jan 2008 11:11:46 -0600 >>>>>>>>>> Subject: remove use_sg_chaining >>>>>>>>>> >>>>>>>>>> With the sg table code, every SCSI driver is now either chain capable >>>>>>>>>> or broken, so there's no need to have a check in the host template. >>>>>>>>>> >>>>>>>>>> Also tidy up the code by moving the scatterlist size defines into the >>>>>>>>>> SCSI includes and permit the last entry of the scatterlist pools not >>>>>>>>>> to be a power of two. >>>>>>>>>> --- >>>>>>>>> I have a theoretical problem that BUGed me from the beginning. >>>>>>>>> >>>>>>>>> Could it happen that a memory critical IO, (that is needed to free >>>>>>>>> memory), be collected into an sg-chained large IO, and the allocation >>>>>>>>> of the multiple sg-pool-allocations fail, thous dead locking on >>>>>>>>> out-of-memory? Is there a mechanism in place that will split large IO's >>>>>>>>> into smaller chunks in the event of out-of-memory condition in prep_fn? >>>>>>>>> >>>>>>>>> Is it possible to call blk_rq_map_sg() with less then what is present >>>>>>>>> at request to only map the starting portion? >>>>>>>> Obviously, that's why I was worrying about mempool size and default >>>>>>>> blocks a while ago. >>>>>>>> >>>>>>>> However, the deadlock only occurs if the device is swap or backing a >>>>>>>> filesystem with memory mapped files. The use cases for this are really >>>>>>>> tapes and other entities that need huge buffers. That's why we're >>>>>>>> keeping the system sector size at 1024 unless you alter it through sysfs >>>>>>>> (here gun, there foot ...) >>>>>>> Alternatively (and much safer, imho), we allow blk_rq_map_sg() return >>>>>>> smaller than nr_phys_segments and just ensure that the request is >>>>>>> continued nicely through the normal 'request if residual' logic. >>>>>>> >>>>>> Thats a grate Idea. I will Q it on my todo list. Thanks >>>>> ok good, thanks :-) >>>> btw, the above is full of typos, my apologies. it should read "requeue >>>> if residual", but I guess you already guessed as much. >>> Something like ... >>> >>> It looks to me like it would make sense to have something like a >>> BLKPREP_SGALLOCFAIL return so the block layer can do this for us ... >>> Alternatively, we'll have to find a way of adjusting the sector count as >>> it goes into the ULD prep functions. >>> >>> James >> By luck this is no problem because it happens exactly before the ULD >> actually prepares the command. sd and sr are already doing these >> adjustments based on bufflen. For BLOCK_PC we will need to fail with >> perhaps a new BLKPREP_SGALLOCFAIL, like you said, and let the >> initiator take care of it. > > Right, the scsi_init_io() takes care of it and adjusts the buflen as > needed, no need to pass this "erro"r back. As far as I'm concerned, > blocking for BLOCK_PC requests should be fine (is anyone using these for > swap?). > I was also thinking of a live-lock as opposed to dead-lock, where thousands of requests are issued to tens/hundreds of devices all large chained IO, so each fails to allocate second order chain segment and all are stuck in a traffic jam. Maybe BLKPREP_SGALLOCFAIL could mean wait for normal BLOCK_PC commands and return if FAIL_FAST. But I guess we can do that much later, after the picture settles. (And some experiments are do) Thanks Jens Boaz - To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
