On Sun, Jan 20 2008 at 21:29 +0200, Jens Axboe <[EMAIL PROTECTED]> 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 <[EMAIL PROTECTED]> >>> wrote: >>>> this patch depends on the sg branch of the block tree >>>> >>>> James >>>> >>>> --- >>>> From: James Bottomley <[EMAIL PROTECTED]> >>>> 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
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