On 2010-09-24 19:05, Malahal Naineni wrote: > Jens Axboe [jaxboe@xxxxxxxxxxxx] wrote: >> On 2010-09-22 00:22, Malahal Naineni wrote: >>> The bounce_pfn of the request queue in 64 bit systems is set to the >>> current max_low_pfn. Adding more memory later makes this incorrect. >> >> Clearly correct. >> >>> Memory allocated beyond this boot time max_low_pfn appear to require >>> bounce buffers (bounce buffers are actually not allocated but used in >>> calculating segments that may result in "over max segments limit" >>> errors). >> >> But I can't quite convince myself that the change is fully correct. You >> don't really explain in your own words what the patch does, just what it >> fixes. > > OK, the problem is we get "over max segments limit" errors from > blk_rq_check_limits() after adding memory. The actual bug is in > blk_phys_contig_segment() where it doesn't check for possibility of > bounce buffers. This results in merging more requests in > ll_merge_requests_fn(). Later, blk_recalc_rq_segments() call from > blk_rq_check_limits() actually uses the possibility of bounce buffers, > so the calculated number of segments exceed q's max_segments resulting > in the above error. > > Fix for the actual problem is posted here: > http://permalink.gmane.org/gmane.linux.kernel.device-mapper.devel/12426 > > So clearly the bug should manifest only when 'bounce buffers' are > involved! Applying the above patch indeed_fixed_ the problem, but I > know there shouldn't be any need for bounce buffers on our system, and > further investigation revealed that DMA limit is not set correctly. > > This patch also _fixed_ our problem. So we are fine with either patch, > but this patch is preferred as it enables more request merges. Also, > both patches maybe needed for some configurations. Plus it doesn't needlessly bounce, that's the real problem you want to fix. I have applied this thread patch to for-2.6.37/core, thanks. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
