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Re: [dm-devel] New -udm?



goggin, edward wrote:
On Mon, 11 Apr 2005 04:53:07 -0700
Mike Christie <mikenc us ibm com>

Lars Marowsky-Bree wrote:

On 2005-04-11T02:27:11, Mike Christie <mikenc us ibm com> wrote:



what is wrong with what you have now where you utilize the

queue/path's


mempool by doing a blk_get_request with GFP_WAIT?


... what if it's trying to free memory by going to swap on

multipath,


and can't, because we're blocked on getting the request with
GFP_WAIT...?

GFP_WAIT does not casue IOs though. That is the difference between waiing on GFP_KERNEL and GFP_WAIT I thought. GFP_KERNEL can cause a page write out which you wait on and then there is a problem since it could be to the same disk you are trying to recover. But if you are just waiting for something to be returned to the mempool like with GFP_WAIT + blk_get_request you should be ok as long as the code below you eventually give up their resources and frees the requests you are waiting on?



A deterministic, fool-proof solution for this case must deal with
the possibility that in order to make progress, one cannot depend
that any memory resource which has previously been used will free
up -- because the freeing of that memory may be dependent on
making progress at this point. Even using GFP_WAIT, it is possible
that all previously allocated bio (not sure about requests) mempool
resources needed are queued waiting for a multipath path to become
usable again.

For requests...
I thought the only way you can have a problem is if userspace is doing sg io directly to the paths, like the path testers, and the lower level drivers do not free resources (like if some evil scsi driver decides to block forever in its error handler while multiple sg io requests are queued for example). There is no one else above us so we do not have to worry about that case where people above dm call blk_get_request on the same queue, deplete the mempool and never free those requests so we block forever on some layer above us which cannot proceed becuase we cannot.


Using blk_get_request + GFP_WAIT allows dm to use the same block layer code path as __make_request for memory allcoation failures, so if there is a problem with that code itself (like needing to preallocate correctly or some other issue like that - I am not saying preallocation is a problem there) it should be solved for all of us there. It makes no sense to have a fool proof multipath but flakey block layer and if we can share code all the better.


I don't see a way around needing to use pre-allocated bio memory which is reserved strictly for this purpose -- albeit it is possible that a single bio could be reserved for making progress in serial fashion across all multipaths which are in this state.


Your patch helps, because it means we need less memory.

But, ultimately, we ought to preallocate the requests

already when the


hw-handler is initialized for a map (because presumably at that time
we'll have enough memory, or can just fail the table

setup). From that


point on, our memory useage should not grow.



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