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SubjectRe: [PATCH 2/2] scsi: Use new __dma_buffer to align sense buffer in scsi_cmnd
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On Mon, 07 Jan 2008 15:25:36 +0200
Boaz Harrosh <bharrosh@panasas.com> wrote:

> On Mon, Jan 07 2008 at 8:53 +0200, FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp> wrote:
> > On Sun, 23 Dec 2007 13:09:05 +0200
> > Boaz Harrosh <bharrosh@panasas.com> wrote:
> >
> >> On Fri, Dec 21 2007 at 4:30 +0200, Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> >>> The sense buffer ins scsi_cmnd can nowadays be DMA'ed into directly
> >>> by some low level drivers (that typically happens with USB mass
> >>> storage).
> >>>
> >>> This is a problem on non cache coherent architectures such as
> >>> embedded PowerPCs where the sense buffer can share cache lines with
> >>> other structure members, which leads to various forms of corruption.
> >>>
> >>> This uses the newly defined __dma_buffer annotation to enforce that
> >>> on such platforms, the sense_buffer is contained within its own
> >>> cache line. This has no effect on cache coherent architectures.
> >>>
> >>> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> >>> ---
> >>>
> >>> include/scsi/scsi_cmnd.h | 2 +-
> >>> 1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> --- linux-merge.orig/include/scsi/scsi_cmnd.h 2007-12-21 13:07:14.000000000 +1100
> >>> +++ linux-merge/include/scsi/scsi_cmnd.h 2007-12-21 13:07:29.000000000 +1100
> >>> @@ -88,7 +88,7 @@ struct scsi_cmnd {
> >>> working on */
> >>>
> >>> #define SCSI_SENSE_BUFFERSIZE 96
> >>> - unsigned char sense_buffer[SCSI_SENSE_BUFFERSIZE];
> >>> + unsigned char sense_buffer[SCSI_SENSE_BUFFERSIZE] __dma_buffer;
> >>> /* obtained by REQUEST SENSE when
> >>> * CHECK CONDITION is received on original
> >>> * command (auto-sense) */
> >> This has the potential of leaving a big fat ugly hole in the middle of
> >> scsi_cmnd. I would suggest of *just* moving the sense_buffer array to be
> >> the *first member* of struct scsi_cmnd. The command itself is already cache
> >> aligned, allocated by the proper flags to it's slab. And put a fat comment
> >> near it's definition.
> >>
> >> This is until a proper fix is sent. I have in my Q a proposition for a
> >> more prominent solution, which I will send next month. Do to short comings
> >> in the sense handling and optimizations, but should definitely cover this
> >> problem.
> >>
> >> The code should have time to be discussed and tested, so it is only 2.6.26
> >> material. For the duration of the 2.6.25 kernel we can live with a reorder
> >> of scsi_cmnd members, if it solves such a grave bug for some ARCHs.
> >>
> >> Boaz
> >> ----
> >> [RFD below]
> >> My proposed solution will be has follows:
> >>
> >> 1. Since the scsi protocol mandates an immediate REQUEST_SENSE after an error
> >> in effect the Q is frozen until the REQUEST_SENSE command returns.
> >>
> >> 2. The scsi-ml needs the sense buffer for its normal operation, independent
> >> from the ULD's request of the sence_buffer or not at request->sense. But
> >> in effect, 90% of all scsi-requests come with ULD's allocated buffer for
> >> sense, that is copied to, on command completion.
> >>
> >> 3. 99% of all commands complete successfully, so if an optimization is
> >> proposed for the successful case, sacrificing a few cycles for the error
> >> case than thats a good thing.
> >>
> >> My suggestion is to have a per Q, driver-overridable, sense buffer that is
> >> DMAed/written to by drivers. At the end of the REQUEST_SENSE command one
> >> of 2 things will be done. Either copy the sense to the ULD's supplied buffer,
> >> or if not available, allocate one from a dedicated mem_cache pool.
> >>
> >> So we are completely saving 92 bytes from scsi_cmnd by replacing the buffer
> >> with a pointer. We can always read the sense into a per Q buffer. And 10% of
> >> %1 of the times we will need to allocate a sense buffer from a dedicated mem_cache
> >> I would say thats a nice optimization.
> >>
> >> The changes to scsi_error/scsi_cmnd and friends, is pretty strait forward. But
> >> it depends on a conversion of 4/5 drivers to the new scsi_eh API for
> >> REQUEST_SENSE. I have only converted these drivers that interfered with the accessors
> >> effort + 1 other places. But there are a few more places that are not converted.
> >> Once done. The implementation can easily change with no affect on drivers.
> >
> > I think that removing the sense_buffer array from scsi_cmnd effects
> > lots of LLDs. As I wrote in other mail, many LLDs assume that
> > scsi_cmnd:sense_buffer is always available. Another big task is to
> > take care about auto sense.
> >
> > Have you already had some patches? I've just started to work on this
> > and I'd like to push that fix into 2.6.25.
>
> Tomo Hi.
> Since you ask to push this into 2.6.25, I have ask permission to
> prioritize this effort, as until now it was on a back burner.

There are no short-term solusions and seems that __dma_buffer will not
be merged. It would be better to fix this soon though it's a bit hard
to fix this before 2.6.25, I think.


> I have only done 3 drivers up to now. (out of something like 15)
>
> I have seen 4 patterns of "sense" use.
>
> 1. Driver allocated sense that is memcpy'ed in interrupt time to
> cmnd->sense.
> The sense is automatically fetched by controller and is
> usually pre-mapped.
>
> 2. dma-map of cmnd->sense prior to each command. similar to case-1
> but DMAed directly into cmnd->sense buffer. (AutoSense)

Not all of them are autosense. #2 is not related with autosense.

I think that the point is that #1 and #2 might use as many sense
buffers as can_queue. The differences are:

- For #1, we can attach sense_buffer only when necessary.

- For #2, we always need to attach sense_buffer before queuecommand.


> 3. Synchronous request for sense, like the places I sent patches
> for. Where cmnd is re-used to map the REQUEST_SENSE read.
>
> 4. Do nothing and let scsi_error issue a request sense asynchronously.
>
> What I did until now is, added a new API for case 1 - scsi_eh_cpy_sense(...) ,
> and 3,4 are covered by the existing (new) APIs. Case-2 is hardest and I'll
> fix it per-driver-case, like in iscsi it was very easy as it already have
> a per command structure allocated.

> I have not yet converted the scsi midlayer to any new system but my plan
> is:
>
> Stage 1
> 1. convert all drivers to some new/old API where:
> Those drivers with case-2 above, that can DMA_FROM_DEVICE
> concurrently into more than one sense buffer, will change
> specifically, to pre-allocate a can_queue number of buffers.

scsi-ml needs more sense buffers than can_queue since
scsi_finish_command uses sense_buffer after calling
scsi_device_unbusy.


> 2. Change mid-layer to:
> a. Pre-allocate one dma-able sense buffer per Q.
> b. copy sense out of above, directly into ULD's allocated sense
> buffer which was put on cmnd->request->sense
> c. Those very *few* requests that come without ULD allocated
> sense, allocate one for them out of a mempool.
> (All regular file-system ULDs allocate a sense buffer)
>
> Stage 2
> I think I can see a way how to shuffle the mid-layer post-sense processing
> directly in copy-sense time. So in case ULD did not "request sense" the
> buffer can be discarded. But I'm not 100% sure about that.
>
> Let me write up a sketch of what I mean which will include 3 drivers
> that demonstrate above cases, and the mid-layer changes.
> Then we can discus it and see if it makes sense (;)) to every body.
> But let me have time until tomorrow night for that.

I see. I'll wait for your patches.


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