[GIT PULL) (xen) stable/for-jens-3.10 - Patches for Linux 3.11

[Konrad Rzeszutek Wilk]

Hey Jens,

I have a branch ready for v3.11 (the same that was for v3.10) with
tons of fixes in it. There are some extra fixes that we are working
through - but they are little one-line fixes (sanity checks).

Since the merge window could open shortly and those little one-line
fixes can be applied later I am hoping you could pull this
branch:

 git://git.kernel.org/pub/scm/linux/kernel/git/konrad/xen.git stable/for-jens-3.10

and then when we are done talking over the little one-line fixes
I can send again a pull with the fixes.

Here is the description of what this git pull contains:
<blurb>
It has the 'feature-max-indirect-segments' implemented in both backend
and frontend. The current problem with the backend and frontend is that the
segment size is limited to 11 pages. It means we can at most squeeze in 44kB per
request. The ring can hold 32 (next power of two below 36) requests, meaning we
can do 1.4M of outstanding requests. Nowadays that is not enough.

The problem in the past was addressed in two ways - but neither one went upstream.
The first solution to this proposed by Justin from Spectralogic was to negotiate
the segment size.  This means that the âstruct blkif_sring_entryâ is now a variable size.
It can expand from 112 bytes (cover 11 pages of data - 44kB) to 1580 bytes
(256 pages of data - so 1MB). It is a simple extension by just making the array in the
request expand from 11 to a variable size negotiated. But it had limits: this extension
still limits the number of segments per request to 255 (as the total number must be
specified in the request, which only has an 8-bit field for that purpose).

The other solution (from Intel - Ronghui) was to create one extra ring that only has the
âstruct blkif_request_segmentâ in them. The âstruct blkif_requestâ would be changed to have
an index in said âsegment ringâ. There is only one segment ring. This means that the size of
the initial ring is still the same. The requests would point to the segment and enumerate out
how many of the indexes it wants to use. The limit is of course the size of the segment.
If one assumes a one-page segment this means we can in one request cover ~4MB.

Those patches were posted as RFC and the author never followed up on the ideas on changing
it to be a bit more flexible.

There is yet another mechanism that could be employed Â(which these patches implement) - and it
borrows from VirtIO protocol. And that is the âindirect descriptorsâ. This very similar to
what Intel suggests, but with a twist. The twist is to negotiate how many of these
'segment' pages (aka indirect descriptor pages) we want to support (in reality we negotiate
how many entries in the segment we want to cover, and we module the number if it is
bigger than the segment size).

This means that with the existing 36 slots in the ring (single page) we can cover:
32 slots * each blkif_request_indirect covers: 512 * 4096 ~= 64M. Since we ample space
in the blkif_request_indirect to span more than one indirect page, that number (64M)
can be also multiplied by eight = 512MB.

Roger Pau Monne took the idea and implemented them in these patches. They work
great and the corner cases (migration between backends with and without this extension)
work nicely. The backend has a limit right now off how many indirect entries
it can handle: one indirect page, and at maximum 256 entries (out of 512 - so  50% of the page
is used). That comes out to 32 slots * 256 entries in a indirect page * 1 indirect page
per request * 4096 = 32MB.

This is a conservative number that can change in the future. Right now it strikes
a good balance between giving excellent performance, memory usage in the backend, and
balancing the needs of many guests.

In the patchset there is also the split of the blkback structure to be per-VBD.
This means that the spinlock contention we had with many guests trying to do I/O and
all the blkback threads hitting the same lock has been eliminated.

Also there are bug-fixes to deal with oddly sized sectors, insane amounts on
th ring, and also a security fix (posted earlier).
</blurb>

Here is the full diffstat and such:


 Documentation/ABI/testing/sysfs-driver-xen-blkback |  17 +
 .../ABI/testing/sysfs-driver-xen-blkfront          |  10 +
 drivers/block/xen-blkback/blkback.c                | 869 +++++++++++++--------
 drivers/block/xen-blkback/common.h                 | 147 +++-
 drivers/block/xen-blkback/xenbus.c                 |  85 ++
 drivers/block/xen-blkfront.c                       | 532 ++++++++++---
 include/xen/interface/io/blkif.h                   |  53 ++
 include/xen/interface/io/ring.h                    |   5 +
 8 files changed, 1297 insertions(+), 421 deletions(-)

Jan Beulich (1):
      xen/io/ring.h: new macro to detect whether there are too many requests on the ring

Konrad Rzeszutek Wilk (5):
      xen-blkfront: Introduce a 'max' module parameter to alter the amount of indirect segments.
      xen-blkback/sysfs: Move the parameters for the persistent grant features
      xen/blkback: Check device permissions before allowing OP_DISCARD
      xen/blkback: Check for insane amounts of request on the ring (v6).
      Merge branch 'stable/for-jens-3.10' into HEAD

Roger Pau Monne (11):
      xen-blkback: print stats about persistent grants
      xen-blkback: use balloon pages for all mappings
      xen-blkback: implement LRU mechanism for persistent grants
      xen-blkback: move pending handles list from blkbk to pending_req
      xen-blkback: make the queue of free requests per backend
      xen-blkback: expand map/unmap functions
      xen-block: implement indirect descriptors
      xen-blkback: allocate list of pending reqs in small chunks
      xen-blkfront: use a different scatterlist for each request
      xen-blkback: workaround compiler bug in gcc 4.1
      xen-blkfront: set blk_queue_max_hw_sectors correctly

Stefan Bader (1):
      xen/blkback: Use physical sector size for setup


 Documentation/ABI/testing/sysfs-driver-xen-blkback |  17 +
 .../ABI/testing/sysfs-driver-xen-blkfront          |  10 +
 drivers/block/xen-blkback/blkback.c                | 869 +++++++++++++--------
 drivers/block/xen-blkback/common.h                 | 147 +++-
 drivers/block/xen-blkback/xenbus.c                 |  85 ++
 drivers/block/xen-blkfront.c                       | 532 ++++++++++---
 include/xen/interface/io/blkif.h                   |  53 ++
 include/xen/interface/io/ring.h                    |   5 +
 8 files changed, 1297 insertions(+), 421 deletions(-)

Jan Beulich (1):
      xen/io/ring.h: new macro to detect whether there are too many requests on the ring

Konrad Rzeszutek Wilk (5):
      xen-blkfront: Introduce a 'max' module parameter to alter the amount of indirect segments.
      xen-blkback/sysfs: Move the parameters for the persistent grant features
      xen/blkback: Check device permissions before allowing OP_DISCARD
      xen/blkback: Check for insane amounts of request on the ring (v6).
      Merge branch 'stable/for-jens-3.10' into HEAD

Roger Pau Monne (11):
      xen-blkback: print stats about persistent grants
      xen-blkback: use balloon pages for all mappings
      xen-blkback: implement LRU mechanism for persistent grants
      xen-blkback: move pending handles list from blkbk to pending_req
      xen-blkback: make the queue of free requests per backend
      xen-blkback: expand map/unmap functions
      xen-block: implement indirect descriptors
      xen-blkback: allocate list of pending reqs in small chunks
      xen-blkfront: use a different scatterlist for each request
      xen-blkback: workaround compiler bug in gcc 4.1
      xen-blkfront: set blk_queue_max_hw_sectors correctly

Stefan Bader (1):
      xen/blkback: Use physical sector size for setup

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