Re: [RFC v2] Adding block layer APIs resembling Linux ZoneBlockDevice ioctls.

[Stefan Hajnoczi]

On Fri, Jun 24, 2022 at 11:14:32AM +0800, Sam Li wrote:
> Hi Stefan,
> 
> Stefan Hajnoczi <stefanha@redhat.com> 于2022年6月20日周一 15:55写道：
> >
> > On Mon, Jun 20, 2022 at 11:36:11AM +0800, Sam Li wrote:
> >
> > Hi Sam,
> > Is this version 2 of "[RFC v1] Add support for zoned device"? Please
> > keep the email subject line the same (except for "v2", "v3", etc) so
> > that it's clear which patch series this new version replaces.
> >
> > > Fix some mistakes before. It can report a range of zones now.
> >
> > This looks like the description of what changed compared to v1. Please
> > put the changelog below "---" in the future. When patch emails are
> > merged by git-am(1) it keeps the text above "---" and discards the text
> > below "---". The changelog is usually no longer useful once the patches
> > are merged, so it should be located below the "---" line.
> >
> > The text above the "---" is the commit description (an explanation of
> > why this commit is necessary). In this case the commit description
> > should explain that this patch adds .bdrv_co_zone_report() and
> > .bdrv_co_zone_mgmt() to BlockDriver so that zoned block devices can be
> > supported.
> >
> > >
> > > Signed-off-by: Sam Li <faithilikerun@gmail.com>
> > > ---
> > >  block/block-backend.c             |  22 ++++
> > >  block/coroutines.h                |   5 +
> > >  block/file-posix.c                | 182 ++++++++++++++++++++++++++++++
> > >  block/io.c                        |  23 ++++
> > >  include/block/block-common.h      |  43 ++++++-
> > >  include/block/block-io.h          |  13 +++
> > >  include/block/block_int-common.h  |  20 ++++
> > >  qemu-io-cmds.c                    | 118 +++++++++++++++++++
> > >  tests/qemu-iotests/tests/zoned.sh |  52 +++++++++
> > >  9 files changed, 477 insertions(+), 1 deletion(-)
> > >  create mode 100644 tests/qemu-iotests/tests/zoned.sh
> > >
> > > diff --git a/block/block-backend.c b/block/block-backend.c
> > > index e0e1aff4b1..20248e4a35 100644
> > > --- a/block/block-backend.c
> > > +++ b/block/block-backend.c
> > > @@ -104,6 +104,8 @@ typedef struct BlockBackendAIOCB {
> > >      int ret;
> > >  } BlockBackendAIOCB;
> > >
> > > +
> > > +
> >
> > Please avoid whitespace changes in code that is otherwise untouched by
> > your patch. Code changes can cause merge conflicts and they make it
> > harder to use git-annotate(1), so only changes that are necessary should
> > be included in a patch.
> >
> > >  static const AIOCBInfo block_backend_aiocb_info = {
> > >      .get_aio_context = blk_aiocb_get_aio_context,
> > >      .aiocb_size = sizeof(BlockBackendAIOCB),
> > > @@ -1810,6 +1812,25 @@ int blk_flush(BlockBackend *blk)
> > >      return ret;
> > >  }
> > >
> >
> > Please add a documentation comment for blk_co_zone_report() that
> > explains how to use the functions and the purpose of the arguments. For
> > example, does offset have to be the first byte in a zone or can it be
> > any byte offset? What are the alignment requirements of offset and len?
> > Why is nr_zones a pointer?
> >
> > > +int blk_co_zone_report(BlockBackend *blk, int64_t offset, int64_t len,
> >
> > Functions that run in coroutine context must be labeled with
> > coroutine_fn:
> >
> >     int coroutine_fn blk_co_zone_report(...)
> >
> > This tells humans and tools that the function can only be called from a
> > coroutine. There is a blog post about coroutines in QEMU here:
> > https://blog.vmsplice.net/2014/01/coroutines-in-qemu-basics.html
> >
> > > +                       int64_t *nr_zones,
> > > +                       struct BlockZoneDescriptor *zones)
> >
> > QEMU coding style uses typedefs when defining structs, so "struct
> > BlockZoneDescriptor *zones" should be written as "BlockZoneDescriptor
> > *zones".
> >
> > > +{
> > > +    int ret;
> >
> > This function is called from the I/O code path, please mark it with:
> >
> >   IO_CODE();
> >
> > From include/block/block-io.h:
> >
> >   * I/O API functions. These functions are thread-safe, and therefore
> >   * can run in any thread as long as the thread has called
> >   * aio_context_acquire/release().
> >   *
> >   * These functions can only call functions from I/O and Common categories,
> >   * but can be invoked by GS, "I/O or GS" and I/O APIs.
> >   *
> >   * All functions in this category must use the macro
> >   * IO_CODE();
> >   * to catch when they are accidentally called by the wrong API.
> >
> > > +    ret = bdrv_co_zone_report(blk->root->bs, offset, len, nr_zones, 
> > > zones);
> >
> > Please add blk_inc_in_flight(blk) and blk_dec_in_flight(blk) around this
> > function call to ensure that zone report requests finish before I/O is
> > drained (see bdrv_drained_begin()). This is necessary so that it's
> > possible to wait for I/O requests, including zone report, to complete.
> >
> > Similar to blk_co_do_preadv() we need blk_wait_while_drained(blk),
> > blk_check_byte_request(), and bdrv_inc_in_flight(bs) before calling
> > bdrv_co_zone_report(). bdrv_dec_in_flight(bs) needs to be called after
> > bdrv_co_zone_report() returns.
> >
> After adding similar structure to blk_co_do_preadv(), zone operation
> command will always fail at blk_wait_while_drained(blk) because
> blk->inflight <= 0. Would it be ok to just remove
> blk_wait_while_drained?

Are you hitting the assertion in
block/block-backend.c:blk_wait_while_drained()?

  assert(blk->in_flight > 0);

If yes, then there is a bug in the code. You need to make sure that
blk_inc_in_flight() is called before blk_wait_while_drained().

> > > +    BLK_ZT_CONV = BLK_ZONE_TYPE_CONVENTIONAL,
> > > +    BLK_ZT_SWR = BLK_ZONE_TYPE_SEQWRITE_REQ,
> > > +    BLK_ZT_SWP = BLK_ZONE_TYPE_SEQWRITE_PREF,
> > > +};
> > > +
> > > +enum zone_cond {
> > > +    BLK_ZS_NOT_WP = BLK_ZONE_COND_NOT_WP,
> > > +    BLK_ZS_EMPTY = BLK_ZONE_COND_EMPTY,
> > > +    BLK_ZS_IOPEN = BLK_ZONE_COND_IMP_OPEN,
> > > +    BLK_ZS_EOPEN = BLK_ZONE_COND_EXP_OPEN,
> > > +    BLK_ZS_CLOSED = BLK_ZONE_COND_CLOSED,
> > > +    BLK_ZS_RDONLY = BLK_ZONE_COND_READONLY,
> > > +    BLK_ZS_FULL = BLK_ZONE_COND_FULL,
> > > +    BLK_ZS_OFFLINE = BLK_ZONE_COND_OFFLINE,
> > > +};
> >
> > This 1:1 correspondence with Linux constants could make the code a
> > little harder to port.
> >
> > Maybe QEMU's block layer should define its own numeric constants so the
> > code doesn't rely on operating system-specific headers.
> > block/file-posix.c #ifdef __linux__ code can be responsible for
> > converting Linux-specific constants to QEMU constants (and the 1:1
> > mapping can be used there).
> >
> Can we define those constants in block-common.h? Because
> BlockZoneDescriptor requires zone_condition, zone_type defined and
> BlockZoneDesicriptor are used in header files and qemu-io
> sub-commands. If we use #ifdef __linux__ in block-common.h, it can be
> responsible for converting Linux constants instead.
> 
> Thanks for reviewing! If there is any problem, please let me know.

I suggest defining the constants in block-common.h. #ifdef __linux__ is
not necessary in block-common.h because the constants should just be an
enum with BLK_ZS_NOT_WP = 0 and so on (no need for Linux headers).

In block/file-posix.c you can define a helper function inside #ifdef
__linux__ that does something like:

  BlkZoneCond zone_cond_from_linux(enum blk_zone_cond val)
  {
      switch (val) {
      case BLK_ZONE_COND_NOT_WP:
          return BLK_ZS_NOT_WP;
      ...
  }

The code in block/file-posix.c should call this helper to convert from
Linux values to QEMU values.

This way the QEMU block layer does not use Linux constants and compiles
on non-Linux machines.

Stefan

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