Re: [Qemu-devel] [PATCH 3/7] block: raw-posix image file reopen

[Jeff Cody <jcody@redhat.com>]

On 09/05/2012 11:30 AM, Kevin Wolf wrote:
> Am 30.08.2012 20:47, schrieb Jeff Cody:
>> This is derived from the Supriya Kannery's reopen patches.
>>
>> This contains the raw-posix driver changes for the bdrv_reopen_*
>> functions.  All changes are staged into a temporary scratch buffer
>> during the prepare() stage, and copied over to the live structure
>> during commit().  Upon abort(), all changes are abandoned, and the
>> live structures are unmodified.
>>
>> The _prepare() will create an extra fd - either by means of a dup,
>> if possible, or opening a new fd if not (for instance, access
>> control changes).  Upon _commit(), the original fd is closed and
>> the new fd is used.  Upon _abort(), the duplicate/new fd is closed.
>>
>> Signed-off-by: Jeff Cody <jcody@redhat.com>
>> ---
>>  block/raw-posix.c | 153 +++++++++++++++++++++++++++++++++++++++++++++++++-----
>>  1 file changed, 139 insertions(+), 14 deletions(-)
>>
>> diff --git a/block/raw-posix.c b/block/raw-posix.c
>> index 6be20b1..48086d7 100644
>> --- a/block/raw-posix.c
>> +++ b/block/raw-posix.c
>> @@ -140,6 +140,15 @@ typedef struct BDRVRawState {
>>  #endif
>>  } BDRVRawState;
>>  
>> +typedef struct BDRVRawReopenState {
>> +    BDRVReopenState reopen_state;
>> +    int fd;
>> +    int open_flags;
>> +    uint8_t *aligned_buf;
>> +    unsigned aligned_buf_size;
>> +    BDRVRawState *stash_s;
>> +} BDRVRawReopenState;
>> +
>>  static int fd_open(BlockDriverState *bs);
>>  static int64_t raw_getlength(BlockDriverState *bs);
>>  
>> @@ -185,6 +194,28 @@ static int raw_normalize_devicepath(const char **filename)
>>  }
>>  #endif
>>  
>> +static void raw_parse_flags(int bdrv_flags, int *open_flags)
>> +{
>> +    assert(open_flags != NULL);
>> +
>> +    *open_flags |= O_BINARY;
>> +    *open_flags &= ~O_ACCMODE;
>> +    if (bdrv_flags & BDRV_O_RDWR) {
>> +        *open_flags |= O_RDWR;
>> +    } else {
>> +        *open_flags |= O_RDONLY;
>> +    }
>> +
>> +    /* Use O_DSYNC for write-through caching, no flags for write-back caching,
>> +     * and O_DIRECT for no caching. */
>> +    if ((bdrv_flags & BDRV_O_NOCACHE)) {
>> +        *open_flags |= O_DIRECT;
>> +    }
>> +    if (!(bdrv_flags & BDRV_O_CACHE_WB)) {
>> +        *open_flags |= O_DSYNC;
>> +    }
>> +}
> 
> The code motion would ideally be a separate patch.
>

OK

>> +
>>  static int raw_open_common(BlockDriverState *bs, const char *filename,
>>                             int bdrv_flags, int open_flags)
>>  {
>> @@ -196,20 +227,8 @@ static int raw_open_common(BlockDriverState *bs, const char *filename,
>>          return ret;
>>      }
>>  
>> -    s->open_flags = open_flags | O_BINARY;
>> -    s->open_flags &= ~O_ACCMODE;
>> -    if (bdrv_flags & BDRV_O_RDWR) {
>> -        s->open_flags |= O_RDWR;
>> -    } else {
>> -        s->open_flags |= O_RDONLY;
>> -    }
>> -
>> -    /* Use O_DSYNC for write-through caching, no flags for write-back caching,
>> -     * and O_DIRECT for no caching. */
>> -    if ((bdrv_flags & BDRV_O_NOCACHE))
>> -        s->open_flags |= O_DIRECT;
>> -    if (!(bdrv_flags & BDRV_O_CACHE_WB))
>> -        s->open_flags |= O_DSYNC;
>> +    s->open_flags = open_flags;
>> +    raw_parse_flags(bdrv_flags, &s->open_flags);
>>  
>>      s->fd = -1;
>>      fd = qemu_open(filename, s->open_flags, 0644);
>> @@ -283,6 +302,109 @@ static int raw_open(BlockDriverState *bs, const char *filename, int flags)
>>      return raw_open_common(bs, filename, flags, 0);
>>  }
>>  
>> +static int raw_reopen_prepare(BDRVReopenState *state, Error **errp)
>> +{
>> +    BDRVRawState *s;
>> +    BDRVRawReopenState *raw_s;
>> +    int ret = 0;
>> +
>> +    assert(state != NULL);
>> +    assert(state->bs != NULL);
>> +
>> +    s = state->bs->opaque;
>> +
>> +    state->opaque = g_malloc0(sizeof(BDRVRawReopenState));
>> +    raw_s = state->opaque;
>> +
>> +    raw_parse_flags(state->flags, &raw_s->open_flags);
>> +
>> +    /*
>> +     * If we didn't have BDRV_O_NOCACHE set before, we may not have allocated
>> +     * aligned_buf
>> +     */
>> +    if ((state->flags & BDRV_O_NOCACHE)) {
>> +        /*
>> +         * Allocate a buffer for read/modify/write cycles.  Choose the size
>> +         * pessimistically as we don't know the block size yet.
>> +         */
>> +        raw_s->aligned_buf_size = 32 * MAX_BLOCKSIZE;
>> +        raw_s->aligned_buf = qemu_memalign(MAX_BLOCKSIZE,
>> +                                           raw_s->aligned_buf_size);
>> +
>> +        if (raw_s->aligned_buf == NULL) {
>> +            ret = -1;
>> +            goto error;
>> +        }
> 
> Even though it's pretty small, I think I would factor this out into a
> small static helper to make sure it's kept in sync with raw_open_common().
> 

OK, and like your suggestion above, I'll put that in a separate code
motion patch.

>> +    }
>> +
>> +    int fcntl_flags = O_APPEND | O_ASYNC | O_NONBLOCK;
>> +#ifdef O_NOATIME
>> +    fcntl_flags |= O_NOATIME;
>> +#endif
>> +    if ((raw_s->open_flags & ~fcntl_flags) == (s->open_flags & ~fcntl_flags)) {
>> +        /* dup the original fd */
>> +        /* TODO: use qemu fcntl wrapper */
>> +        raw_s->fd = fcntl(s->fd, F_DUPFD_CLOEXEC, 0);
>> +        if (raw_s->fd == -1) {
>> +            ret = -1;
>> +            goto error;
>> +        }
>> +        ret = fcntl_setfl(raw_s->fd, raw_s->open_flags);
>> +    } else {
>> +        raw_s->fd = qemu_open(state->bs->filename, raw_s->open_flags, 0644);
>> +        if (raw_s->fd == -1) {
>> +            ret = -1;
>> +        }
> 
> Ignoring this part for now, with qemu_dup_flags() it's going to look a
> bit different. In particular, I'm hoping that we don't get a second
> fcntl_flags enumeration here, but can just fall back to qemu_open()
> whenever qemu_dup_flags() fails.

That will require modification to qemu_dup_flags()... I believe
qemu_dup_flags() silently filters out fcntl incompatible flags.

Maybe it would be best to create a small helper function in osdep.c, that
fetches the fcntl_flags.  Then qemu_dup_flags() and this function would
use the same helper to fetch fcntl_flags.  The results of that would
determine if we call qemu_dup_flags() or qemu_open().

Although, I do think it makes sense to always try qemu_open() if
qemu_dup_flags() fails for some reason.

> 
> If we do need to keep fcntl_flags here, we'll probably want to add
> O_DIRECT to it.
> 

>> +    }
>> +error:
>> +    return ret;
>> +}
>> +
>> +
>> +static void raw_reopen_commit(BDRVReopenState *state)
>> +{
>> +    BDRVRawReopenState *raw_s = state->opaque;
>> +    BDRVRawState *s = state->bs->opaque;
>> +
>> +    if (raw_s->aligned_buf != NULL) {
>> +        if (s->aligned_buf) {
>> +            qemu_vfree(s->aligned_buf);
>> +        }
>> +        s->aligned_buf      = raw_s->aligned_buf;
>> +        s->aligned_buf_size = raw_s->aligned_buf_size;
>> +    }
>> +
>> +    s->open_flags = raw_s->open_flags;
>> +
>> +    close(s->fd);
>> +    s->fd = raw_s->fd;
>> +
>> +    g_free(state->opaque);
>> +    state->opaque = NULL;
>> +}
> 
> I think s->use_aio must be changed as well, it depends on
> BDRV_O_NOCACHE. Maybe we even need to do it in prepare, laio_init() may
> need to be called.

Yes, good catch, thanks.

> 
> Kevin
>