Re: [RFC PATCH v3 15/30] io: Add a pwritev/preadv version that takes a discontiguous iovec

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Mon, Nov 27, 2023 at 05:25:57PM -0300, Fabiano Rosas wrote:
>> For the upcoming support to fixed-ram migration with multifd, we need
>> to be able to accept an iovec array with non-contiguous data.
>> 
>> Add a pwritev and preadv version that splits the array into contiguous
>> segments before writing. With that we can have the ram code continue
>> to add pages in any order and the multifd code continue to send large
>> arrays for reading and writing.
>> 
>> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> ---
>> - split the API that was merged into a single function
>> - use uintptr_t for compatibility with 32-bit
>> ---
>>  include/io/channel.h | 26 ++++++++++++++++
>>  io/channel.c         | 70 ++++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 96 insertions(+)
>> 
>> diff --git a/include/io/channel.h b/include/io/channel.h
>> index 7986c49c71..25383db5aa 100644
>> --- a/include/io/channel.h
>> +++ b/include/io/channel.h
>> @@ -559,6 +559,19 @@ int qio_channel_close(QIOChannel *ioc,
>>  ssize_t qio_channel_pwritev(QIOChannel *ioc, const struct iovec *iov,
>>                              size_t niov, off_t offset, Error **errp);
>>  
>> +/**
>> + * qio_channel_pwritev_all:
>> + * @ioc: the channel object
>> + * @iov: the array of memory regions to write data from
>> + * @niov: the length of the @iov array
>> + * @offset: the iovec offset in the file where to write the data
>> + * @errp: pointer to a NULL-initialized error object
>> + *
>> + * Returns: 0 if all bytes were written, or -1 on error
>> + */
>> +int qio_channel_pwritev_all(QIOChannel *ioc, const struct iovec *iov,
>> +                            size_t niov, off_t offset, Error **errp);
>> +
>>  /**
>>   * qio_channel_pwrite
>>   * @ioc: the channel object
>> @@ -595,6 +608,19 @@ ssize_t qio_channel_pwrite(QIOChannel *ioc, char *buf, 
>> size_t buflen,
>>  ssize_t qio_channel_preadv(QIOChannel *ioc, const struct iovec *iov,
>>                             size_t niov, off_t offset, Error **errp);
>>  
>> +/**
>> + * qio_channel_preadv_all:
>> + * @ioc: the channel object
>> + * @iov: the array of memory regions to read data to
>> + * @niov: the length of the @iov array
>> + * @offset: the iovec offset in the file from where to read the data
>> + * @errp: pointer to a NULL-initialized error object
>> + *
>> + * Returns: 0 if all bytes were read, or -1 on error
>> + */
>> +int qio_channel_preadv_all(QIOChannel *ioc, const struct iovec *iov,
>> +                           size_t niov, off_t offset, Error **errp);
>> +
>>  /**
>>   * qio_channel_pread
>>   * @ioc: the channel object
>> diff --git a/io/channel.c b/io/channel.c
>> index a1f12f8e90..2f1745d052 100644
>> --- a/io/channel.c
>> +++ b/io/channel.c
>> @@ -472,6 +472,69 @@ ssize_t qio_channel_pwritev(QIOChannel *ioc, const 
>> struct iovec *iov,
>>      return klass->io_pwritev(ioc, iov, niov, offset, errp);
>>  }
>>  
>> +static int qio_channel_preadv_pwritev_contiguous(QIOChannel *ioc,
>> +                                                 const struct iovec *iov,
>> +                                                 size_t niov, off_t offset,
>> +                                                 bool is_write, Error 
>> **errp)
>> +{
>> +    ssize_t ret = -1;
>> +    int i, slice_idx, slice_num;
>> +    uintptr_t base, next, file_offset;
>> +    size_t len;
>> +
>> +    slice_idx = 0;
>> +    slice_num = 1;
>> +
>> +    /*
>> +     * If the iov array doesn't have contiguous elements, we need to
>> +     * split it in slices because we only have one (file) 'offset' for
>> +     * the whole iov. Do this here so callers don't need to break the
>> +     * iov array themselves.
>> +     */
>> +    for (i = 0; i < niov; i++, slice_num++) {
>> +        base = (uintptr_t) iov[i].iov_base;
>> +
>> +        if (i != niov - 1) {
>> +            len = iov[i].iov_len;
>> +            next = (uintptr_t) iov[i + 1].iov_base;
>> +
>> +            if (base + len == next) {
>> +                continue;
>> +            }
>> +        }
>> +
>> +        /*
>> +         * Use the offset of the first element of the segment that
>> +         * we're sending.
>> +         */
>> +        file_offset = offset + (uintptr_t) iov[slice_idx].iov_base;
>> +
>> +        if (is_write) {
>> +            ret = qio_channel_pwritev(ioc, &iov[slice_idx], slice_num,
>> +                                      file_offset, errp);
>> +        } else {
>> +            ret = qio_channel_preadv(ioc, &iov[slice_idx], slice_num,
>> +                                     file_offset, errp);
>> +        }
>> +
>> +        if (ret < 0) {
>> +            break;
>> +        }
>> +
>> +        slice_idx += slice_num;
>> +        slice_num = 0;
>> +    }
>> +
>> +    return (ret < 0) ? -1 : 0;
>> +}
>> +
>> +int qio_channel_pwritev_all(QIOChannel *ioc, const struct iovec *iov,
>> +                            size_t niov, off_t offset, Error **errp)
>> +{
>> +    return qio_channel_preadv_pwritev_contiguous(ioc, iov, niov,
>> +                                                 offset, true, errp);
>> +}
>
> I'm not sure how Dan thinks about this, but I don't think this is pretty..
>
> With this implementation, iochannels' preadv/pwritev is completely not
> compatible with most OSes now, afaiu.

This is internal QEMU code. I hope no one is expecting qio_channel_foo()
to behave like some OS's foo() system call. We cannot guarantee that
compatibility save for the simplest of wrappers.

>
> The definition of offset in preadv/pwritev of current iochannel is hard to
> understand.. if I read it right it'll later be set to:
>       
>                 /*
>                  * If we subtract the host page now, we don't need to
>                  * pass it into qio_channel_pwritev_all() below.
>                  */
>                 write_base = p->pages->block->pages_offset -
>                     (uintptr_t)p->pages->block->host;
>
> Which I cannot easily tell what it is.. besides being an unsigned int.

This description was unfortunately dropped along the way:

"Since iovs can be non contiguous, we'd need a separate array on the
side to carry an extra file offset for each of them, so I'm relying on
the fact that iovs are all within a same host page and passing in an
encoded offset that takes the host page into account."

> IIUC it's also based on the assumption that the host address of each iov
> entry is linear to its offset in the file, but it may not be true for
> future iochannel users of such interface called as pwritev/preadv.  So
> error prone.

Yes, but it's also our choice whether to make this a generic API. We may
have good reasons to consider a migration-specific function here.

> Would it be possible we keep using the offset array (p->pages->offset[x])?
> We have it already anyway, right?  Wouldn't that be clearer?
>

We'd have to make a copy of the array because p->pages is expected to
change while the IO happens. And while we already have a copy in
p->normal, my intention for multifd was to eliminate p->normal in the
future, so it would be nice if we could avoid it.

Also, we cannot use p->pages->offset alone because we still need the
pages_offset, i.e. the file offset where that ramblocks's pages begin.
So that means also adding that to each element of the new array.

It would probably be overall clearer and less wasteful to pass in the
host page address instead of an array of offsets. I don't see an issue
with restricting the iovs to the same host page. The migration code is
the only user for this code and AFAIK we don't have plans to change that
invariant.

> It doesn't need to be called pwritev/preadv, but taking two arrays: the
> host address array and another offset array on that file.  It can still do
> the range merge, do another sanity check on the offsets to make sure the
> offsets are also continuous (and should be true in our case).
>
>> +
>>  ssize_t qio_channel_pwrite(QIOChannel *ioc, char *buf, size_t buflen,
>>                             off_t offset, Error **errp)
>>  {
>> @@ -501,6 +564,13 @@ ssize_t qio_channel_preadv(QIOChannel *ioc, const 
>> struct iovec *iov,
>>      return klass->io_preadv(ioc, iov, niov, offset, errp);
>>  }
>>  
>> +int qio_channel_preadv_all(QIOChannel *ioc, const struct iovec *iov,
>> +                           size_t niov, off_t offset, Error **errp)
>> +{
>> +    return qio_channel_preadv_pwritev_contiguous(ioc, iov, niov,
>> +                                                 offset, false, errp);
>> +}
>> +
>>  ssize_t qio_channel_pread(QIOChannel *ioc, char *buf, size_t buflen,
>>                            off_t offset, Error **errp)
>>  {
>> -- 
>> 2.35.3
>>