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

[Fabiano Rosas <farosas@suse.de>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Wed, Jan 17, 2024 at 12:39:26PM +0000, Daniel P. Berrangé wrote:
>> 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);
>> > +        }
>> 
>> iov_base is the address of a pointer in RAM, so could be
>> potentially any 64-bit value.
>> 
>> We're assigning file_offset to this pointer address with an
>> user supplied offset, and then using it as an offset on disk.
>> First this could result in 64-bit overflow when 'offset' is
>> added to 'iov_base', and second this could result in a file
>> that's 16 Exabytes in size (with holes of course).
>> 
>> I don't get how this is supposed to work, or be used ?
>
> I feel like this whole method might become clearer if we separated
> out the logic for merging memory adjacent iovecs.
>
> How about adding a 'iov_collapse' method in iov.h / iov.c to do
> the merging and then let the actual I/O code be simpler ?

I think if we add a migration-specific wrapper like we're discussing
with Peter earlier in the thread (on this same message), that would be
enough to keep the migration assumptions contained and not have to
pollute the IO code with any of this logic.

>> 
>> > +
>> > +        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);
>> > +}
>> > +
>> >  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
>> > 
>> 
>> With regards,
>> Daniel
>> -- 
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>> 
>> 
>
> With regards,
> Daniel