On 2011-07-02 03:11, Steven Lang wrote:
> Hello,
>
> What is the intended difference between these two flags? The former
> seems to carry some specific semantic meaning while the latter seems
> like it is merely a statement of requirement. However, within the
> code they seem to be used synonymously, to the point that if an IO
> engine uses FIO_RAWIO and not FIO_MEMALIGN it would potentially crash.
>
> From memory.c:196
> if (td->o.odirect || td->o.mem_align ||
> (td->io_ops->flags & FIO_MEMALIGN)) {
> total_mem += page_mask;
> if (td->o.mem_align && td->o.mem_align > page_size)
> total_mem += td->o.mem_align - page_size;
> }
>
> From fio.c:830
> if (td->o.odirect || td->o.mem_align ||
> (td->io_ops->flags & FIO_RAWIO))
> p = PAGE_ALIGN(td->orig_buffer) + td->o.mem_align;
>
> These two pieces of code seem like they are a pair that should always
> execute together, yet they have slightly different conditions. The
> result seems like if FIO_RAWIO is used without FIO_MEMALIGN, it will
> try to page align the buffer (Which of course involves just padding it
> to the next alignment) without the actual memory available to do so,
> resulting in the IO buffer overwriting other parts of the heap.
>
> If this is changed one way or the other, however, it would result in
> an unused flag. The condition in memory.c is the only place
> FIO_MEMALIGN appears, and FIO_RAWIO only makes two other
> appearances...
>
> From init.c:320
> if (o->bs_unaligned && (o->odirect || td->io_ops->flags & FIO_RAWIO))
> log_err("fio: bs_unaligned may not work with raw io\n");
> From init.c:585
> if (td->o.odirect)
> td->io_ops->flags |= FIO_RAWIO;
>
> The former doesn't actually do anything but print a message and run
> anyway (And should probably use the same form of the flag as the other
> two anyway), and the latter is redundant since the same check for
> FIO_RAWIO/FIO_MEMALIGN also checks for o.direct.
Sorry for missing this email. Yes, the two flags are a bit of a mix. I
think what used to be the case was that there used to be other
differences. But your analysis looks correct, FIO_RAWIO without MEMALIGN
would be buggy at this point.
Feel free to send a patch to unify and cleanup those flags.
--
Jens Axboe
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