Re: [Qemu-devel] [PATCH 2/2] raw-posix: SEEK_HOLE suffices, get rid of FIEMAP

[Max Reitz <mreitz@redhat.com>]

On 2014-11-13 at 12:40, Kevin Wolf wrote:
> Am 13.11.2014 um 00:25 hat Eric Blake geschrieben:
>> On 11/12/2014 01:27 PM, Markus Armbruster wrote:
>>> +    /* in hole, end not yet known */
>>> +    offs = lseek(s->fd, start, SEEK_DATA);
>>> +    if (offs < 0) {
>>> +        /* no idea where the hole ends, give up (unlikely to happen) */
>>> +        goto dunno;
>>> +    }
>>> +    assert(offs >= start);
>>> +    *hole = start;
>>> +    *data = offs;
>> This assertion feels like an off-by-one.  The same offset cannot be both
>> a hole and data (except in some racy situation where some other process
>> is writing data to that offset in between our two lseek calls, but
>> that's already in no-man's land because no one else should be writing
>> the file while qemu has it open).  Is it worth using 'assert(offs >
>> start)' instead?
> As soon as you say "except", it's wrong to assert this at all. We can't
> guarantee that the condition is true and it's not a programming error
> in qemu if it's false. Sounds to me as if it should be a normal error
> check rather than an assertion.
>
> Also, what happens after EOF? I haven't read the patch yet, maybe it
> handles the situation already earlier, but if it doesn't, won't we get
> offset == start then?

raw_co_get_block_status() already bails out if start is at or beyond EOF.

Max