Re: [libvirt] [PULL 01/19] util/hbitmap: strict hbitmap_reset

On 10/11/19 7:18 PM, John Snow wrote: > > > On 10/11/19 5:48 PM, Eric Blake wrote: >> On 10/11/19 4:25 PM, John Snow wrote: >>> From: Vladimir Sementsov-Ogievskiy <vsementsov(a)virtuozzo.com&gt; >>> >>> hbitmap_reset has an unobvious property: it rounds requested region up. >>> It may provoke bugs, like in recently fixed write-blocking mode of >>> mirror: user calls reset on unaligned region, not keeping in mind that >>> there are possible unrelated dirty bytes, covered by rounded-up region >>> and information of this unrelated "dirtiness" will be lost. >>> >>> Make hbitmap_reset strict: assert that arguments are aligned, allowing >>> only one exception when @start + @count == hb->orig_size. It's needed >>> to comfort users of hbitmap_next_dirty_area, which cares about >>> hb->orig_size. >>> >>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov(a)virtuozzo.com&gt; >>> Reviewed-by: Max Reitz <mreitz(a)redhat.com&gt; >>> Message-Id: <20190806152611.280389-1-vsementsov(a)virtuozzo.com&gt; >>> [Maintainer edit: Max's suggestions from on-list. --js] >>> Signed-off-by: John Snow <jsnow(a)redhat.com&gt; >>> --- >>>   include/qemu/hbitmap.h | 5 +++++ >>>   tests/test-hbitmap.c   | 2 +- >>>   util/hbitmap.c         | 4 ++++ >>>   3 files changed, 10 insertions(+), 1 deletion(-) >>> >> >>> +++ b/util/hbitmap.c >>> @@ -476,6 +476,10 @@ void hbitmap_reset(HBitmap *hb, uint64_t start, >>> uint64_t count) >>>       /* Compute range in the last layer.  */ >>>       uint64_t first; >>>       uint64_t last = start + count - 1; >>> +    uint64_t gran = 1ULL << hb->granularity; >>> + >>> +    assert(!(start & (gran - 1))); >>> +    assert(!(count & (gran - 1)) || (start + count == hb->orig_size)); >> >> I know I'm replying a bit late (since this is now a pull request), but >> would it be worth using the dedicated macro: >> >> assert(QEMU_IS_ALIGNED(start, gran)); >> assert(QEMU_IS_ALIGNED(count, gran) || start + count == hb->orig_size); >> >> instead of open-coding it?  (I would also drop the extra () around the >> right half of ||). If we want it, that would now be a followup patch. I've noticed that seasoned C programmers hate extra parentheses a lot. I've noticed that I cannot remember operator precedence enough to ever feel like this is actually an improvement. Something about a nice weighted tree of ((expr1) || (expr2)) feels soothing to my weary eyes. So, if it's not terribly important, I'd prefer to leave it as-is.