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Re: [Qemu-devel] [PATCH] util/hbitmap: fix unaligned reset
From: |
Vladimir Sementsov-Ogievskiy |
Subject: |
Re: [Qemu-devel] [PATCH] util/hbitmap: fix unaligned reset |
Date: |
Mon, 5 Aug 2019 09:45:56 +0000 |
03.08.2019 0:19, Max Reitz wrote:
> On 02.08.19 20:58, Vladimir Sementsov-Ogievskiy wrote:
>> hbitmap_reset is broken: it rounds up the requested region. It leads to
>> the following bug, which is shown by fixed test:
>>
>> assume granularity = 2
>> set(0, 3) # count becomes 4
>> reset(0, 1) # count becomes 2
>>
>> But user of the interface assume that virtual bit 1 should be still
>> dirty, so hbitmap should report count to be 4!
>>
>> In other words, because of granularity, when we set one "virtual" bit,
>> yes, we make all "virtual" bits in same chunk to be dirty. But this
>> should not be so for reset.
>>
>> Fix this, aligning bound correctly.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
>> ---
>>
>> Hi all!
>>
>> Hmm, is it a bug or feature? :)
>> I don't have a test for mirror yet, but I think that sync mirror may be
>> broken
>> because of this, as do_sync_target_write() seems to be using unaligned reset.
>
> Crap.
>
>
> Yes, you’re right. This would fix it, and it wouldn’t fix it in the
> worst way.
>
> But I don’t know whether this patch is the best way forward still. I
> think call hbitmap_reset() with unaligned boundaries generally calls for
> trouble, as John has laid out. If mirror’s do_sync_target_write() is
> the only offender right now, I’d prefer for hbitmap_reset() to assert
> that the boundaries are aligned (for 4.2),
OK, agree that asserting this is better.
and for
> do_sync_target_write() to be fixed (for 4.1? :-/).
>
> (A practical problem with this patch is that do_sync_target_write() will
> still do the write, but it won’t change anything in the bitmap, so the
> copy operation was effectively useless.)
>
> I don’t know how to fix mirror exactly, though. I have four ideas:
>
> (A) Quick fix 1: do_sync_target_write() should shrink [offset, offset +
> bytes) such that it is aligned. This would make it skip writes that
> don’t fill one whole chunk.
>
> +: Simple fix. Could go into 4.1.
> -: Makes copy-mode=write-blocking equal to copy-mode=background unless
> you set the granularity to like 512. (Still beats just being
> completely broken.)
>
> (B) Quick fix 2: Setting the request_alignment block limit to the job’s
> granularity when in write-blocking mode.
>
> +: Very simple fix. Could go into 4.1.
> +: Every write will trigger a RMW cycle, which copies the whole chunk to
> the target, so write-blocking will do what it’s supposed to do.
> -: request_alignment forces everything to have the same granularity, so
> this slows down reads needlessly. (But only for write-blocking.)
>
> (C) Maybe the right fix 1: Let do_sync_target_write() expand [offset,
> offset + bytes) such that it is aligned and read head and tail from the
> source node. (So it would do the RMW itself.)
>
> + Doesn’t slow reads down.
> + Writes to dirty areas will make them clean – which is what
> write-blocking is for.
> - Probably more complicated. Nothing for 4.1.
This is how backup works.
>
> (D) Maybe the right fix 2: Split BlockLimits.request_alignment into
> read_alignment and write_alignment. Then do (B).
Now it's OK, but if we implement bitmap mode for mirror (which is upcoming
anyway, I think), it will slow down all writes, when we are interested only
in which are touching dirty parts.
>
> In effect, this is more or less the same as (C), but probably in a
> simpler way. Still not simple enough for 4.1, though.
>
>
> So... I tend to do either (A) or (B) now, and then probably (D) for
> 4.2? (And because (D) is an extension to (B), it would make sense to do
> (B) now, unless you’d prefer (A).)
>
> Max
>
--
Best regards,
Vladimir
Re: [Qemu-devel] [PATCH] util/hbitmap: fix unaligned reset, Max Reitz, 2019/08/05