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Re: [Qemu-block] [PATCH 09/25] block/dirty-bitmap: add readonly field to

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH 09/25] block/dirty-bitmap: add readonly field to BdrvDirtyBitmap
Date: Wed, 31 May 2017 16:44:39 +0200
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On 2017-05-31 16:29, Vladimir Sementsov-Ogievskiy wrote:
> 31.05.2017 16:43, Max Reitz wrote:
>> On 2017-05-30 08:50, Vladimir Sementsov-Ogievskiy wrote:
>>> Thank you for this scenario. Hmm.
>>> So, as I need guarantee that image and bitmap are unchanged,
>>> bdrv_set_dirty should return error and fail the whole write. Ok?
>> I don't know. That would mean that you couldn't commit to an image that
>> has a persistent auto-loading bitmap, which doesn't seem very nice to me.
>> I'm not quite sure what to do myself. So first I'd definitely want the
>> commit operation to succeed. That means we'd have to automatically make
>> the bitmap non-readonly once we write to it. The "readonly" flag would
>> then be an "unchanged" flag, rather, to signify that the bitmap has not
>> been changed since it was loaded, which means that it does not need to
>> be written back to the image file.
>> Now the issue remains that if you modify a persistent bitmap that is
>> stored in an image file that is opened RO when it's closed, you won't be
>> able to write the modifications back.
>> So in addition, I guess we'd need to "flush" all persistent bitmaps
>> (that is, write all modifications back to the file and set the
>> "unchanged" flag (you could also call it "dirty" and then mean the
>> opposite) for each bitmap) not only when the image is closed or
>> invalidated, but also when it is reopened read-only.
>> (block-commit reopens the backing BDS R/W, then writes to them, thus
>> modifying the dirty bitmaps, and finally reopens the BDS as read-only;
>> before that happens, we will have to flush the modified bitmap data.)
> Ok, understand.
> We need to consider also setting in_use flag in the image. We _must not_
> write to image with dirty bitmap,
> if in_use flag of this dirty bitmap is not set, as in case of something
> fail we will have image with wrong bitmap with
> unset in_use flag (which looks ok).


> I see two ways to handle it:
> variant 1:
> 1. readonly field stays as is (see v19, with normal errors, not only
> asserts)
> 2. immediately after reopening r/w we do "reopening bitmaps r/w", i.e.
> set in_use in the image and set BdrvDirtyBitmap.readonly = false
> 3. in reopen_prepare, if reopening r-o do "reopening bitmaps r-o", i.e.
> save them into the image and set BdrvDirtyBitmap.readonly = true

Sounds good, yes.

> variant 2:
> 1. instead of 'readonly' add 'dirty' field, set dirty to 0 for all
> bitmaps on create
> 2. before write/discard check this field in all related bitmaps, and if
> dirty=0 (and persistent=1), write IN_USE flag into the image first, set
> dirty=1, and only then do write. (if writing IN_USE=1 failed, fail the
> whole write)
> 3. in reopen_prepare, if reopening r-o do "reopening bitmaps r-o", i.e.
> save them into the image and set BdrvDirtyBitmap.dirty = 0

Works, too.

I think the second variant would the more "efficient" way (because you
only have to flush out dirty dirty bitmaps), but the first one would be
simpler and has the great advantage of not requiring a write to the
image file when you just want to set a bit in the in-memory dirty
bitmap. So I'd personally go for the first variant.


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