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From: | Eric Blake |
Subject: | Re: [Qemu-devel] [PATCH v5 4/6] nbd/server: implement dirty bitmap export |
Date: | Wed, 20 Jun 2018 10:58:32 -0500 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0 |
On 06/20/2018 10:43 AM, Eric Blake wrote:
On 06/20/2018 06:24 AM, Eric Blake wrote:+/* Set several extents, describing region of given @length with given @flags.+ * Do not set more than @nb_extents, return number of set extents. + */ +static unsigned add_extents(NBDExtent *extents, unsigned nb_extents, + uint64_t length, uint32_t flags) +{ + unsigned i = 0; + uint32_t max_extent = QEMU_ALIGN_DOWN(INT32_MAX, BDRV_SECTOR_SIZE);This is too small of a granularity wrong when the server advertised 4k alignment during NBD_OPT_GO; it should probably refer to bs->bl.request_alignment.In fact, we can just use INT32_MAX. The dirty bitmap has a granularity at least as large as the sector size, but no smaller than the request_alignment. We don't have to worry about alignment here, as the extents will already be naturally aligned when converting from the bitmap into extents in the caller.
Oh, I see. The NBD protocol can only ask for a length of up to 32 bits, but if you learn via bitmap query that the entire rest of the image has the same state, you can indeed call add_extents() with a value larger than 32 bits, that needs to be fragmented back down into sub-32-bit chunks. INT32_MAX isn't quite right, but rounding may still pick the wrong granularity on an export with 4k alignment; however, INT32_MAX + 1U is just fine, and unlikely to run into alignment issues.
-- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3266 Virtualization: qemu.org | libvirt.org
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