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Re: [Qemu-block] [PATCH v13 02/20] block: Drop consistent read perm if o
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-block] [PATCH v13 02/20] block: Drop consistent read perm if opened unsafe |
Date: |
Thu, 20 Apr 2017 12:58:29 +0200 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
Am 20.04.2017 um 09:52 hat Fam Zheng geschrieben:
> Signed-off-by: Fam Zheng <address@hidden>
> ---
> block.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/block.c b/block.c
> index 1fbbb8d..f5182d8 100644
> --- a/block.c
> +++ b/block.c
> @@ -1722,9 +1722,15 @@ void bdrv_format_default_perms(BlockDriverState *bs,
> BdrvChild *c,
> }
>
> /* bs->file always needs to be consistent because of the metadata. We
> - * can never allow other users to resize or write to it. */
> - perm |= BLK_PERM_CONSISTENT_READ;
> - shared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
> + * cannot allow other users to resize or write to it unless the
> caller
> + * explicitly expects unsafe readings. */
> + if (!(bdrv_get_flags(bs) & BDRV_O_UNSAFE_READ)) {
We have already spent considerable time to get rid of flags and instead
convert them into options passed in the QDict, so that they become
configurable with things like blockdev-add. Adding new flags potentially
moves in the opposite direction, so we have to be careful there.
I would be okay with patch 1, because in this case it's basically just a
shortcut for callers of blk_new_open(), which is fine. As soon as we
start querying the flag later and even rely on it being inherited, like
in this patch, I think it becomes a problem.
So if we need the flag in all nodes, can we make it an option that is
parsed in bdrv_open_common() into a bool bs->unsafe_read and inherited
explicitly in bdrv_inherited_options() and bdrv_backing_options()?
> + perm |= BLK_PERM_CONSISTENT_READ;
> + shared &= ~(BLK_PERM_WRITE | BLK_PERM_RESIZE);
> + } else {
> + perm &= ~BLK_PERM_CONSISTENT_READ;
> + shared |= BLK_PERM_WRITE | BLK_PERM_RESIZE;
> + }
I'm not completely sure why we would be interested in CONSISTENT_READ
anyway, isn't allowing shared writes what we really need? (Which you
already do here in addition to dropping CONSISTENT_READ, without it
being mentioned in the commit message.)
Also, another thought: Being only at the start of the series, I'm not
sure what this will be used for, but can we make sure that unsafe_read
is only set if the image is opened read-only? If this is for the
libguestfs use case, this restriction should be fine.
Kevin
- [Qemu-block] [PATCH v13 00/20] block: Image locking series, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 01/20] block: Introduce BDRV_O_UNSAFE_READ, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 02/20] block: Drop consistent read perm if opened unsafe, Fam Zheng, 2017/04/20
- Re: [Qemu-block] [PATCH v13 02/20] block: Drop consistent read perm if opened unsafe,
Kevin Wolf <=
- [Qemu-block] [PATCH v13 03/20] block: Don't require BLK_PERM_CONSISTENT_READ when unsafe open, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 04/20] qemu-img: Add --unsafe-read option to subcommands, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 05/20] qemu-img: Update documentation for --unsafe-read, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 06/20] qemu-io: Add --unsafe-read option, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 07/20] iotests: 030: Prepare for image locking, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 08/20] iotests: 046: Prepare for image locking, Fam Zheng, 2017/04/20
- [Qemu-block] [PATCH v13 09/20] iotests: 055: Don't attach the target image already for drive-backup, Fam Zheng, 2017/04/20