Hi
This was already fixed in 2.6.31 with commit
8cbeb67ad50f7d68e5e83be2cb2284de8f9c03b5.
See this piece of code in dm.c:
/*
* If the target doesn't support merge method and some of the devices
* provided their merge_bvec method (we know this by looking at
* queue_max_hw_sectors), then we can't allow bios with multiple vector
* entries. So always set max_size to 0, and the code below allows
* just one page.
*/
else if (queue_max_hw_sectors(q) <= PAGE_SIZE >> 9)
max_size = 0;
Mikulas
On Sun, 1 Sep 2013, Ben Hutchings wrote:
> This is analogous to commit 627a2d3c29427637f4c5d31ccc7fcbd8d312cd71
> ('md: deal with merge_bvec_fn in component devices better.'),
> which does the same for md-devices at the top of the stack. The
> following explanation is taken from that commit. Thanks to Neil Brown
> <neilb@xxxxxxx> for the advice.
>
> If a component device has a merge_bvec_fn then as we never call it
> we must ensure we never need to. Currently this is done by setting
> max_sector to 1 PAGE, however this does not stop a bio being created
> with several sub-page iovecs that would violate the merge_bvec_fn.
>
> So instead set max_segments to 1 and set the segment boundary to the
> same as a page boundary to ensure there is only ever one single-page
> segment of IO requested at a time.
>
> This can particularly be an issue when 'xen' is used as it is
> known to submit multiple small buffers in a single bio.
>
> References: http://bugs.debian.org/604457
> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
> ---
> We've been carrying this in Debian for about 3 years and it's about time
> it got reviewed... It's basically Neil's work so should probably have
> his sign-off as well.
>
> Ben.
>
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -536,13 +536,15 @@ int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
> (unsigned long long) start << SECTOR_SHIFT);
>
> /*
> - * Check if merge fn is supported.
> - * If not we'll force DM to use PAGE_SIZE or
> - * smaller I/O, just to be safe.
> + * If we don't call merge_bvec_fn, we must never risk
> + * violating it, so limit max_phys_segments to 1 lying within
> + * a single page.
> */
> - if (dm_queue_merge_is_compulsory(q) && !ti->type->merge)
> - blk_limits_max_hw_sectors(limits,
> - (unsigned int) (PAGE_SIZE >> 9));
> + if (dm_queue_merge_is_compulsory(q) && !ti->type->merge) {
> + limits->max_segments = 1;
> + limits->seg_boundary_mask = PAGE_CACHE_SIZE - 1;
> + }
> +
> return 0;
> }
> EXPORT_SYMBOL_GPL(dm_set_device_limits);
>
> --
> Ben Hutchings
> The most exhausting thing in life is being insincere. - Anne Morrow Lindberg
>
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