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
<ne...@suse.de> 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 <b...@decadent.org.uk> --- 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
signature.asc
Description: This is a digitally signed message part
