Hi,
在 2026/9/18 15:16, Christoph Hellwig 写道:
> On Fri, Sep 18, 2026 at 12:17:43PM +0800, Yu Kuai wrote:
>> static inline void dm_bio_record(struct dm_bio_details *bd, struct bio
>> *bio)
>> {
>> bd->bi_bdev = bio->bi_bdev;
>> - bd->bi_flags = bio->bi_flags;
>> + /*
>> + * Restoring BIO_BLKG_REF would not recreate its associated reference.
>> + */
>> + bd->bi_flags = bio->bi_flags & ~(1U << BIO_BLKG_REF);
> But do we need to restore the reference? I have to admit that
> this surrounding code is very confusing and I would really like
> to have the dm maintainers look over it.
dm_bio_record() will store the bi_flags for current bio, later the flag is
restored
by dm_bio_restore(). For example like dm-raid1 read error path:
issue path, if blk-throttle is enabled, flag can be set before bio issuing to
dm-raid1
mirror_map
dm_bio_record
bd->bi_flags = bio->bi_flags
// contain the new flag
bio_endio
bio_clear_blkcg
// new flag is cleared
clone_endio
mirror_end_io
// if this IO failed and there is another mirror to read
dm_bio_restore
bio->bi_flags = bd->bi_flags
// new flag is set without get a blkg reference
So I think we don't need to restore the reference, BTW, there is another
approach to
avoid such problem in blk-throttle I don't take for now:
For bio entering blk-throttle, we can clear the new flag and blkg reference
directly
after the bio is throttled and before this bio is issuing to disk, because bio
will
not enter blk-throttle in the completion path. And since blk-throttle is the
only rq_qos
policy that can be enabled for bio-based disk, then dm_bio_record() will never
see a
bio with the new flag set.
--
Thanks,
Kuai
