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    SubjectRe: [PATCH v7 2/3] block: add bdev_interposer
    On Wed, Mar 17 2021 at  2:14pm -0400,
    Sergei Shtepa <sergei.shtepa@veeam.com> wrote:

    > The 03/17/2021 18:04, Mike Snitzer wrote:
    > > On Wed, Mar 17 2021 at 8:22am -0400,
    > > Sergei Shtepa <sergei.shtepa@veeam.com> wrote:
    > >
    > > > The 03/17/2021 06:03, Ming Lei wrote:
    > > > > On Tue, Mar 16, 2021 at 07:35:44PM +0300, Sergei Shtepa wrote:
    > > > > > The 03/16/2021 11:09, Ming Lei wrote:
    > > > > > > On Fri, Mar 12, 2021 at 06:44:54PM +0300, Sergei Shtepa wrote:
    > > > > > > > bdev_interposer allows to redirect bio requests to another devices.
    > > > > > > >
    > > > > > > > Signed-off-by: Sergei Shtepa <sergei.shtepa@veeam.com>
    > > > > > > > ---
    > > > > > > > block/bio.c | 2 ++
    > > > > > > > block/blk-core.c | 57 +++++++++++++++++++++++++++++++++++++++
    > > > > > > > block/genhd.c | 54 +++++++++++++++++++++++++++++++++++++
    > > > > > > > include/linux/blk_types.h | 3 +++
    > > > > > > > include/linux/blkdev.h | 9 +++++++
    > > > > > > > 5 files changed, 125 insertions(+)
    > > > > > > >
    > > > > > > > diff --git a/block/bio.c b/block/bio.c
    > > > > > > > index a1c4d2900c7a..0bfbf06475ee 100644
    > > > > > > > --- a/block/bio.c
    > > > > > > > +++ b/block/bio.c
    > > > > > > > @@ -640,6 +640,8 @@ void __bio_clone_fast(struct bio *bio, struct bio *bio_src)
    > > > > > > > bio_set_flag(bio, BIO_THROTTLED);
    > > > > > > > if (bio_flagged(bio_src, BIO_REMAPPED))
    > > > > > > > bio_set_flag(bio, BIO_REMAPPED);
    > > > > > > > + if (bio_flagged(bio_src, BIO_INTERPOSED))
    > > > > > > > + bio_set_flag(bio, BIO_INTERPOSED);
    > > > > > > > bio->bi_opf = bio_src->bi_opf;
    > > > > > > > bio->bi_ioprio = bio_src->bi_ioprio;
    > > > > > > > bio->bi_write_hint = bio_src->bi_write_hint;
    > > > > > > > diff --git a/block/blk-core.c b/block/blk-core.c
    > > > > > > > index fc60ff208497..da1abc4c27a9 100644
    > > > > > > > --- a/block/blk-core.c
    > > > > > > > +++ b/block/blk-core.c
    > > > > > > > @@ -1018,6 +1018,55 @@ static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
    > > > > > > > return ret;
    > > > > > > > }
    > > > > > > >
    > > > > > > > +static noinline blk_qc_t submit_bio_interposed(struct bio *bio)
    > > > > > > > +{
    > > > > > > > + blk_qc_t ret = BLK_QC_T_NONE;
    > > > > > > > + struct bio_list bio_list[2] = { };
    > > > > > > > + struct gendisk *orig_disk;
    > > > > > > > +
    > > > > > > > + if (current->bio_list) {
    > > > > > > > + bio_list_add(&current->bio_list[0], bio);
    > > > > > > > + return BLK_QC_T_NONE;
    > > > > > > > + }
    > > > > > > > +
    > > > > > > > + orig_disk = bio->bi_bdev->bd_disk;
    > > > > > > > + if (unlikely(bio_queue_enter(bio)))
    > > > > > > > + return BLK_QC_T_NONE;
    > > > > > > > +
    > > > > > > > + current->bio_list = bio_list;
    > > > > > > > +
    > > > > > > > + do {
    > > > > > > > + struct block_device *interposer = bio->bi_bdev->bd_interposer;
    > > > > > > > +
    > > > > > > > + if (unlikely(!interposer)) {
    > > > > > > > + /* interposer was removed */
    > > > > > > > + bio_list_add(&current->bio_list[0], bio);
    > > > > > > > + break;
    > > > > > > > + }
    > > > > > > > + /* assign bio to interposer device */
    > > > > > > > + bio_set_dev(bio, interposer);
    > > > > > > > + bio_set_flag(bio, BIO_INTERPOSED);
    > > > > > > > +
    > > > > > > > + if (!submit_bio_checks(bio))
    > > > > > > > + break;
    > > > > > > > + /*
    > > > > > > > + * Because the current->bio_list is initialized,
    > > > > > > > + * the submit_bio callback will always return BLK_QC_T_NONE.
    > > > > > > > + */
    > > > > > > > + interposer->bd_disk->fops->submit_bio(bio);
    > > > > > >
    > > > > > > Given original request queue may become live when calling attach() and
    > > > > > > detach(), see below comment. bdev_interposer_detach() may be run
    > > > > > > when running ->submit_bio(), meantime the interposer device is
    > > > > > > gone during the period, then kernel oops.
    > > > > >
    > > > > > I think that since the bio_queue_enter() function was called,
    > > > > > q->q_usage_counter will not allow the critical code in the attach/detach
    > > > > > functions to be executed, which is located between the blk_freeze_queue
    > > > > > and blk_unfreeze_queue calls.
    > > > > > Please correct me if I'm wrong.
    > > > > >
    > > > > > >
    > > > > > > > + } while (false);
    > > > > > > > +
    > > > > > > > + current->bio_list = NULL;
    > > > > > > > +
    > > > > > > > + blk_queue_exit(orig_disk->queue);
    > > > > > > > +
    > > > > > > > + /* Resubmit remaining bios */
    > > > > > > > + while ((bio = bio_list_pop(&bio_list[0])))
    > > > > > > > + ret = submit_bio_noacct(bio);
    > > > > > > > +
    > > > > > > > + return ret;
    > > > > > > > +}
    > > > > > > > +
    > > > > > > > /**
    > > > > > > > * submit_bio_noacct - re-submit a bio to the block device layer for I/O
    > > > > > > > * @bio: The bio describing the location in memory and on the device.
    > > > > > > > @@ -1029,6 +1078,14 @@ static blk_qc_t __submit_bio_noacct_mq(struct bio *bio)
    > > > > > > > */
    > > > > > > > blk_qc_t submit_bio_noacct(struct bio *bio)
    > > > > > > > {
    > > > > > > > + /*
    > > > > > > > + * Checking the BIO_INTERPOSED flag is necessary so that the bio
    > > > > > > > + * created by the bdev_interposer do not get to it for processing.
    > > > > > > > + */
    > > > > > > > + if (bdev_has_interposer(bio->bi_bdev) &&
    > > > > > > > + !bio_flagged(bio, BIO_INTERPOSED))
    > > > > > > > + return submit_bio_interposed(bio);
    > > > > > > > +
    > > > > > > > if (!submit_bio_checks(bio))
    > > > > > > > return BLK_QC_T_NONE;
    > > > > > > >
    > > > > > > > diff --git a/block/genhd.c b/block/genhd.c
    > > > > > > > index c55e8f0fced1..c840ecffea68 100644
    > > > > > > > --- a/block/genhd.c
    > > > > > > > +++ b/block/genhd.c
    > > > > > > > @@ -30,6 +30,11 @@
    > > > > > > > static struct kobject *block_depr;
    > > > > > > >
    > > > > > > > DECLARE_RWSEM(bdev_lookup_sem);
    > > > > > > > +/*
    > > > > > > > + * Prevents different block-layer interposers from attaching or detaching
    > > > > > > > + * to the block device at the same time.
    > > > > > > > + */
    > > > > > > > +static DEFINE_MUTEX(bdev_interposer_attach_lock);
    > > > > > > >
    > > > > > > > /* for extended dynamic devt allocation, currently only one major is used */
    > > > > > > > #define NR_EXT_DEVT (1 << MINORBITS)
    > > > > > > > @@ -1940,3 +1945,52 @@ static void disk_release_events(struct gendisk *disk)
    > > > > > > > WARN_ON_ONCE(disk->ev && disk->ev->block != 1);
    > > > > > > > kfree(disk->ev);
    > > > > > > > }
    > > > > > > > +
    > > > > > > > +int bdev_interposer_attach(struct block_device *original,
    > > > > > > > + struct block_device *interposer)
    > > > > > > > +{
    > > > > > > > + int ret = 0;
    > > > > > > > +
    > > > > > > > + if (WARN_ON(((!original) || (!interposer))))
    > > > > > > > + return -EINVAL;
    > > > > > > > + /*
    > > > > > > > + * interposer should be simple, no a multi-queue device
    > > > > > > > + */
    > > > > > > > + if (!interposer->bd_disk->fops->submit_bio)
    > > > > > > > + return -EINVAL;
    > > > > > > > +
    > > > > > > > + if (WARN_ON(!blk_mq_is_queue_frozen(original->bd_disk->queue)))
    > > > > > > > + return -EPERM;
    > > > > > >
    > > > > > > The original request queue may become live now...
    > > > > >
    > > > > > Yes.
    > > > > > I will remove the blk_mq_is_queue_frozen() function and use a different
    > > > > > approach.
    > > > >
    > > > > Looks what attach and detach needs is that queue is kept as frozen state
    > > > > instead of being froze simply at the beginning of the two functions, so
    > > > > you can simply call freeze/unfreeze inside the two functions.
    > > > >
    > > > > But what if 'original' isn't a MQ queue? queue usage counter is just
    > > > > grabed when calling ->submit_bio(), and queue freeze doesn't guarantee there
    > > > > isn't any io activity, is that a problem for bdev_interposer use case?
    > > > >
    > > > > --
    > > > > Ming
    > > > >
    > > >
    > > > It makes sense to add freeze_bdev/thaw_bdev. This will be useful.
    > > > For the main file systems, the freeze functions are defined
    > > > sb->s_op->freeze_super() or sb - >s_op->freeze_fs()
    > > > (btrfs, ext2, ext4, f2fs, jfs, nilfs2, reiserfs, xfs).
    > > > If the file system is frozen, then no new requests should be received.
    > > >
    > > > But if the file system does not support freeze or the disk is used without
    > > > a file system, as for some databases, freeze_bdev seems useless to me.
    > > > In this case, we will need to stop working with the disk from user-space,
    > > > for example, to freeze the database itself.
    > > >
    > > > I can add dm_suspend() before bdev_interposer_detach(). This will ensure that
    > > > all intercepted requests have been processed. Applying dm_suspend() before
    > > > bdev_interposer_attach() is pointless. The attachment is made when the target
    > > > is created, and at this time the target is not ready to work yet.
    > > > There shouldn't be any bio requests, I suppose. In addition,
    > > > sb->s_op->freeze_fs() for the interposer will not be called, because the file
    > > > system is not mounted for the interposer device. It should not be able to
    > > > be mounted. To do this, I will add an exclusive opening of the interposer
    > > > device.
    > > >
    > > > I'll add freeze_bdev() for the original device and dm_suspend() for the
    > > > interposer to the DM code. For normal operation of bdev_interposer,
    > > > it is enough to transfer blk_mq_freeze_queue and blk_mq_quiesce_queue to
    > > > bdev_interposer_attach/bdev_interposer_detach.
    > > > The lock on the counter q->q_usage_counter is enough to not catch NULL in
    > > > bd_interposer.
    > > >
    > > > Do you think this is enough?
    > > > I think there are no other ways to stop the block device queue.
    > >
    > > Either you're pretty confused, or I am... regardless.. I think we need
    > > to cover the basics of how interposer is expected to be paired with
    > > an "original" device.
    >
    > Thank you Mike for your patience. I really appreciate it.
    > I really may not understand something. Let me get this straight.
    >
    > >
    > > Those "original" device are already active and potentially in use
    > > right? They may be either request-based blk-mq _or_ bio-based.
    >
    > Yes. Exactly.
    >
    > >
    > > So what confuses me is that you're making assertions about how actively
    > > used bio-based DM devices aren't in use until the interposed device
    > > create happens... this is all getting very muddled.
    >
    > The original device is indeed already actively used and already mounted.
    > This is most likely not a DM device.
    > If it is a request-based blk-mq, then it is enough to stop its queue by
    > blk_mq_freeze_queue().
    > If it is a bio-based device, then we can try to stop it by freeze_bdev.
    > But in both cases, if the blk_mq_freeze_bdev() function was called, bio cannot
    > get into the critical section between bio_queue_enter() and blk_queue_exit().
    > This allows to safely change the value of original->bd_interposer.

    Even though bios cannot get into underlying blk-mq they are already
    inflight on behalf of the upper-layer bio-based device. I'll look closer
    at the code but it seems like there is potential for the original
    device's bios to still be queued to original, past the ->submit_bio
    entry, and waiting for blk-mq to unfreeze. Meaning upon return from
    what I _think_ you're saying will be sufficient: DM bio-based device
    will carry on submitting IO to the blk-mq device that has since been
    interposed.. that IO will _not_ complete in terms of the interposed
    device.. so you'll have a split-brain dual completion of IO from the
    original bio-based DM device _and_ the interposed device (for any new io
    that hits ->submit_bio after the interposed device is in place).

    I think you need to have original bio-based DM suspend, interpose
    device, and then resume the original. Anything entering original's
    ->submit_bio from that point will all get sent to interposed
    device. Right?

    > To intercept requests to the original device, we create a new md with
    > the DM_INTERPOSE_FLAG flag. It is this interposer device that has not
    > yet been initialized by this time. It just runs DM_TABLE_LOAD_CMD.
    > That is why I think that the queue of this device should not be stopped,
    > since this device has not yet been initialized.
    >
    > >
    > > And your lack of understanding of these various IO flushing methods
    > > (freeze/thaw, suspend/resume, etc) is showing. Please slow down and
    > > approach this more systematically.
    >
    > For any block device, we can call the freeze_bdev() function. It will
    > allow to wait until the processing of previously sent requests is
    > completed and block the sending of new ones. blk_mq_freeze_queue()
    > allows to change the bd_interposer variable. This allow to attach/detach
    > the interposer to original device.

    freeze_bdev/thaw_bdev are only relevant if a filesystem is layered
    ontop. A bio-based DM device can be used directly (by a database or
    whatever).

    > dm_suspend() is used to stop mapped device. This is what I plan to use
    > before detaching the interposer. It will allow to wait for the
    > completion of all the bios that were sent for the interposer.

    Yes, but you need to suspend before attaching the interposer too, to
    flush any in-flight bios that might be in-flight within the various DM
    target code.

    DM should be able to internalize all this when handling the
    DM_INTERPOSE_FLAG during the new table load. It'd call into
    dm_internal_suspend_fast and then dm_internal_resume_fast for the
    original md device.

    Mike

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