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Message-ID: <20210316163544.GA31272@veeam.com>
Date: Tue, 16 Mar 2021 19:35:44 +0300
From: Sergei Shtepa <sergei.shtepa@...am.com>
To: Ming Lei <ming.lei@...hat.com>
CC: Christoph Hellwig <hch@...radead.org>,
Mike Snitzer <snitzer@...hat.com>,
Alasdair Kergon <agk@...hat.com>,
Hannes Reinecke <hare@...e.de>, Jens Axboe <axboe@...nel.dk>,
"dm-devel@...hat.com" <dm-devel@...hat.com>,
"linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
Pavel Tide <Pavel.TIde@...am.com>
Subject: Re: [PATCH v7 2/3] block: add bdev_interposer
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@...am.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(¤t->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(¤t->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.
>
> > +
> > + mutex_lock(&bdev_interposer_attach_lock);
> > +
> > + if (bdev_has_interposer(original))
> > + ret = -EBUSY;
> > + else {
> > + original->bd_interposer = bdgrab(interposer);
> > + if (!original->bd_interposer)
> > + ret = -ENODEV;
> > + }
> > +
> > + mutex_unlock(&bdev_interposer_attach_lock);
> > +
> > + return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(bdev_interposer_attach);
> > +
> > +void bdev_interposer_detach(struct block_device *original)
> > +{
> > + if (WARN_ON(!original))
> > + return;
> > +
> > + if (WARN_ON(!blk_mq_is_queue_frozen(original->bd_disk->queue)))
> > + return;
>
> The original request queue may become live now...
>
>
> --
> Ming
>
--
Sergei Shtepa
Veeam Software developer.
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