linux-kernel - Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze

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Message-ID: <CAB0TPYEfGYWcUNAZCNAJDAKCUDp+XdiUstS+cDkfeeJe7q8xEw@mail.gmail.com>
Date:   Mon, 19 Aug 2019 11:26:06 +0200
From:   Martijn Coenen <maco@...roid.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg KH <gregkh@...uxfoundation.org>, kernel-team@...roid.com,
        Narayan Kamath <narayan@...gle.com>,
        Dario Freni <dariofreni@...gle.com>,
        Nikita Ioffe <ioffe@...gle.com>,
        Jiyong Park <jiyong@...gle.com>,
        Martijn Coenen <maco@...gle.com>
Subject: Re: [PATCH] RFC: loop: Avoid calling blk_mq_freeze_queue() when possible.

On Mon, Aug 19, 2019 at 11:06 AM Martijn Coenen <maco@...roid.com> wrote:
> One idea to fix is to call blk_queue_logical_block_size() as part of
> LOOP_SET_FD, to match the block size of the backing fs in case the
> backing file is opened with O_DIRECT; you could argue that if the
> backing file is opened with O_DIRECT, this is what the user wanted
> anyway. This would allow us to get rid of the latter two ioctl's and
> already save quite some time.

Basically:

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index ab7ca5989097a..ad3db72fbd729 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -994,6 +994,12 @@ static int loop_set_fd(struct loop_device *lo,
fmode_t mode,
        if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
                blk_queue_write_cache(lo->lo_queue, true, false);

+       if(io_is_direct(lo->lo_backing_file) && inode->i_sb->s_bdev) {
+               /* In case of direct I/O, match underlying block size */
+               blk_queue_logical_block_size(lo->lo_queue,
+                               bdev_logical_block_size(inode->i_sb->s_bdev));
+       }
+
        loop_update_rotational(lo);
        loop_update_dio(lo);

>
> Thanks,
> Martijn
>
> >
> > Something like the following patch:
> >
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index a7461f482467..8791f9242583 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -1015,6 +1015,9 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> >          */
> >         bdgrab(bdev);
> >         mutex_unlock(&loop_ctl_mutex);
> > +
> > +       percpu_ref_switch_to_percpu(&lo->lo_queue->q_usage_counter);
> > +
> >         if (partscan)
> >                 loop_reread_partitions(lo, bdev);
> >         if (claimed_bdev)
> > @@ -1171,6 +1174,8 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
> >         lo->lo_state = Lo_unbound;
> >         mutex_unlock(&loop_ctl_mutex);
> >
> > +       percpu_ref_switch_to_atomic(&lo->lo_queue->q_usage_counter, NULL);
> > +
> >         /*
> >          * Need not hold loop_ctl_mutex to fput backing file.
> >          * Calling fput holding loop_ctl_mutex triggers a circular
> > @@ -2003,6 +2008,12 @@ static int loop_add(struct loop_device **l, int i)
> >         }
> >         lo->lo_queue->queuedata = lo;
> >
> > +       /*
> > +        * cheat block layer for not switching to q_usage_counter's
> > +        * percpu mode before loop becomes Lo_bound
> > +        */
> > +       blk_queue_flag_set(QUEUE_FLAG_INIT_DONE, lo->lo_queue);
> > +
> >         blk_queue_max_hw_sectors(lo->lo_queue, BLK_DEF_MAX_SECTORS);
> >
> >         /*
> >
> >
> > thanks,
> > Ming