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From:   Ignat Korchagin <ignat@cloudflare.com>
Date:   Wed, 8 Mar 2023 13:55:34 +0000
Message-ID: <CALrw=nFrbWF2ZhQtK9gx5ZFHK4Cd9outwEQqByJgmb7ryOoCgA@mail.gmail.com>
Subject: Re: dm crypt: initialize tasklet in crypt_io_init()
To:     Hou Tao <houtao@huaweicloud.com>
Cc:     Mike Snitzer <snitzer@kernel.org>, dm-devel@redhat.com,
        houtao1@huawei.com, linux-kernel@vger.kernel.org,
        Alasdair Kergon <agk@redhat.com>, mpatocka@redhat.com
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On Wed, Mar 8, 2023 at 2:56=E2=80=AFAM Hou Tao <houtao@huaweicloud.com> wro=
te:
>
> Hi,
>
> On 3/7/2023 10:47 PM, Mike Snitzer wrote:
> > On Mon, Mar 06 2023 at  9:12P -0500,
> > Hou Tao <houtao@huaweicloud.com> wrote:
> >
> >> Hi,
> >>
> >> On 3/7/2023 3:31 AM, Mike Snitzer wrote:
> >>> On Mon, Mar 06 2023 at  8:49P -0500,
> >>> Hou Tao <houtao@huaweicloud.com> wrote:
> >>>
> >>>> From: Hou Tao <houtao1@huawei.com>
> >>>>
> >>>> When neither no_read_workqueue nor no_write_workqueue are enabled,
> >>>> tasklet_trylock() in crypt_dec_pending() may still return false due =
to
> >>>> an uninitialized state, and dm-crypt will do io completion in io_que=
ue
> >>>> instead of current context unnecessarily.
> >>> Have you actually experienced this?
> >> Yes. I had written a bpftrace script to check the completion context o=
f
> >> blkdev_bio_end_io_simple() when doing direct io read on dm-crypt devic=
e. The
> >> expected context should be unbound workers of crypt_queue, but sometim=
es the
> >> context is the bound worker of io_queue.
> > OK, thanks for clarifying.  Curious to know the circumstance (I
> > thought per-bio-data is zero'd -- but it may be I'm mistaken).
> The circumstance is just a normal qemu VM running the vanilla kernel for =
test
> purpose. According to the implementation of bio_alloc_bioset(), the front=
 pad of
> bio is not initialized and only bio itself is initialized. AFAIK if
> CONFIG_INIT_ON_ALLOC_DEFAULT_ON is enabled, per-bio-data may be zeroed.
> >
> > I won't be marking this commit for stable@ but if others feel
> > differently please let me know and I'll do so. (We can always propose
> > it to stable@, after the fact, even if the commit header doesn't Cc
> > stable@)
> >
> >>>> Fix it by initializing io->tasklet in crypt_io_init().
> >>> Really would rather avoid always calling tasklet_init(). But I can
> >>> optimize it away with a later patch.
> >> My first though was "io->tasklet.state =3D 0", but it may be fragile b=
ecause it
> >> operated on the internal status of tasklet, so I switch to tasklet_ini=
t().
> > Yes, I looked into it and came up with the same hack.. and I too felt
> > it was too fragile due to open-coding direct access to the tasklet's
> > members.
> >
> > I have a patch I just staged that staged that uses jump_labels to
> > optimize this code.  If you might review/test/verify it works well for
> > you that'd be appreciated:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.=
git/commit/?h=3Ddm-6.3&id=3Dae75a25bd83f7c541240449d2fff3a44433e506b
> >
> > It builds on your patch, which I added a comment to:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.=
git/commit/?h=3Ddm-6.3&id=3Dd9fe0a98a2e0a1cf585e8a6555afb33be968bd13
> Thanks for the comments. It is fine to me.
> >
> > From: Mike Snitzer <snitzer@kernel.org>
> > Date: Mon, 6 Mar 2023 15:58:33 -0500
> > Subject: [PATCH] dm crypt: conditionally enable code needed for tasklet=
 usecases
> >
> > Use jump_label to limit the need for branching, and tasklet_init(),
> > unless either of the optional "no_read_workqueue" and/or
> > "no_write_workqueue" features are used.
> >
> > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > ---
> >  drivers/md/dm-crypt.c | 35 +++++++++++++++++++++++++++--------
> >  1 file changed, 27 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
> > index 641457e72603..2d0309ca07f5 100644
> > --- a/drivers/md/dm-crypt.c
> > +++ b/drivers/md/dm-crypt.c
> > @@ -40,6 +40,7 @@
> >  #include <keys/user-type.h>
> >  #include <keys/encrypted-type.h>
> >  #include <keys/trusted-type.h>
> > +#include <linux/jump_label.h>
> >
> >  #include <linux/device-mapper.h>
> >
> > @@ -85,6 +86,8 @@ struct dm_crypt_io {
> >       struct rb_node rb_node;
> >  } CRYPTO_MINALIGN_ATTR;
> >
> > +static DEFINE_STATIC_KEY_FALSE(use_tasklet_enabled);
> > +
> >  struct dm_crypt_request {
> >       struct convert_context *ctx;
> >       struct scatterlist sg_in[4];
> > @@ -1730,12 +1733,15 @@ static void crypt_io_init(struct dm_crypt_io *i=
o, struct crypt_config *cc,
> >       io->sector =3D sector;
> >       io->error =3D 0;
> >       io->ctx.r.req =3D NULL;
> > -     /*
> > -      * tasklet_init() here to ensure crypt_dec_pending()'s
> > -      * tasklet_trylock() doesn't incorrectly return false
> > -      * even when tasklet isn't in use.
> > -      */
> > -     tasklet_init(&io->tasklet, kcryptd_crypt_tasklet, (unsigned long)=
&io->work);
> > +     if (static_branch_unlikely(&use_tasklet_enabled)) {
> > +             /*
> > +              * tasklet_init() here to ensure crypt_dec_pending()'s
> > +              * tasklet_trylock() doesn't incorrectly return false
> > +              * even when tasklet isn't in use.
> > +              */
> > +             tasklet_init(&io->tasklet, kcryptd_crypt_tasklet,
> > +                          (unsigned long)&io->work);
> > +     }
> >       io->integrity_metadata =3D NULL;
> >       io->integrity_metadata_from_pool =3D false;
> >       atomic_set(&io->io_pending, 0);
> > @@ -1775,6 +1781,10 @@ static void crypt_dec_pending(struct dm_crypt_io=
 *io)
> >               kfree(io->integrity_metadata);
> >
> >       base_bio->bi_status =3D error;
> > +     if (!static_branch_unlikely(&use_tasklet_enabled)) {
> > +             bio_endio(base_bio);
> > +             return;
> > +     }
> Because use_tasklet_enabled can be enabled concurrently, so I think it is=
 still
> possible that crypt_dec_pending will try-lock an unitialized tasklet if
> use_tasklet_enabled is enabled when invoking crypt_dec_pending().

Perhaps instead we can just pass an additional flag from
tasklet_schedule to indicate to the function that we're running in a
tasklet. I originally have chosen the tasklet_trylock/unlock hack to
avoid passing an extra flag. But unitialized memory makes sense as
well as the desire to avoid calling tasklet_init unconditionally. So
an extra member in dm_crypt_io might be the most straightforward here.

Ignat

> >       /*
> >        * If we are running this function from our tasklet,
> > @@ -2232,8 +2242,9 @@ static void kcryptd_queue_crypt(struct dm_crypt_i=
o *io)
> >  {
> >       struct crypt_config *cc =3D io->cc;
> >
> > -     if ((bio_data_dir(io->base_bio) =3D=3D READ && test_bit(DM_CRYPT_=
NO_READ_WORKQUEUE, &cc->flags)) ||
> > -         (bio_data_dir(io->base_bio) =3D=3D WRITE && test_bit(DM_CRYPT=
_NO_WRITE_WORKQUEUE, &cc->flags))) {
> > +     if (static_branch_unlikely(&use_tasklet_enabled) &&
> > +         ((bio_data_dir(io->base_bio) =3D=3D READ && test_bit(DM_CRYPT=
_NO_READ_WORKQUEUE, &cc->flags)) ||
> > +          (bio_data_dir(io->base_bio) =3D=3D WRITE && test_bit(DM_CRYP=
T_NO_WRITE_WORKQUEUE, &cc->flags)))) {
> >               /*
> >                * in_hardirq(): Crypto API's skcipher_walk_first() refus=
es to work in hard IRQ context.
> >                * irqs_disabled(): the kernel may run some IO completion=
 from the idle thread, but
> > @@ -2746,6 +2757,10 @@ static void crypt_dtr(struct dm_target *ti)
> >       crypt_calculate_pages_per_client();
> >       spin_unlock(&dm_crypt_clients_lock);
> >
> > +     if (test_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags) ||
> > +         test_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags))
> > +             static_branch_dec(&use_tasklet_enabled);
> > +
> >       dm_audit_log_dtr(DM_MSG_PREFIX, ti, 1);
> >  }
> >
> > @@ -3375,6 +3390,10 @@ static int crypt_ctr(struct dm_target *ti, unsig=
ned int argc, char **argv)
> >       ti->limit_swap_bios =3D true;
> >       ti->accounts_remapped_io =3D true;
> >
> > +     if (test_bit(DM_CRYPT_NO_READ_WORKQUEUE, &cc->flags) ||
> > +         test_bit(DM_CRYPT_NO_WRITE_WORKQUEUE, &cc->flags))
> > +             static_branch_inc(&use_tasklet_enabled);
> > +
> >       dm_audit_log_ctr(DM_MSG_PREFIX, ti, 1);
> >       return 0;
> >
>