Re: [dm-devel] dm crypt: initialize tasklet in crypt_io_init()

[Mike Snitzer <snitzer@kernel.org>]

On Wed, Mar 08 2023 at  8:55P -0500,
Ignat Korchagin <ignat@cloudflare.com> wrote:

> On Wed, Mar 8, 2023 at 2:56 AM Hou Tao <houtao@huaweicloud.com> wrote:
> >
> > 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_queue
> > >>>> instead of current context unnecessarily.
> > >>> Have you actually experienced this?
> > >> Yes. I had written a bpftrace script to check the completion context of
> > >> blkdev_bio_end_io_simple() when doing direct io read on dm-crypt device. The
> > >> expected context should be unbound workers of crypt_queue, but sometimes 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.

OK.

> > > 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 *io, struct crypt_config *cc,
> > >       io->sector = sector;
> > >       io->error = 0;
> > >       io->ctx.r.req = 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 = NULL;
> > >       io->integrity_metadata_from_pool = 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 = 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().

Good point, while I think it is probably acceptable given the worst
case is punting the bio_endio to a workqueue for a time ...

> 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.

... I think we should certainly evaluate the use of an extra flag.

Ignat: I'll have a look at implementing it but if you have a patch
already developed please do share.

Thanks,
Mike

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