Re: [RFC PATCH 0/1] dm-crypt excessive overhead

[Ignat Korchagin <ignat@cloudflare.com>]

Do you think it may be better to break it in two flags: one for read
path and one for write? So, depending on the needs and workflow these
could be enabled independently?

Regards,
Ignat

On Tue, Jun 23, 2020 at 4:01 PM Mike Snitzer <snitzer@redhat.com> wrote:
>
> On Sun, Jun 21 2020 at  8:45pm -0400,
> Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
>
> > On 2020/06/20 1:56, Mike Snitzer wrote:
> > > On Fri, Jun 19 2020 at 12:41pm -0400,
> > > Ignat Korchagin <ignat@cloudflare.com> wrote:
> > >
> > >> This is a follow up from the long-forgotten [1], but with some more convincing
> > >> evidence. Consider the following script:
> > >>
> > >> #!/bin/bash -e
> > >>
> > >> # create 4G ramdisk
> > >> sudo modprobe brd rd_nr=1 rd_size=4194304
> > >>
> > >> # create a dm-crypt device with NULL cipher on top of /dev/ram0
> > >> echo '0 8388608 crypt capi:ecb(cipher_null) - 0 /dev/ram0 0' | sudo dmsetup create eram0
> > >>
> > >> # create a dm-crypt device with NULL cipher and custom force_inline flag
> > >> echo '0 8388608 crypt capi:ecb(cipher_null) - 0 /dev/ram0 0 1 force_inline' | sudo dmsetup create inline-eram0
> > >>
> > >> # read all data from /dev/ram0
> > >> sudo dd if=/dev/ram0 bs=4k iflag=direct | sha256sum
> > >>
> > >> # read the same data from /dev/mapper/eram0
> > >> sudo dd if=/dev/mapper/eram0 bs=4k iflag=direct | sha256sum
> > >>
> > >> # read the same data from /dev/mapper/inline-eram0
> > >> sudo dd if=/dev/mapper/inline-eram0 bs=4k iflag=direct | sha256sum
> > >>
> > >> This script creates a ramdisk (to eliminate hardware bias in the benchmark) and
> > >> two dm-crypt instances on top. Both dm-crypt instances use the NULL cipher
> > >> to eliminate potentially expensive crypto bias (the NULL cipher just uses memcpy
> > >> for "encyption"). The first instance is the current dm-crypt implementation from
> > >> 5.8-rc1, the second is the dm-crypt instance with a custom new flag enabled from
> > >> the patch attached to this thread. On my VM (Debian in VirtualBox with 4 cores
> > >> on 2.8 GHz Quad-Core Intel Core i7) I get the following output (formatted for
> > >> better readability):
> > >>
> > >> # plain ram0
> > >> 1048576+0 records in
> > >> 1048576+0 records out
> > >> 4294967296 bytes (4.3 GB, 4.0 GiB) copied, 21.2305 s, 202 MB/s
> > >> 8479e43911dc45e89f934fe48d01297e16f51d17aa561d4d1c216b1ae0fcddca  -
> > >>
> > >> # eram0 (current dm-crypt)
> > >> 1048576+0 records in
> > >> 1048576+0 records out
> > >> 4294967296 bytes (4.3 GB, 4.0 GiB) copied, 53.2212 s, 80.7 MB/s
> > >> 8479e43911dc45e89f934fe48d01297e16f51d17aa561d4d1c216b1ae0fcddca  -
> > >>
> > >> # inline-eram0 (patched dm-crypt)
> > >> 1048576+0 records in
> > >> 1048576+0 records out
> > >> 4294967296 bytes (4.3 GB, 4.0 GiB) copied, 21.3472 s, 201 MB/s
> > >> 8479e43911dc45e89f934fe48d01297e16f51d17aa561d4d1c216b1ae0fcddca  -
> > >>
> > >> As we can see, current dm-crypt implementation creates a significant IO
> > >> performance overhead (at least on small IO block sizes) for both latency and
> > >> throughput. We suspect offloading IO request processing into workqueues and
> > >> async threads is more harmful these days with the modern fast storage. I also
> > >> did some digging into the dm-crypt git history and much of this async processing
> > >> is not needed anymore, because the reasons it was added are mostly gone from the
> > >> kernel. More details can be found in [2] (see "Git archeology" section).
> > >>
> > >> We have been running the attached patch on different hardware generations in
> > >> more than 200 datacentres on both SATA SSDs and NVME SSDs and so far were very
> > >> happy with the performance benefits.
> > >>
> > >> [1]: https://www.spinics.net/lists/dm-crypt/msg07516.html
> > >> [2]: https://blog.cloudflare.com/speeding-up-linux-disk-encryption/
> > >>
> > >> Ignat Korchagin (1):
> > >>   Add DM_CRYPT_FORCE_INLINE flag to dm-crypt target
> > >>
> > >>  drivers/md/dm-crypt.c | 55 +++++++++++++++++++++++++++++++++----------
> > >>  1 file changed, 43 insertions(+), 12 deletions(-)
> > >>
> > >> --
> > >> 2.20.1
> > >>
> > >
> > > Hi,
> > >
> > > I saw [2] and have been expecting something from cloudflare ever since.
> > > Nice to see this submission.
> > >
> > > There is useful context in your 0th patch header.  I'll likely merge
> > > parts of this patch header with the more terse 1/1 header (reality is
> > > there only needed to be a single patch submission).
> > >
> > > Will review and stage accordingly if all looks fine to me.  Mikulas,
> > > please have a look too.
> >
> > Very timely: I was about to send a couple of patches to add zoned block device
> > support to dm-crypt :)
> >
> > I used [1] work as a base to have all _write_ requests be processed inline in
> > the submitter context so that the submission order is preserved, avoiding the
> > potential reordering of sequential writes that the normal workqueue based
> > processing can generate. This inline processing is done only for writes. Reads
> > are unaffected.
> >
> > To do this, I added a "inline_io" flag to struct convert_context which is
> > initialized in crypt_io_init() based on the BIO op.
> > kcryptd_crypt_write_io_submit() then uses this flag to call directly
> > generic_make_request() if inline_io is true.
> >
> > This simplifies things compared to [1] since reads can still be processed as is,
> > so there are no issued with irq context and no need for a tasklet.
> >
> > Should I send these patches as RFC to see what can be merged ? Or I can wait for
> > these patches and rebase on top.
>
> It'd be ideal for this inline capability to address both Ignat's and
> your needs.  Given Ignat's changes _should_ enable yours (and Ignat
> clarified that having reads inline is actually important) then I think it
> best if you review Ignat's patch closely, rebase on it and test that it
> meets your needs.
>
> I'll wait for you to do this work so that I can get your feedback on
> whether Ignat's changes look good for you too.  We have some time before
> the 5.9 merge window opens, lets just keep the communication going and
> make sure what we send upstream addresses everyone's needs and concerns.
>
> Thanks,
> Mike
>