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SubjectRe: dm: add secdel target
On Sun, Oct 14 2018 at  7:24am -0400,
Vitaly Chikunov <vt@altlinux.org> wrote:

> Report to the upper level ability to discard, and translate arriving
> discards to the writes of random or zero data to the underlying level.
>
> Signed-off-by: Vitaly Chikunov <vt@altlinux.org>
> ---
> This target is the same as the linear target except that is reports ability to
> discard to the upper level and translates arriving discards into sector
> overwrites with random (or zero) data.

There is a fair amount of code duplication between dm-linear.c and this
new target.

Something needs to give, ideally you'd factor out methods that are
shared by both targets, but those methods must _not_ introduce overhead
to dm-linear.

Could be that dm-linear methods just get called by the wrapper
dm-sec-erase target (more on the "dm-sec-erase" name below).

> The target does not try to determine if the underlying drive reliably supports
> data overwrites, this decision is solely on the discretion of a user.
>
> It may be useful to create a secure deletion setup when filesystem when
> unlinking a file sends discards to its sectors, in this target they are
> translated to writes that wipe deleted data on the underlying drive.
>
> Tested on x86.

All of this extra context and explanation needs to be captured in the
actual patch header. Not as a tangent in that "cut" section of your
patch header.

> Documentation/device-mapper/dm-secdel.txt | 24 ++
> drivers/md/Kconfig | 14 ++
> drivers/md/Makefile | 2 +
> drivers/md/dm-secdel.c | 399 ++++++++++++++++++++++++++++++
> 4 files changed, 439 insertions(+)
> create mode 100644 Documentation/device-mapper/dm-secdel.txt
> create mode 100644 drivers/md/dm-secdel.c

<snip>

Shouldn't this target be implementing all that is needed for
REQ_OP_SECURE_ERASE support? And the resulting DM device would
advertise its capability using QUEUE_FLAG_SECERASE?

And this is why I think the target should be named "dm-sec-erase" or
even "dm-secure-erase".

> diff --git a/drivers/md/dm-secdel.c b/drivers/md/dm-secdel.c
> new file mode 100644
> index 000000000000..9aeaf3f243c0
> --- /dev/null
> +++ b/drivers/md/dm-secdel.c

<snip>

> +/*
> + * Send amount of masking data to the device
> + * @mode: 0 to write zeros, otherwise to write random data
> + */
> +static int issue_erase(struct block_device *bdev, sector_t sector,
> + sector_t nr_sects, gfp_t gfp_mask, enum secdel_mode mode)
> +{
> + int ret = 0;
> +
> + while (nr_sects) {
> + struct bio *bio;
> + unsigned int nrvecs = min(nr_sects,
> + (sector_t)BIO_MAX_PAGES >> 3);
> +
> + bio = bio_alloc(gfp_mask, nrvecs);

You should probably be using your own bioset to allocate these bios.

> + if (!bio) {
> + DMERR("%s %lu[%lu]: no memory to allocate bio (%u)",
> + __func__, sector, nr_sects, nrvecs);
> + ret = -ENOMEM;
> + break;
> + }
> +
> + bio->bi_iter.bi_sector = sector;
> + bio_set_dev(bio, bdev);
> + bio->bi_end_io = bio_end_erase;
> +
> + while (nr_sects != 0) {
> + unsigned int sn;
> + struct page *page = NULL;
> +
> + sn = min((sector_t)PAGE_SIZE >> 9, nr_sects);
> + if (mode == SECDEL_MODE_RAND) {
> + page = alloc_page(gfp_mask);
> + if (!page) {
> + DMERR("%s %lu[%lu]: no memory to allocate page for random data",
> + __func__, sector, nr_sects);
> + /* will fallback to zero filling */

In general, performing memory allocations to service IO is something all
DM core and DM targets must work to avoid. This smells bad.

...

> +
> +/* convert discards into writes */
> +static int secdel_map_discard(struct dm_target *ti, struct bio *sbio)
> +{
> + struct secdel_c *lc = ti->private;
> + struct block_device *bdev = lc->dev->bdev;
> + sector_t sector = sbio->bi_iter.bi_sector;
> + sector_t nr_sects = bio_sectors(sbio);
> +
> + lc->requests++;
> + if (!bio_sectors(sbio))
> + return 0;
> + if (!op_discard(sbio))
> + return 0;
> + lc->discards++;
> + if (WARN_ON(sbio->bi_vcnt != 0))
> + return -1;
> + DMDEBUG("DISCARD %lu: %u sectors M%d", sbio->bi_iter.bi_sector,
> + bio_sectors(sbio), lc->mode);
> + bio_endio(sbio);
> +
> + if (issue_erase(bdev, sector, nr_sects, GFP_NOFS, lc->mode))

At a minimum this should be GFP_NOIO. You don't want to recurse into
block (and potentially yourself) in the face of low memory.

> +static int secdel_end_io(struct dm_target *ti, struct bio *bio,
> + blk_status_t *error)
> +{
> + struct secdel_c *lc = ti->private;
> +
> + if (!*error && bio_op(bio) == REQ_OP_ZONE_REPORT)
> + dm_remap_zone_report(ti, bio, lc->start);
> +
> + return DM_ENDIO_DONE;
> +}

Perfect example of why this new target shouldn't be doing a point in
time copy of dm-linear code.

With 4.20's upcoming zoned device changes dm-linear no longer even has
an end_io hook (which was introduced purely for REQ_OP_ZONE_REPORT's
benefit).

Mike

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