Re: [PATCH v7 1/8] block: ensure bios are not split in middle of crypto data unit

Eric Biggers
2020-11-25T22:12:40Z

On Wed, Nov 18, 2020 at 12:38:15AM +0000, Satya Tangirala wrote:
> > > +/**
> > > + * update_aligned_sectors_and_segs() - Ensures that *@aligned_sectors is aligned
> > > + * to @bio_sectors_alignment, and that
> > > + * *@aligned_segs is the value of nsegs
> > > + * when sectors reached/first exceeded that
> > > + * value of *@aligned_sectors.
> > > + *
> > > + * @nsegs: [in] The current number of segs
> > > + * @sectors: [in] The current number of sectors
> > > + * @aligned_segs: [in,out] The number of segments that make up @aligned_sectors
> > > + * @aligned_sectors: [in,out] The largest number of sectors <= @sectors that is
> > > + * aligned to @sectors
> > > + * @bio_sectors_alignment: [in] The alignment requirement for the number of
> > > + * sectors
> > > + *
> > > + * Updates *@aligned_sectors to the largest number <= @sectors that is also a
> > > + * multiple of @bio_sectors_alignment. This is done by updating *@aligned_sectors
> > > + * whenever @sectors is at least @bio_sectors_alignment more than
> > > + * *@aligned_sectors, since that means we can increment *@aligned_sectors while
> > > + * still keeping it aligned to @bio_sectors_alignment and also keeping it <=
> > > + * @sectors. *@aligned_segs is updated to the value of nsegs when @sectors first
> > > + * reaches/exceeds any value that causes *@aligned_sectors to be updated.
> > > + */
> > > +static inline void update_aligned_sectors_and_segs(const unsigned int nsegs,
> > > + const unsigned int sectors,
> > > + unsigned int *aligned_segs,
> > > + unsigned int *aligned_sectors,
> > > + const unsigned int bio_sectors_alignment)
> > > +{
> > > + if (sectors - *aligned_sectors < bio_sectors_alignment)
> > > + return;
> > > + *aligned_sectors = round_down(sectors, bio_sectors_alignment);
> > > + *aligned_segs = nsegs;
> > > +}
> > > +
> > > /**
> > > * bvec_split_segs - verify whether or not a bvec should be split in the middle
> > > * @q: [in] request queue associated with the bio associated with @bv
> > > @@ -195,9 +232,12 @@ static inline unsigned get_max_segment_size(const struct request_queue *q,
> > > * the block driver.
> > > */
> > > static bool bvec_split_segs(const struct request_queue *q,
> > > - const struct bio_vec *bv, unsigned *nsegs,
> > > - unsigned *sectors, unsigned max_segs,
> > > - unsigned max_sectors)
> > > + const struct bio_vec *bv, unsigned int *nsegs,
> > > + unsigned int *sectors, unsigned int *aligned_segs,
> > > + unsigned int *aligned_sectors,
> > > + unsigned int bio_sectors_alignment,
> > > + unsigned int max_segs,
> > > + unsigned int max_sectors)
> > > {
> > > unsigned max_len = (min(max_sectors, UINT_MAX >> 9) - *sectors) << 9;
> > > unsigned len = min(bv->bv_len, max_len);
> > > @@ -211,6 +251,11 @@ static bool bvec_split_segs(const struct request_queue *q,
> > >
> > > (*nsegs)++;
> > > total_len += seg_size;
> > > + update_aligned_sectors_and_segs(*nsegs,
> > > + *sectors + (total_len >> 9),
> > > + aligned_segs,
> > > + aligned_sectors,
> > > + bio_sectors_alignment);
> > > len -= seg_size;
> > >
> > > if ((bv->bv_offset + total_len) & queue_virt_boundary(q))
> > > @@ -235,6 +280,8 @@ static bool bvec_split_segs(const struct request_queue *q,
> > > * following is guaranteed for the cloned bio:
> > > * - That it has at most get_max_io_size(@q, @bio) sectors.
> > > * - That it has at most queue_max_segments(@q) segments.
> > > + * - That the number of sectors in the returned bio is aligned to
> > > + * blk_crypto_bio_sectors_alignment(@bio)
> > > *
> > > * Except for discard requests the cloned bio will point at the bi_io_vec of
> > > * the original bio. It is the responsibility of the caller to ensure that the
> > > @@ -252,6 +299,9 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> > > unsigned nsegs = 0, sectors = 0;
> > > const unsigned max_sectors = get_max_io_size(q, bio);
> > > const unsigned max_segs = queue_max_segments(q);
> > > + const unsigned int bio_sectors_alignment =
> > > + blk_crypto_bio_sectors_alignment(bio);
> > > + unsigned int aligned_segs = 0, aligned_sectors = 0;
> > >
> > > bio_for_each_bvec(bv, bio, iter) {
> > > /*
> > > @@ -266,8 +316,14 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> > > bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
> > > nsegs++;
> > > sectors += bv.bv_len >> 9;
> > > - } else if (bvec_split_segs(q, &bv, &nsegs, &sectors, max_segs,
> > > - max_sectors)) {
> > > + update_aligned_sectors_and_segs(nsegs, sectors,
> > > + &aligned_segs,
> > > + &aligned_sectors,
> > > + bio_sectors_alignment);
> > > + } else if (bvec_split_segs(q, &bv, &nsegs, &sectors,
> > > + &aligned_segs, &aligned_sectors,
> > > + bio_sectors_alignment, max_segs,
> > > + max_sectors)) {
> > > goto split;
> > > }
> > >
> > > @@ -275,11 +331,24 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
> > > bvprvp = &bvprv;
> > > }
> > >
> > > + /*
> > > + * The input bio's number of sectors is assumed to be aligned to
> > > + * bio_sectors_alignment. If that's the case, then this function should
> > > + * ensure that aligned_segs == nsegs and aligned_sectors == sectors if
> > > + * the bio is not going to be split.
> > > + */
> > > + WARN_ON(aligned_segs != nsegs || aligned_sectors != sectors);
> > > *segs = nsegs;
> > > return NULL;
> > > split:
> > > - *segs = nsegs;
> > > - return bio_split(bio, sectors, GFP_NOIO, bs);
> > > + *segs = aligned_segs;
> > > + if (WARN_ON(aligned_sectors == 0))
> > > + goto err;
> > > + return bio_split(bio, aligned_sectors, GFP_NOIO, bs);
> > > +err:
> > > + bio->bi_status = BLK_STS_IOERR;
> > > + bio_endio(bio);
> > > + return bio;
> > > }
> >
> > This part is pretty complex. Are you sure it's needed? How was alignment to
> > logical_block_size ensured before?
> >
> Afaict, alignment to logical_block_size (lbs) is done by assuming that
> bv->bv_len is always lbs aligned (among other things). Is that not the
> case?

I believe that's the case; bvecs are logical_block_size aligned.

So the new thing (with data_unit_size > logical_block_size) is that
bvec boundaries aren't necessarily valid split points anymore.

>
> If it is the case, that's what we're trying to avoid with this patch (we
> want to be able to submit bios that have 2 bvecs that together make up a
> single crypto data unit, for example). And this is complex because
> multiple segments could "add up" to make up a single crypto data unit,
> but this function's job is to limit both the number of segments *and*
> the number of sectors - so when ensuring that the number of sectors is
> aligned to crypto data unit size, we also want the smallest number of
> segments that can make up that aligned number of sectors.

Does the number of physical segments that is calculated have to be exact, or
could it be a slight overestimate? If the purpose of the calculation is just to
size scatterlists and to avoid exceeding the hardware limit on the number of
physical segments (and at a quick glance that seems to be the purpose, though I
didn't look at everything), it seems that a slight overestimate would be okay.

If so, couldn't the number of sectors could simply be rounded down to
blk_crypto_bio_sectors_alignment(bio) when blk_bio_segment_split() actually
calls bio_split()? That would be much simpler; why doesn't that work?

- Eric