[PATCH v3 16/16] bcache: avoid extra memory consumption in struct bbio for large bucket size

colyli
2020-07-15T14:31:07Z

Bcache uses struct bbio to do I/Os for meta data pages like uuids,
disk_buckets, prio_buckets, and btree nodes.

Example writing a btree node onto cache device, the process is,
- Allocate a struct bbio from mempool c->bio_meta.
- Inside struct bbio embedded a struct bio, initialize bi_inline_vecs
for this embedded bio.
- Call bch_bio_map() to map each meta data page to each bv from the
inlined bi_io_vec table.
- Call bch_submit_bbio() to submit the bio into underlying block layer.
- When the I/O completed, only release the struct bbio, don't touch the
reference counter of the meta data pages.

The struct bbio is defined as,
738 struct bbio {
739 unsigned int submit_time_us;
[snipped]
748 struct bio bio;
749 };

Because struct bio is embedded at the end of struct bbio, therefore the
actual size of struct bbio is sizeof(struct bio) + size of the embedded
bio->bi_inline_vecs.

Now all the meta data bucket size are limited to meta_bucket_pages(), if
the bucket size is large than meta_bucket_pages()*PAGE_SECTORS, rested
space in the bucket is unused. Therefore the most used space in meta
bucket is (1<<MAX_ORDER) pages, or (1<<CONFIG_FORCE_MAX_ZONEORDER) if it
is configured.

Therefore for large bucket size, it is unnecessary to calculate the
allocation size of mempool c->bio_meta as,
mempool_init_kmalloc_pool(&c->bio_meta, 2,
sizeof(struct bbio) +
sizeof(struct bio_vec) * bucket_pages(c))
It is too large, neither the Linux buddy allocator cannot allocate so
much continuous pages, nor the extra allocated pages are wasted.

This patch replace bucket_pages() to meta_bucket_pages() in two places,
- In bch_cache_set_alloc(), when initialize mempool c->bio_meta, uses
sizeof(struct bbio) + sizeof(struct bio_vec) * bucket_pages(c) to set
the allocating object size.
- In bch_bbio_alloc(), when calling bio_init() to set inline bvec talbe
bi_inline_bvecs, uses meta_bucket_pages() to indicate number of the
inline bio vencs number.

Now the maximum size of embedded bio inside struct bbio exactly matches
the limit of meta_bucket_pages(), no extra page wasted.

Signed-off-by: Coly Li <colyli@suse.de>
---
drivers/md/bcache/io.c | 2 +-
drivers/md/bcache/super.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/md/bcache/io.c b/drivers/md/bcache/io.c
index b25ee33b0d0b..a14a445618b4 100644
--- a/drivers/md/bcache/io.c
+++ b/drivers/md/bcache/io.c
@@ -26,7 +26,7 @@ struct bio *bch_bbio_alloc(struct cache_set *c)
struct bbio *b = mempool_alloc(&c->bio_meta, GFP_NOIO);
struct bio *bio = &b->bio;

- bio_init(bio, bio->bi_inline_vecs, bucket_pages(c));
+ bio_init(bio, bio->bi_inline_vecs, meta_bucket_pages(&c->sb));

return bio;
}
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 90494c7dead8..cade3f09661d 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1920,7 +1920,7 @@ struct cache_set *bch_cache_set_alloc(struct cache_sb *sb)

if (mempool_init_kmalloc_pool(&c->bio_meta, 2,
sizeof(struct bbio) +
- sizeof(struct bio_vec) * bucket_pages(c)))
+ sizeof(struct bio_vec) * meta_bucket_pages(&c->sb)))
goto err;

if (mempool_init_kmalloc_pool(&c->fill_iter, 1, iter_size))
--
2.26.2