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Subject[PATCH v3 5/7] zram: support idle/huge page writeback
Date
This patch supports new feature "zram idle/huge page writeback".
On zram-swap usecase, zram has usually many idle/huge swap pages.
It's pointless to keep in memory(ie, zram).

To solve the problem, this feature introduces idle/huge page
writeback to backing device so the goal is to save more memory
space on embedded system.

Normal sequence to use idle/huge page writeback feature is as follows,

while (1) {
# mark allocated zram slot to idle
echo all > /sys/block/zram0/idle
# leave system working for several hours
# Unless there is no access for some blocks on zram,
# they are still IDLE marked pages.

echo "idle" > /sys/block/zram0/writeback
or/and
echo "huge" > /sys/block/zram0/writeback
# write the IDLE or/and huge marked slot into backing device
# and free the memory.
}

By per discussion:
https://lore.kernel.org/lkml/20181122065926.GG3441@jagdpanzerIV/T/#u,

This patch removes direct incommpressibe page writeback feature
(d2afd25114f4, zram: write incompressible pages to backing device)
so we could regard it as regression because incompressible pages
doesn't go to backing storage automatically. Instead, usre should
do it via "echo huge" > /sys/block/zram/writeback" manually.

If we hear some regression, we could restore the function.

Reviewed-by: Joey Pabalinas <joeypabalinas@gmail.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
---
Documentation/ABI/testing/sysfs-block-zram | 7 +
Documentation/blockdev/zram.txt | 28 ++-
drivers/block/zram/Kconfig | 5 +-
drivers/block/zram/zram_drv.c | 247 +++++++++++++++------
drivers/block/zram/zram_drv.h | 1 +
5 files changed, 209 insertions(+), 79 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-block-zram b/Documentation/ABI/testing/sysfs-block-zram
index 04c9a5980bc7..d1f80b077885 100644
--- a/Documentation/ABI/testing/sysfs-block-zram
+++ b/Documentation/ABI/testing/sysfs-block-zram
@@ -106,3 +106,10 @@ Contact: Minchan Kim <minchan@kernel.org>
idle file is write-only and mark zram slot as idle.
If system has mounted debugfs, user can see which slots
are idle via /sys/kernel/debug/zram/zram<id>/block_state
+
+What: /sys/block/zram<id>/writeback
+Date: November 2018
+Contact: Minchan Kim <minchan@kernel.org>
+Description:
+ The writeback file is write-only and trigger idle and/or
+ huge page writeback to backing device.
diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt
index f3bcd716d8a9..806cdaabac83 100644
--- a/Documentation/blockdev/zram.txt
+++ b/Documentation/blockdev/zram.txt
@@ -238,11 +238,31 @@ The stat file represents device's mm statistics. It consists of a single

= writeback

-With incompressible pages, there is no memory saving with zram.
-Instead, with CONFIG_ZRAM_WRITEBACK, zram can write incompressible page
+With CONFIG_ZRAM_WRITEBACK, zram can write idle/incompressible page
to backing storage rather than keeping it in memory.
-User should set up backing device via /sys/block/zramX/backing_dev
-before disksize setting.
+To use the feature, admin should set up backing device via
+
+ "echo /dev/sda5 > /sys/block/zramX/backing_dev"
+
+before disksize setting. It supports only partition at this moment.
+If admin want to use incompressible page writeback, they could do via
+
+ "echo huge > /sys/block/zramX/write"
+
+To use idle page writeback, first, user need to declare zram pages
+as idle.
+
+ "echo all > /sys/block/zramX/idle"
+
+From now on, any pages on zram are idle pages. The idle mark
+will be removed until someone request access of the block.
+IOW, unless there is access request, those pages are still idle pages.
+
+Admin can request writeback of those idle pages at right timing via
+
+ "echo idle > /sys/block/zramX/writeback"
+
+With the command, zram writeback idle pages from memory to the storage.

= memory tracking

diff --git a/drivers/block/zram/Kconfig b/drivers/block/zram/Kconfig
index fcd055457364..1ffc64770643 100644
--- a/drivers/block/zram/Kconfig
+++ b/drivers/block/zram/Kconfig
@@ -15,7 +15,7 @@ config ZRAM
See Documentation/blockdev/zram.txt for more information.

config ZRAM_WRITEBACK
- bool "Write back incompressible page to backing device"
+ bool "Write back incompressible or idle page to backing device"
depends on ZRAM
help
With incompressible page, there is no memory saving to keep it
@@ -23,6 +23,9 @@ config ZRAM_WRITEBACK
For this feature, admin should set up backing device via
/sys/block/zramX/backing_dev.

+ With /sys/block/zramX/{idle,writeback}, application could ask
+ idle page's writeback to the backing device to save in memory.
+
See Documentation/blockdev/zram.txt for more information.

config ZRAM_MEMORY_TRACKING
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 180613b478a6..6b5a886c8f32 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -52,6 +52,9 @@ static unsigned int num_devices = 1;
static size_t huge_class_size;

static void zram_free_page(struct zram *zram, size_t index);
+static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
+ u32 index, int offset, struct bio *bio);
+

static int zram_slot_trylock(struct zram *zram, u32 index)
{
@@ -73,13 +76,6 @@ static inline bool init_done(struct zram *zram)
return zram->disksize;
}

-static inline bool zram_allocated(struct zram *zram, u32 index)
-{
-
- return (zram->table[index].flags >> (ZRAM_FLAG_SHIFT + 1)) ||
- zram->table[index].handle;
-}
-
static inline struct zram *dev_to_zram(struct device *dev)
{
return (struct zram *)dev_to_disk(dev)->private_data;
@@ -138,6 +134,13 @@ static void zram_set_obj_size(struct zram *zram,
zram->table[index].flags = (flags << ZRAM_FLAG_SHIFT) | size;
}

+static inline bool zram_allocated(struct zram *zram, u32 index)
+{
+ return zram_get_obj_size(zram, index) ||
+ zram_test_flag(zram, index, ZRAM_SAME) ||
+ zram_test_flag(zram, index, ZRAM_WB);
+}
+
#if PAGE_SIZE != 4096
static inline bool is_partial_io(struct bio_vec *bvec)
{
@@ -308,10 +311,14 @@ static ssize_t idle_store(struct device *dev,
}

for (index = 0; index < nr_pages; index++) {
+ /*
+ * Do not mark ZRAM_UNDER_WB slot as ZRAM_IDLE to close race.
+ * See the comment in writeback_store.
+ */
zram_slot_lock(zram, index);
- if (!zram_allocated(zram, index))
+ if (!zram_allocated(zram, index) ||
+ zram_test_flag(zram, index, ZRAM_UNDER_WB))
goto next;
-
zram_set_flag(zram, index, ZRAM_IDLE);
next:
zram_slot_unlock(zram, index);
@@ -546,6 +553,158 @@ static int read_from_bdev_async(struct zram *zram, struct bio_vec *bvec,
return 1;
}

+#define HUGE_WRITEBACK 0x1
+#define IDLE_WRITEBACK 0x2
+
+static ssize_t writeback_store(struct device *dev,
+ struct device_attribute *attr, const char *buf, size_t len)
+{
+ struct zram *zram = dev_to_zram(dev);
+ unsigned long nr_pages = zram->disksize >> PAGE_SHIFT;
+ unsigned long index;
+ struct bio bio;
+ struct bio_vec bio_vec;
+ struct page *page;
+ ssize_t ret, sz;
+ char mode_buf[8];
+ unsigned long mode = -1UL;
+ unsigned long blk_idx = 0;
+
+ sz = strscpy(mode_buf, buf, sizeof(mode_buf));
+ if (sz <= 0)
+ return -EINVAL;
+
+ /* ignore trailing newline */
+ if (mode_buf[sz - 1] == '\n')
+ mode_buf[sz - 1] = 0x00;
+
+ if (!strcmp(mode_buf, "idle"))
+ mode = IDLE_WRITEBACK;
+ else if (!strcmp(mode_buf, "huge"))
+ mode = HUGE_WRITEBACK;
+
+ if (mode == -1UL)
+ return -EINVAL;
+
+ down_read(&zram->init_lock);
+ if (!init_done(zram)) {
+ ret = -EINVAL;
+ goto release_init_lock;
+ }
+
+ if (!zram->backing_dev) {
+ ret = -ENODEV;
+ goto release_init_lock;
+ }
+
+ page = alloc_page(GFP_KERNEL);
+ if (!page) {
+ ret = -ENOMEM;
+ goto release_init_lock;
+ }
+
+ for (index = 0; index < nr_pages; index++) {
+ struct bio_vec bvec;
+
+ bvec.bv_page = page;
+ bvec.bv_len = PAGE_SIZE;
+ bvec.bv_offset = 0;
+
+ if (!blk_idx) {
+ blk_idx = alloc_block_bdev(zram);
+ if (!blk_idx) {
+ ret = -ENOSPC;
+ break;
+ }
+ }
+
+ zram_slot_lock(zram, index);
+ if (!zram_allocated(zram, index))
+ goto next;
+
+ if (zram_test_flag(zram, index, ZRAM_WB) ||
+ zram_test_flag(zram, index, ZRAM_SAME) ||
+ zram_test_flag(zram, index, ZRAM_UNDER_WB))
+ goto next;
+
+ if ((mode & IDLE_WRITEBACK &&
+ !zram_test_flag(zram, index, ZRAM_IDLE)) &&
+ (mode & HUGE_WRITEBACK &&
+ !zram_test_flag(zram, index, ZRAM_HUGE)))
+ goto next;
+ /*
+ * Clearing ZRAM_UNDER_WB is duty of caller.
+ * IOW, zram_free_page never clear it.
+ */
+ zram_set_flag(zram, index, ZRAM_UNDER_WB);
+ /* Need for hugepage writeback racing */
+ zram_set_flag(zram, index, ZRAM_IDLE);
+ zram_slot_unlock(zram, index);
+ if (zram_bvec_read(zram, &bvec, index, 0, NULL)) {
+ zram_slot_lock(zram, index);
+ zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+ zram_clear_flag(zram, index, ZRAM_IDLE);
+ zram_slot_unlock(zram, index);
+ continue;
+ }
+
+ bio_init(&bio, &bio_vec, 1);
+ bio_set_dev(&bio, zram->bdev);
+ bio.bi_iter.bi_sector = blk_idx * (PAGE_SIZE >> 9);
+ bio.bi_opf = REQ_OP_WRITE | REQ_SYNC;
+
+ bio_add_page(&bio, bvec.bv_page, bvec.bv_len,
+ bvec.bv_offset);
+ /*
+ * XXX: A single page IO would be inefficient for write
+ * but it would be not bad as starter.
+ */
+ ret = submit_bio_wait(&bio);
+ if (ret) {
+ zram_slot_lock(zram, index);
+ zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+ zram_clear_flag(zram, index, ZRAM_IDLE);
+ zram_slot_unlock(zram, index);
+ continue;
+ }
+
+ /*
+ * We released zram_slot_lock so need to check if the slot was
+ * changed. If there is freeing for the slot, we can catch it
+ * easily by zram_allocated.
+ * A subtle case is the slot is freed/reallocated/marked as
+ * ZRAM_IDLE again. To close the race, idle_store doesn't
+ * mark ZRAM_IDLE once it found the slot was ZRAM_UNDER_WB.
+ * Thus, we could close the race by checking ZRAM_IDLE bit.
+ */
+ zram_slot_lock(zram, index);
+ if (!zram_allocated(zram, index) ||
+ !zram_test_flag(zram, index, ZRAM_IDLE)) {
+ zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+ zram_clear_flag(zram, index, ZRAM_IDLE);
+ goto next;
+ }
+
+ zram_free_page(zram, index);
+ zram_clear_flag(zram, index, ZRAM_UNDER_WB);
+ zram_set_flag(zram, index, ZRAM_WB);
+ zram_set_element(zram, index, blk_idx);
+ blk_idx = 0;
+ atomic64_inc(&zram->stats.pages_stored);
+next:
+ zram_slot_unlock(zram, index);
+ }
+
+ if (blk_idx)
+ free_block_bdev(zram, blk_idx);
+ ret = len;
+ __free_page(page);
+release_init_lock:
+ up_read(&zram->init_lock);
+
+ return ret;
+}
+
struct zram_work {
struct work_struct work;
struct zram *zram;
@@ -603,57 +762,8 @@ static int read_from_bdev(struct zram *zram, struct bio_vec *bvec,
else
return read_from_bdev_async(zram, bvec, entry, parent);
}
-
-static int write_to_bdev(struct zram *zram, struct bio_vec *bvec,
- u32 index, struct bio *parent,
- unsigned long *pentry)
-{
- struct bio *bio;
- unsigned long entry;
-
- bio = bio_alloc(GFP_ATOMIC, 1);
- if (!bio)
- return -ENOMEM;
-
- entry = alloc_block_bdev(zram);
- if (!entry) {
- bio_put(bio);
- return -ENOSPC;
- }
-
- bio->bi_iter.bi_sector = entry * (PAGE_SIZE >> 9);
- bio_set_dev(bio, zram->bdev);
- if (!bio_add_page(bio, bvec->bv_page, bvec->bv_len,
- bvec->bv_offset)) {
- bio_put(bio);
- free_block_bdev(zram, entry);
- return -EIO;
- }
-
- if (!parent) {
- bio->bi_opf = REQ_OP_WRITE | REQ_SYNC;
- bio->bi_end_io = zram_page_end_io;
- } else {
- bio->bi_opf = parent->bi_opf;
- bio_chain(bio, parent);
- }
-
- submit_bio(bio);
- *pentry = entry;
-
- return 0;
-}
-
#else
static inline void reset_bdev(struct zram *zram) {};
-static int write_to_bdev(struct zram *zram, struct bio_vec *bvec,
- u32 index, struct bio *parent,
- unsigned long *pentry)
-
-{
- return -EIO;
-}
-
static int read_from_bdev(struct zram *zram, struct bio_vec *bvec,
unsigned long entry, struct bio *parent, bool sync)
{
@@ -1006,7 +1116,8 @@ static void zram_free_page(struct zram *zram, size_t index)
atomic64_dec(&zram->stats.pages_stored);
zram_set_handle(zram, index, 0);
zram_set_obj_size(zram, index, 0);
- WARN_ON_ONCE(zram->table[index].flags & ~(1UL << ZRAM_LOCK));
+ WARN_ON_ONCE(zram->table[index].flags &
+ ~(1UL << ZRAM_LOCK | 1UL << ZRAM_UNDER_WB));
}

static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
@@ -1115,7 +1226,6 @@ static int __zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
struct page *page = bvec->bv_page;
unsigned long element = 0;
enum zram_pageflags flags = 0;
- bool allow_wb = true;

mem = kmap_atomic(page);
if (page_same_filled(mem, &element)) {
@@ -1140,21 +1250,8 @@ static int __zram_bvec_write(struct zram *zram, struct bio_vec *bvec,
return ret;
}

- if (unlikely(comp_len >= huge_class_size)) {
+ if (comp_len >= huge_class_size)
comp_len = PAGE_SIZE;
- if (zram->backing_dev && allow_wb) {
- zcomp_stream_put(zram->comp);
- ret = write_to_bdev(zram, bvec, index, bio, &element);
- if (!ret) {
- flags = ZRAM_WB;
- ret = 1;
- goto out;
- }
- allow_wb = false;
- goto compress_again;
- }
- }
-
/*
* handle allocation has 2 paths:
* a) fast path is executed with preemption disabled (for
@@ -1643,6 +1740,7 @@ static DEVICE_ATTR_RW(max_comp_streams);
static DEVICE_ATTR_RW(comp_algorithm);
#ifdef CONFIG_ZRAM_WRITEBACK
static DEVICE_ATTR_RW(backing_dev);
+static DEVICE_ATTR_WO(writeback);
#endif

static struct attribute *zram_disk_attrs[] = {
@@ -1657,6 +1755,7 @@ static struct attribute *zram_disk_attrs[] = {
&dev_attr_comp_algorithm.attr,
#ifdef CONFIG_ZRAM_WRITEBACK
&dev_attr_backing_dev.attr,
+ &dev_attr_writeback.attr,
#endif
&dev_attr_io_stat.attr,
&dev_attr_mm_stat.attr,
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index a84611b97867..1ad74f030b6d 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -47,6 +47,7 @@ enum zram_pageflags {
ZRAM_LOCK = ZRAM_FLAG_SHIFT,
ZRAM_SAME, /* Page consists the same element */
ZRAM_WB, /* page is stored on backing_device */
+ ZRAM_UNDER_WB, /* page is under writeback */
ZRAM_HUGE, /* Incompressible page */
ZRAM_IDLE, /* not accessed page since last idle marking */

--
2.20.0.rc0.387.gc7a69e6b6c-goog
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