Message-ID: <20250325034210.3337080-1-senozhatsky@chromium.org>
Date: Tue, 25 Mar 2025 12:42:04 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan@...nel.org>
Cc: Brian Geffon <bgeffon@...gle.com>,
	linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: [PATCH] zram: modernize writeback interface

The writeback interface supports a page_index=N parameter which
performs writeback of the given page.  Since we rarely need to
writeback just one single page, the typical use case involves
a number of writeback calls, each performing writeback of one
page:

    echo page_index=100 > zram0/writeback
    ...
    echo page_index=200 > zram0/writeback
    echo page_index=500 > zram0/writeback
    ...
    echo page_index=700 > zram0/writeback

One obvious downside of this is that it increases the number of
syscalls.  Less obvious, but a significantly more important downside,
is that when given only one page to post-process zram cannot perform
an optimal target selection.  This becomes a critical limitation
when writeback_limit is enabled, because under writeback_limit we
want to guarantee the highest memory savings hence we first need
to writeback pages that release the highest amount of zsmalloc pool
memory.

This patch adds page_index_range=LOW-HIGH parameter to the writeback
interface:

    echo page_index_range=100-200 \
 	 page_index_range=500-700 > zram0/writeback

This gives zram a chance to apply an optimal target selection
strategy on each iteration of the writeback loop.

Apart from that the patch also unifies parameters passing and resembles
other "modern" zram device attributes (e.g. recompression), while the
old interface used a mixed scheme: values-less parameters for mode and
a key=value format for page_index.  We still support the "old" value-less
format for compatibility reasons.

Signed-off-by: Sergey Senozhatsky <senozhatsky@...omium.org>
---
 Documentation/admin-guide/blockdev/zram.rst |  11 +
 drivers/block/zram/zram_drv.c               | 321 +++++++++++++-------
 2 files changed, 227 insertions(+), 105 deletions(-)

diff --git a/Documentation/admin-guide/blockdev/zram.rst b/Documentation/admin-guide/blockdev/zram.rst
index 9bdb30901a93..9dca86365a4d 100644
--- a/Documentation/admin-guide/blockdev/zram.rst
+++ b/Documentation/admin-guide/blockdev/zram.rst
@@ -369,6 +369,17 @@ they could write a page index into the interface::
 
 	echo "page_index=1251" > /sys/block/zramX/writeback
 
+In Linux 6.16 this interface underwent some rework.  First, the interface
+now supports `key=value` format for all of its parameters (`type=huge_idle`,
+etc.)  Second, the support for `page_index_range` was introduced, which
+specify `LOW-HIGH` range (or ranges) of pages to be written-back.  This
+reduces the number of syscalls, but more importantly this enables optimal
+post-processing target selection strategy. Usage example::
+
+	echo "type=idle" > /sys/block/zramX/writeback
+	echo "page_index_range=1-100 page_index_range=200-300" > \
+		/sys/block/zramX/writeback
+
 If there are lots of write IO with flash device, potentially, it has
 flash wearout problem so that admin needs to design write limitation
 to guarantee storage health for entire product life.
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index fda7d8624889..2c39d12bd2d4 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -734,114 +734,19 @@ static void read_from_bdev_async(struct zram *zram, struct page *page,
 	submit_bio(bio);
 }
 
-#define PAGE_WB_SIG "page_index="
-
-#define PAGE_WRITEBACK			0
-#define HUGE_WRITEBACK			(1<<0)
-#define IDLE_WRITEBACK			(1<<1)
-#define INCOMPRESSIBLE_WRITEBACK	(1<<2)
-
-static int scan_slots_for_writeback(struct zram *zram, u32 mode,
-				    unsigned long nr_pages,
-				    unsigned long index,
-				    struct zram_pp_ctl *ctl)
+static int zram_writeback_slots(struct zram *zram, struct zram_pp_ctl *ctl)
 {
-	for (; nr_pages != 0; index++, nr_pages--) {
-		bool ok = true;
-
-		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))
-			goto next;
-
-		if (mode & IDLE_WRITEBACK &&
-		    !zram_test_flag(zram, index, ZRAM_IDLE))
-			goto next;
-		if (mode & HUGE_WRITEBACK &&
-		    !zram_test_flag(zram, index, ZRAM_HUGE))
-			goto next;
-		if (mode & INCOMPRESSIBLE_WRITEBACK &&
-		    !zram_test_flag(zram, index, ZRAM_INCOMPRESSIBLE))
-			goto next;
-
-		ok = place_pp_slot(zram, ctl, index);
-next:
-		zram_slot_unlock(zram, index);
-		if (!ok)
-			break;
-	}
-
-	return 0;
-}
-
-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;
-	struct zram_pp_ctl *ctl = NULL;
+	unsigned long blk_idx = 0;
+	struct page *page = NULL;
 	struct zram_pp_slot *pps;
-	unsigned long index = 0;
-	struct bio bio;
 	struct bio_vec bio_vec;
-	struct page *page = NULL;
-	ssize_t ret = len;
-	int mode, err;
-	unsigned long blk_idx = 0;
-
-	if (sysfs_streq(buf, "idle"))
-		mode = IDLE_WRITEBACK;
-	else if (sysfs_streq(buf, "huge"))
-		mode = HUGE_WRITEBACK;
-	else if (sysfs_streq(buf, "huge_idle"))
-		mode = IDLE_WRITEBACK | HUGE_WRITEBACK;
-	else if (sysfs_streq(buf, "incompressible"))
-		mode = INCOMPRESSIBLE_WRITEBACK;
-	else {
-		if (strncmp(buf, PAGE_WB_SIG, sizeof(PAGE_WB_SIG) - 1))
-			return -EINVAL;
-
-		if (kstrtol(buf + sizeof(PAGE_WB_SIG) - 1, 10, &index) ||
-				index >= nr_pages)
-			return -EINVAL;
-
-		nr_pages = 1;
-		mode = PAGE_WRITEBACK;
-	}
-
-	down_read(&zram->init_lock);
-	if (!init_done(zram)) {
-		ret = -EINVAL;
-		goto release_init_lock;
-	}
-
-	/* Do not permit concurrent post-processing actions. */
-	if (atomic_xchg(&zram->pp_in_progress, 1)) {
-		up_read(&zram->init_lock);
-		return -EAGAIN;
-	}
-
-	if (!zram->backing_dev) {
-		ret = -ENODEV;
-		goto release_init_lock;
-	}
+	struct bio bio;
+	int ret, err;
+	u32 index;
 
 	page = alloc_page(GFP_KERNEL);
-	if (!page) {
-		ret = -ENOMEM;
-		goto release_init_lock;
-	}
-
-	ctl = init_pp_ctl();
-	if (!ctl) {
-		ret = -ENOMEM;
-		goto release_init_lock;
-	}
-
-	scan_slots_for_writeback(zram, mode, nr_pages, index, ctl);
+	if (!page)
+		return -ENOMEM;
 
 	while ((pps = select_pp_slot(ctl))) {
 		spin_lock(&zram->wb_limit_lock);
@@ -929,10 +834,216 @@ static ssize_t writeback_store(struct device *dev,
 
 	if (blk_idx)
 		free_block_bdev(zram, blk_idx);
-
-release_init_lock:
 	if (page)
 		__free_page(page);
+
+	return ret;
+}
+
+#define PAGE_WRITEBACK			0
+#define HUGE_WRITEBACK			(1 << 0)
+#define IDLE_WRITEBACK			(1 << 1)
+#define INCOMPRESSIBLE_WRITEBACK	(1 << 2)
+
+static int parse_page_index(char *val, unsigned long nr_pages,
+			    unsigned long *lo, unsigned long *hi)
+{
+	int ret;
+
+	ret = kstrtoul(val, 10, lo);
+	if (ret)
+		return ret;
+	*hi = *lo + 1;
+	if (*lo >= nr_pages || *hi > nr_pages)
+		return -ERANGE;
+	return 0;
+}
+
+static int parse_page_index_range(char *val, unsigned long nr_pages,
+				  unsigned long *lo, unsigned long *hi)
+{
+	char *delim;
+	int ret;
+
+	delim = strchr(val, '-');
+	if (!delim)
+		return -EINVAL;
+
+	*delim = 0x00;
+	ret = kstrtoul(val, 10, lo);
+	if (ret)
+		return ret;
+	if (*lo >= nr_pages)
+		return -ERANGE;
+
+	ret = kstrtoul(delim + 1, 10, hi);
+	if (ret)
+		return ret;
+	if (*hi >= nr_pages || *lo > *hi)
+		return -ERANGE;
+	*hi += 1;
+	return 0;
+}
+
+static int parse_mode(char *val, u32 *mode)
+{
+	*mode = 0;
+
+	if (!strcmp(val, "idle"))
+		*mode = IDLE_WRITEBACK;
+	if (!strcmp(val, "huge"))
+		*mode = HUGE_WRITEBACK;
+	if (!strcmp(val, "huge_idle"))
+		*mode = IDLE_WRITEBACK | HUGE_WRITEBACK;
+	if (!strcmp(val, "incompressible"))
+		*mode = INCOMPRESSIBLE_WRITEBACK;
+
+	if (*mode == 0)
+		return -EINVAL;
+	return 0;
+}
+
+static int scan_slots_for_writeback(struct zram *zram, u32 mode,
+				    unsigned long lo, unsigned long hi,
+				    struct zram_pp_ctl *ctl)
+{
+	u32 index = lo;
+
+	while (index < hi) {
+		bool ok = true;
+
+		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))
+			goto next;
+
+		if (mode & IDLE_WRITEBACK &&
+		    !zram_test_flag(zram, index, ZRAM_IDLE))
+			goto next;
+		if (mode & HUGE_WRITEBACK &&
+		    !zram_test_flag(zram, index, ZRAM_HUGE))
+			goto next;
+		if (mode & INCOMPRESSIBLE_WRITEBACK &&
+		    !zram_test_flag(zram, index, ZRAM_INCOMPRESSIBLE))
+			goto next;
+
+		ok = place_pp_slot(zram, ctl, index);
+next:
+		zram_slot_unlock(zram, index);
+		if (!ok)
+			break;
+		index++;
+	}
+
+	return 0;
+}
+
+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);
+	u64 nr_pages = zram->disksize >> PAGE_SHIFT;
+	unsigned long lo = 0, hi = nr_pages;
+	struct zram_pp_ctl *ctl = NULL;
+	char *args, *param, *val;
+	ssize_t ret = len;
+	int err, mode = 0;
+
+	down_read(&zram->init_lock);
+	if (!init_done(zram)) {
+		up_read(&zram->init_lock);
+		return -EINVAL;
+	}
+
+	/* Do not permit concurrent post-processing actions. */
+	if (atomic_xchg(&zram->pp_in_progress, 1)) {
+		up_read(&zram->init_lock);
+		return -EAGAIN;
+	}
+
+	if (!zram->backing_dev) {
+		ret = -ENODEV;
+		goto release_init_lock;
+	}
+
+	ctl = init_pp_ctl();
+	if (!ctl) {
+		ret = -ENOMEM;
+		goto release_init_lock;
+	}
+
+	args = skip_spaces(buf);
+	while (*args) {
+		args = next_arg(args, &param, &val);
+
+		/*
+		 * Workaround to support the old writeback interface.
+		 *
+		 * The old writeback interface has a minor inconsistency and
+		 * requires key=value only for page_index parameter, while the
+		 * writeback mode is a valueless parameter.
+		 *
+		 * This is not the case anymore and now all parameters are
+		 * required to have values, however, we need to support the
+		 * legacy writeback interface format so we check if we can
+		 * recognize a valueless parameter as the (legacy) writeback
+		 * mode.
+		 */
+		if (!val || !*val) {
+			err = parse_mode(param, &mode);
+			if (err) {
+				ret = err;
+				goto release_init_lock;
+			}
+
+			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			break;
+		}
+
+		if (!strcmp(param, "type")) {
+			err = parse_mode(val, &mode);
+			if (err) {
+				ret = err;
+				goto release_init_lock;
+			}
+
+			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			break;
+		}
+
+		if (!strcmp(param, "page_index")) {
+			err = parse_page_index(val, nr_pages, &lo, &hi);
+			if (err) {
+				ret = err;
+				goto release_init_lock;
+			}
+
+			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			break;
+		}
+
+		/* There can be several page index ranges */
+		if (!strcmp(param, "page_index_range")) {
+			err = parse_page_index_range(val, nr_pages, &lo, &hi);
+			if (err) {
+				ret = err;
+				goto release_init_lock;
+			}
+
+			scan_slots_for_writeback(zram, mode, lo, hi, ctl);
+			continue;
+		}
+	}
+
+	err = zram_writeback_slots(zram, ctl);
+	if (err)
+		ret = err;
+
+release_init_lock:
 	release_pp_ctl(zram, ctl);
 	atomic_set(&zram->pp_in_progress, 0);
 	up_read(&zram->init_lock);
-- 
2.49.0.395.g12beb8f557-goog

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