zram: rework bdev block allocation

First, writeback bdev ->bitmap bits are set only from one context, as we
can have only one single task performing writeback, so we cannot race with
anything else.  Remove retry path.

Second, we always check ZRAM_WB flag to distinguish writtenback slots, so
we should not confuse 0 bdev block index and 0 handle.  We can use first
bdev block (0 bit) for writeback as well.

While at it, give functions slightly more accurate names, as we don't
alloc/free anything there, we reserve a block for async writeback or
release the block.

Link: https://lkml.kernel.org/r/20251122074029.3948921-6-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Reviewed-by: Brian Geffon <bgeffon@google.com>
Cc: Minchan Kim <minchan@google.com>
Cc: Richard Chang <richardycc@google.com>
Cc: Yuwen Chen <ywen.chen@foxmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 8064972256031f07df6cdd2edd804160a57cb000..1f7e9e914d345fdd091e15404a1f2f8ba36e14bf 100644 (file)
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -500,6 +500,8 @@ out:
 }
 
 #ifdef CONFIG_ZRAM_WRITEBACK
+#define INVALID_BDEV_BLOCK             (~0UL)
+
 struct zram_wb_ctl {
        /* idle list is accessed only by the writeback task, no concurency */
        struct list_head idle_reqs;
@@ -746,23 +748,20 @@ out:
        return err;
 }
 
-static unsigned long alloc_block_bdev(struct zram *zram)
+static unsigned long zram_reserve_bdev_block(struct zram *zram)
 {
-       unsigned long blk_idx = 1;
-retry:
-       /* skip 0 bit to confuse zram.handle = 0 */
-       blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, blk_idx);
-       if (blk_idx == zram->nr_pages)
-               return 0;
+       unsigned long blk_idx;
 
-       if (test_and_set_bit(blk_idx, zram->bitmap))
-               goto retry;
+       blk_idx = find_next_zero_bit(zram->bitmap, zram->nr_pages, 0);
+       if (blk_idx == zram->nr_pages)
+               return INVALID_BDEV_BLOCK;
 
+       set_bit(blk_idx, zram->bitmap);
        atomic64_inc(&zram->stats.bd_count);
        return blk_idx;
 }
 
-static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
+static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
 {
        int was_set;
 
@@ -887,7 +886,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
                 * (if enabled).
                 */
                zram_account_writeback_rollback(zram);
-               free_block_bdev(zram, req->blk_idx);
+               zram_release_bdev_block(zram, req->blk_idx);
                return err;
        }
 
@@ -901,7 +900,7 @@ static int zram_writeback_complete(struct zram *zram, struct zram_wb_req *req)
         * finishes.
         */
        if (!zram_test_flag(zram, index, ZRAM_PP_SLOT)) {
-               free_block_bdev(zram, req->blk_idx);
+               zram_release_bdev_block(zram, req->blk_idx);
                goto out;
        }
 
@@ -990,8 +989,8 @@ static int zram_writeback_slots(struct zram *zram,
                                struct zram_pp_ctl *ctl,
                                struct zram_wb_ctl *wb_ctl)
 {
+       unsigned long blk_idx = INVALID_BDEV_BLOCK;
        struct zram_wb_req *req = NULL;
-       unsigned long blk_idx = 0;
        struct zram_pp_slot *pps;
        int ret = 0, err = 0;
        u32 index = 0;
@@ -1023,9 +1022,9 @@ static int zram_writeback_slots(struct zram *zram,
                                ret = err;
                }
 
-               if (!blk_idx) {
-                       blk_idx = alloc_block_bdev(zram);
-                       if (!blk_idx) {
+               if (blk_idx == INVALID_BDEV_BLOCK) {
+                       blk_idx = zram_reserve_bdev_block(zram);
+                       if (blk_idx == INVALID_BDEV_BLOCK) {
                                ret = -ENOSPC;
                                break;
                        }
@@ -1059,7 +1058,7 @@ static int zram_writeback_slots(struct zram *zram,
                __bio_add_page(&req->bio, req->page, PAGE_SIZE, 0);
 
                zram_submit_wb_request(zram, wb_ctl, req);
-               blk_idx = 0;
+               blk_idx = INVALID_BDEV_BLOCK;
                req = NULL;
                cond_resched();
                continue;
@@ -1366,7 +1365,7 @@ static int read_from_bdev(struct zram *zram, struct page *page,
        return -EIO;
 }
 
-static void free_block_bdev(struct zram *zram, unsigned long blk_idx)
+static void zram_release_bdev_block(struct zram *zram, unsigned long blk_idx)
 {
 }
 #endif
@@ -1890,7 +1889,7 @@ static void zram_free_page(struct zram *zram, size_t index)
 
        if (zram_test_flag(zram, index, ZRAM_WB)) {
                zram_clear_flag(zram, index, ZRAM_WB);
-               free_block_bdev(zram, zram_get_handle(zram, index));
+               zram_release_bdev_block(zram, zram_get_handle(zram, index));
                goto out;
        }