Re: [RFC PATCHv5 0/6] zram: introduce writeback bio batching

[Yuwen Chen <ywen.chen@foxmail.com>]

On Fri, 21 Nov 2025 16:58:41 +0900, Sergey Senozhatsky wrote:
> No problem.  I wonder if the effect is more visible on larger data sets.
> 0.3 second sounds like a very short write.  In my VM tests I couldn't get
> more than 2 inflight requests at a time, I guess because decompression
> was much slower than IO.  I wonder how many inflight requests you had in
> your tests.

I used the following code for testing here, and the result was 32.

code:
@@ -983,6 +983,7 @@ static int zram_writeback_slots(struct zram *zram,
        struct zram_pp_slot *pps;
        int ret = 0, err = 0;
        u32 index = 0;
+       int inflight = 0;
 
        while ((pps = select_pp_slot(ctl))) {
                spin_lock(&zram->wb_limit_lock);
@@ -993,6 +994,9 @@ static int zram_writeback_slots(struct zram *zram,
                }
                spin_unlock(&zram->wb_limit_lock);
 
+               if (inflight < atomic_read(&wb_ctl->num_inflight))
+                       inflight = atomic_read(&wb_ctl->num_inflight);
+
                while (!req) {
                        req = zram_select_idle_req(wb_ctl);
                        if (req)
@@ -1074,6 +1078,7 @@ next:
                        ret = err;
        }
 
+       pr_err("%s: inflight max: %d\n", __func__, inflight);
        return ret;
 }

log: 
[3741949.842927] zram: zram_writeback_slots: inflight max: 32

Changing ZRAM_WB_REQ_CNT to 64 didn't shorten the overall time.

> I think page-fault latency of a written-back page is expected to be
> higher, that's a trade-off that we agree on.  Off the top of my head,
> I don't think we can do anything about it.
>
> Is loop device always used as for writeback targets?

On the Android platform, currently only the loop device is supported as
the backend for writeback, possibly for security reasons. I noticed that
EROFS has implemented a CONFIG_EROFS_FS_BACKED_BY_FILE to reduce this
latency. I think ZRAM might also be able to do this.