Re: [RFC PATCH] zram: avoid preemption with CPU-based compression backends

["Barry Song (Xiaomi)" <baohua@kernel.org>]

On Wed, Aug 5, 2026 at 3:50 PM Barry Song <baohua@kernel.org> wrote:
>
> On Wed, Aug 5, 2026 at 1:21 PM Sergey Senozhatsky
> <senozhatsky@chromium.org> wrote:
> >
> > Hi Barry,
> >
> > On (26/08/05 17:09), Barry Song wrote:
> > > > > This report shows that the zram mutex has become the top lock
> > > > > contributing to UI frame drops, even surpassing mmap_lock, which we
> > > > > are also addressing in multiple threads. :-)
> > > >
> > > > Any chance you can share more details?  Are there perhaps RT tasks
> > > > in the mix, priority inversion, starvations and so on?  Can proxy
> > > > execution address any of those (if it has relevance to the report
> > > > you are looking at)?
> > >
> > > Hi Sergey,
> > >
> > > talked with our engineers reporting the issue. i believe it is all
> > > about priority inversion.
> > > proxy execution wont resolve it as we have a sleepable zs-malloc
> > > within the mutex.
> > >  i believe i need v2 to release the mutex before doing the 2nd stage
> > > zs_malloc with
> > > direct reclaim.
> >
> > Well, we cannot just drop the stream mutex and do sleepable zsmalloc
> > allocation, because this will invalidate compression buffer.  So we
> > then will need to do re-compression.  Something that I was really
> > happy to drop [1].
>
> We used to do that by an temp GFP_ATOMIC buffer and memcpy:
> https://lore.kernel.org/all/1611035683-12732-2-git-send-email-tiantao6@hisilicon.com/
>
> As long as we copy `zstrm->buffer` to a temporary buffer, we are
> free to go anywhere afterwards.
>

Hi Sergey,

Just as a proof of concept, I changed one path and it seems to work.
We release the mutex before calling zs_malloc(), which may enter
direct reclaim, and we no longer need the mutex afterwards.
also, we can avoid re-compression:

From d1a4cbe63fc2f7336c23bc268b1dffe15b0e7444 Mon Sep 17 00:00:00 2001
From: "Barry Song (Xiaomi)" <baohua@kernel.org>
Date: Wed, 5 Aug 2026 17:53:49 +0800
Subject: [PATCH] zram: avoid doing zs_malloc() with direct reclaim within
 mutex

Signed-off-by: Barry Song (Xiaomi) <baohua@kernel.org>
---
 drivers/block/zram/zram_drv.c | 43 ++++++++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 8 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index cfa98846ac48..e00d896a101f 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -2218,6 +2218,35 @@ static int write_same_filled_page(struct zram *zram, unsigned long fill,
 	return 0;
 }
 
+/*
+ * try non-sleepable allocation for !async backend, then try
+ * sleepable allocation; for async backend, we always begin
+ * from sleepable allocation
+ */
+static unsigned long zram_zs_malloc(struct zram *zram, struct zcomp_strm *zstrm,
+		size_t comp_len, const int nid, void **bounce)
+{
+	unsigned long handle;
+
+	handle = zs_malloc(zram->mem_pool, comp_len,
+			__GFP_KSWAPD_RECLAIM | __GFP_NOWARN |
+			__GFP_HIGHMEM | __GFP_MOVABLE, nid);
+	if (!IS_ERR_VALUE(handle))
+		return handle;
+
+	*bounce = kmalloc(comp_len, GFP_ATOMIC);
+	if (!*bounce)
+		return (unsigned long)ERR_PTR(-ENOMEM);
+	memcpy(*bounce, zstrm->buffer, comp_len);
+
+	/* Don't hold mutex to do a sleepable allocation */
+	zcomp_stream_put(zstrm);
+	handle = zs_malloc(zram->mem_pool, comp_len,
+			GFP_NOIO | __GFP_NOWARN |
+			__GFP_HIGHMEM | __GFP_MOVABLE, nid);
+	return handle;
+}
+
 static int write_incompressible_page(struct zram *zram, struct page *page,
 				     u32 index)
 {
@@ -2264,7 +2293,7 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
 	int ret = 0;
 	unsigned long handle;
 	unsigned int comp_len;
-	void *mem;
+	void *mem, *bounce = NULL;
 	struct zcomp_strm *zstrm;
 	unsigned long element;
 	bool same_filled;
@@ -2292,22 +2321,20 @@ static int zram_write_page(struct zram *zram, struct page *page, u32 index)
 		return write_incompressible_page(zram, page, index);
 	}
 
-	handle = zs_malloc(zram->mem_pool, comp_len,
-			   GFP_NOIO | __GFP_NOWARN |
-			   __GFP_HIGHMEM | __GFP_MOVABLE, page_to_nid(page));
+	handle = zram_zs_malloc(zram, zstrm, comp_len, page_to_nid(page), &bounce);
 	if (IS_ERR_VALUE(handle)) {
-		zcomp_stream_put(zstrm);
+		bounce ? kfree(bounce) : zcomp_stream_put(zstrm);
 		return PTR_ERR((void *)handle);
 	}
 
 	if (!zram_can_store_page(zram)) {
-		zcomp_stream_put(zstrm);
+		bounce ? kfree(bounce) : zcomp_stream_put(zstrm);
 		zs_free(zram->mem_pool, handle);
 		return -ENOMEM;
 	}
 
-	zs_obj_write(zram->mem_pool, handle, zstrm->buffer, comp_len);
-	zcomp_stream_put(zstrm);
+	zs_obj_write(zram->mem_pool, handle, bounce ? : zstrm->buffer, comp_len);
+	bounce ? kfree(bounce) : zcomp_stream_put(zstrm);
 
 	slot_lock(zram, index);
 	slot_free(zram, index);
-- 
2.39.3 (Apple Git-146)