Change the dstmem size from 2 * PAGE_SIZE to only one page since
we only need at most one page when compress, and the "dlen" is also
PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
we don't wanna store the output in zswap anyway.

So change it to one page, and delete the stale comment.

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
 mm/zswap.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index edb8b45ed5a1..fa186945010d 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -707,7 +707,7 @@ static int zswap_dstmem_prepare(unsigned int cpu)
 	struct mutex *mutex;
 	u8 *dst;
 
-	dst = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
+	dst = kmalloc_node(PAGE_SIZE, GFP_KERNEL, cpu_to_node(cpu));
 	if (!dst)
 		return -ENOMEM;
 
@@ -1662,8 +1662,7 @@ bool zswap_store(struct folio *folio)
 	sg_init_table(&input, 1);
 	sg_set_page(&input, page, PAGE_SIZE, 0);
 
-	/* zswap_dstmem is of size (PAGE_SIZE * 2). Reflect same in sg_list */
-	sg_init_one(&output, dst, PAGE_SIZE * 2);
+	sg_init_one(&output, dst, PAGE_SIZE);
 	acomp_request_set_params(acomp_ctx->req, &input, &output, PAGE_SIZE, dlen);
 	/*
 	 * it maybe looks a little bit silly that we send an asynchronous request,

-- 
b4 0.10.1

On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
<zhouchengming@bytedance.com> wrote:
>
> Change the dstmem size from 2 * PAGE_SIZE to only one page since
> we only need at most one page when compress, and the "dlen" is also
> PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
> we don't wanna store the output in zswap anyway.
>
> So change it to one page, and delete the stale comment.

I couldn't find the history of why we needed 2 * PAGE_SIZE, it would
be nice if someone has the context, perhaps one of the maintainers.

One potential reason is that we used to store a zswap header
containing the swap entry in the compressed page for writeback
purposes, but we don't do that anymore. Maybe we wanted to be able to
handle the case where an incompressible page would exceed PAGE_SIZE
because of that?

On Wed, Dec 13, 2023 at 3:34 PM Yosry Ahmed <yosryahmed@google.com> wrote:
>
> On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
> >
> > Change the dstmem size from 2 * PAGE_SIZE to only one page since
> > we only need at most one page when compress, and the "dlen" is also
> > PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
> > we don't wanna store the output in zswap anyway.
> >
> > So change it to one page, and delete the stale comment.
>
> I couldn't find the history of why we needed 2 * PAGE_SIZE, it would
> be nice if someone has the context, perhaps one of the maintainers.

It'd be very nice indeed.

>
> One potential reason is that we used to store a zswap header
> containing the swap entry in the compressed page for writeback
> purposes, but we don't do that anymore. Maybe we wanted to be able to
> handle the case where an incompressible page would exceed PAGE_SIZE
> because of that?

It could be hmm. I didn't study the old zswap architecture too much,
but it has been 2 * PAGE_SIZE since the time zswap was first merged
last I checked.
I'm not 100% comfortable ACK-ing the undoing of something that looks
so intentional, but FTR, AFAICT, this looks correct to me.