Re: [PATCH v2 2/3] mm/zswap: remove the memcpy if acomp is not sleepable

On Sat, Feb 17, 2024 at 8:36 AM Yosry Ahmed [off-list ref] wrote:

On Fri, Feb 16, 2024 at 11:10:04PM +1300, Barry Song wrote:

quoted

On Fri, Feb 16, 2024 at 9:30 PM Yosry Ahmed [off-list ref] wrote:

quoted

On Fri, Feb 16, 2024 at 05:08:14PM +1300, Barry Song wrote:

quoted

From: Barry Song <redacted>

Most compressors are actually CPU-based and won't sleep during
compression and decompression. We should remove the redundant
memcpy for them.

Signed-off-by: Barry Song <redacted>
Tested-by: Chengming Zhou <redacted>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
---
 mm/zswap.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 350dd2fc8159..6319d2281020 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c

@@ -168,6 +168,7 @@ struct crypto_acomp_ctx {
      struct crypto_wait wait;
      u8 *buffer;
      struct mutex mutex;
+     bool is_sleepable;
 };

 /*

@@ -716,6 +717,7 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
              goto acomp_fail;
      }
      acomp_ctx->acomp = acomp;
+     acomp_ctx->is_sleepable = acomp_is_sleepable(acomp);

Just one question here. In patch 1, sleepable seems to mean "not async".
IIUC, even a synchronous algorithm may sleep (e.g. if there is a
cond_resched or waiting for a mutex). Does sleepable in acomp terms the
same as "atomic" in scheduling/preemption terms?

I think the answer is yes though async and sleepable are slightly
different semantically
generally speaking. but for comp cases, they are equal.

We have two backends for compression/ decompression - scomp and acomp. if comp
is using scomp backend, we can safely think they are not sleepable at
least from the
below three facts.

1. in zRAM, we are using scomp APIs only - crypto_comp_decompress()/
crypto_comp_compress(),  which are definitely scomp, we have never considered
sleeping problem in zram drivers:
static int zram_read_from_zspool(struct zram *zram, struct page *page,
                                 u32 index)
{
        struct zcomp_strm *zstrm;
        unsigned long handle;
        unsigned int size;
        void *src, *dst;
        u32 prio;
        int ret;

        handle = zram_get_handle(zram, index);
        ...
        src = zs_map_object(zram->mem_pool, handle, ZS_MM_RO);
        if (size == PAGE_SIZE) {
                dst = kmap_local_page(page);
                memcpy(dst, src, PAGE_SIZE);
                kunmap_local(dst);
                ret = 0;
        } else {
                dst = kmap_local_page(page);
                ret = zcomp_decompress(zstrm, src, size, dst);
                kunmap_local(dst);
                zcomp_stream_put(zram->comps[prio]);
        }
        zs_unmap_object(zram->mem_pool, handle);
        return ret;
}

2. zswap used to only support scomp before we moved to use
crypto_acomp_compress()
and crypto_acomp_decompress() APIs whose backends can be either scomp
or acomp, thus new hardware-based compression drivers can be used in zswap.

But before we moved to these new APIs in commit  1ec3b5fe6eec782 ("mm/zswap:
move to use crypto_acomp API for hardware acceleration") , zswap had
never considered
sleeping problems just like zRAM.

3. There is no sleeping in drivers using scomp backend.

$ git grep crypto_register_scomp
crypto/842.c:   ret = crypto_register_scomp(&scomp);
crypto/deflate.c:       ret = crypto_register_scomp(&scomp);
crypto/lz4.c:   ret = crypto_register_scomp(&scomp);
crypto/lz4hc.c: ret = crypto_register_scomp(&scomp);
crypto/lzo-rle.c:       ret = crypto_register_scomp(&scomp);
crypto/lzo.c:   ret = crypto_register_scomp(&scomp);
crypto/zstd.c:  ret = crypto_register_scomp(&scomp);
drivers/crypto/cavium/zip/zip_main.c:   ret =
crypto_register_scomp(&zip_scomp_deflate);
drivers/crypto/cavium/zip/zip_main.c:   ret =
crypto_register_scomp(&zip_scomp_lzs);

which are the most common cases.

Thanks for explaining. Ideally we should be able to catch any violations
with proper debug options as you mentioned. Please include more info the
commit message about sleepability, a summarized version of what you
described above.

ok. I will enhance the commit message of patch 1/3 with the summary.

Thanks
Barry