Re: [PATCH 1/5] mm/zswap: reuse dstmem when decompress

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2023/12/14 07:24, Yosry Ahmed wrote:
> On Tue, Dec 12, 2023 at 8:18 PM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> In the !zpool_can_sleep_mapped() case such as zsmalloc, we need to first
>> copy the entry->handle memory to a temporary memory, which is allocated
>> using kmalloc.
>>
>> Obviously we can reuse the per-compressor dstmem to avoid allocating
>> every time, since it's percpu-compressor and protected in mutex.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
>> ---
>>  mm/zswap.c | 29 +++++++++--------------------
>>  1 file changed, 9 insertions(+), 20 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 7ee54a3d8281..edb8b45ed5a1 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -1772,9 +1772,9 @@ bool zswap_load(struct folio *folio)
>>         struct zswap_entry *entry;
>>         struct scatterlist input, output;
>>         struct crypto_acomp_ctx *acomp_ctx;
>> -       u8 *src, *dst, *tmp;
>> +       unsigned int dlen = PAGE_SIZE;
>> +       u8 *src, *dst;
>>         struct zpool *zpool;
>> -       unsigned int dlen;
>>         bool ret;
>>
>>         VM_WARN_ON_ONCE(!folio_test_locked(folio));
>> @@ -1796,27 +1796,18 @@ bool zswap_load(struct folio *folio)
>>                 goto stats;
>>         }
>>
>> -       zpool = zswap_find_zpool(entry);
>> -       if (!zpool_can_sleep_mapped(zpool)) {
>> -               tmp = kmalloc(entry->length, GFP_KERNEL);
>> -               if (!tmp) {
>> -                       ret = false;
>> -                       goto freeentry;
>> -               }
>> -       }
>> -
>>         /* decompress */
>> -       dlen = PAGE_SIZE;
>> -       src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
>> +       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
>> +       mutex_lock(acomp_ctx->mutex);
>>
>> +       zpool = zswap_find_zpool(entry);
>> +       src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
>>         if (!zpool_can_sleep_mapped(zpool)) {
>> -               memcpy(tmp, src, entry->length);
>> -               src = tmp;
>> +               memcpy(acomp_ctx->dstmem, src, entry->length);
>> +               src = acomp_ctx->dstmem;
> 
> I don't like that we are now using acomp_ctx->dstmem and
> acomp_ctx->mutex now for purposes other than what the naming suggests.

The "mutex" name is coherent, "dstmem" depends on how we use IMHO.
Change to just "mem"? Or do you have a better name to replace?

> 
> How about removing these two fields from acomp_ctx, and directly using
> zswap_dstmem and zswap_mutex in both the load and store paths, rename
> them, and add proper comments above their definitions that they are
> for generic percpu buffering on the load and store paths?

Yes, they are percpu memory and lock, but they are used by per acomp_ctx,
and the cpu maybe changing in the middle, so maybe better to keep them.

Thanks!