Re: [PATCH v4 2/6] mm/zswap: reuse dstmem when decompress

[Chengming Zhou]

On 2023/12/27 09:24, Barry Song wrote:
> On Wed, Dec 27, 2023 at 4:56 AM Chengming Zhou
> <zhouchengming@xxxxxxxxxxxxx> 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 percpu mutex.
> 
> what is the benefit of this since we are actually increasing lock contention
> by reusing this buffer between multiple compression and decompression
> threads?

This mutex is already reused in all compress/decompress paths even before
the reuse optimization. I think the best way maybe to use separate crypto_acomp
for compression and decompression.

Do you think the lock contention will be increased because we now put zpool_map_handle()
and memcpy() in the lock section? Actually, we can move zpool_map_handle() before
the lock section if needed, but that memcpy() should be protected in lock section.

> 
> this mainly affects zsmalloc which can't sleep? do we have performance
> data?

Right, last time when test I remembered there is very minor performance difference.
The main benefit here is to simply the code much and delete one failure case.

> 
> and it seems this patch is also negatively affecting z3fold and zbud.c
> which actually don't need to allocate a tmp buffer.
> 

As for z3fold and zbud, the influence should be much less since the only difference
here is zpool_map_handle() moved in lock section, which could be moved out if needed
as noted above. And also no evident performance regression in the testing.

Thanks.

>>
>> Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>
>> Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
>> Acked-by: Chris Li <chrisl@xxxxxxxxxx> (Google)
>> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>> ---
>>  mm/zswap.c | 44 ++++++++++++--------------------------------
>>  1 file changed, 12 insertions(+), 32 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 976f278aa507..6b872744e962 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -1417,19 +1417,13 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>>         struct crypto_acomp_ctx *acomp_ctx;
>>         struct zpool *pool = zswap_find_zpool(entry);
>>         bool page_was_allocated;
>> -       u8 *src, *tmp = NULL;
>> +       u8 *src;
>>         unsigned int dlen;
>>         int ret;
>>         struct writeback_control wbc = {
>>                 .sync_mode = WB_SYNC_NONE,
>>         };
>>
>> -       if (!zpool_can_sleep_mapped(pool)) {
>> -               tmp = kmalloc(PAGE_SIZE, GFP_KERNEL);
>> -               if (!tmp)
>> -                       return -ENOMEM;
>> -       }
>> -
>>         /* try to allocate swap cache page */
>>         mpol = get_task_policy(current);
>>         page = __read_swap_cache_async(swpentry, GFP_KERNEL, mpol,
>> @@ -1465,15 +1459,15 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>>         /* decompress */
>>         acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
>>         dlen = PAGE_SIZE;
>> +       mutex_lock(acomp_ctx->mutex);
>>
>>         src = zpool_map_handle(pool, entry->handle, ZPOOL_MM_RO);
>>         if (!zpool_can_sleep_mapped(pool)) {
>> -               memcpy(tmp, src, entry->length);
>> -               src = tmp;
>> +               memcpy(acomp_ctx->dstmem, src, entry->length);
>> +               src = acomp_ctx->dstmem;
>>                 zpool_unmap_handle(pool, entry->handle);
>>         }
>>
>> -       mutex_lock(acomp_ctx->mutex);
>>         sg_init_one(&input, src, entry->length);
>>         sg_init_table(&output, 1);
>>         sg_set_page(&output, page, PAGE_SIZE, 0);
>> @@ -1482,9 +1476,7 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>>         dlen = acomp_ctx->req->dlen;
>>         mutex_unlock(acomp_ctx->mutex);
>>
>> -       if (!zpool_can_sleep_mapped(pool))
>> -               kfree(tmp);
>> -       else
>> +       if (zpool_can_sleep_mapped(pool))
>>                 zpool_unmap_handle(pool, entry->handle);
>>
>>         BUG_ON(ret);
>> @@ -1508,9 +1500,6 @@ static int zswap_writeback_entry(struct zswap_entry *entry,
>>         return ret;
>>
>>  fail:
>> -       if (!zpool_can_sleep_mapped(pool))
>> -               kfree(tmp);
>> -
>>         /*
>>          * If we get here because the page is already in swapcache, a
>>          * load may be happening concurrently. It is safe and okay to
>> @@ -1771,7 +1760,7 @@ bool zswap_load(struct folio *folio)
>>         struct zswap_entry *entry;
>>         struct scatterlist input, output;
>>         struct crypto_acomp_ctx *acomp_ctx;
>> -       u8 *src, *dst, *tmp;
>> +       u8 *src, *dst;
>>         struct zpool *zpool;
>>         unsigned int dlen;
>>         bool ret;
>> @@ -1796,26 +1785,19 @@ bool zswap_load(struct folio *folio)
>>         }
>>
>>         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);
>>
>> +       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;
>>                 zpool_unmap_handle(zpool, entry->handle);
>>         }
>>
>> -       acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
>> -       mutex_lock(acomp_ctx->mutex);
>>         sg_init_one(&input, src, entry->length);
>>         sg_init_table(&output, 1);
>>         sg_set_page(&output, page, PAGE_SIZE, 0);
>> @@ -1826,15 +1808,13 @@ bool zswap_load(struct folio *folio)
>>
>>         if (zpool_can_sleep_mapped(zpool))
>>                 zpool_unmap_handle(zpool, entry->handle);
>> -       else
>> -               kfree(tmp);
>>
>>         ret = true;
>>  stats:
>>         count_vm_event(ZSWPIN);
>>         if (entry->objcg)
>>                 count_objcg_event(entry->objcg, ZSWPIN);
>> -freeentry:
>> +
>>         spin_lock(&tree->lock);
>>         if (ret && zswap_exclusive_loads_enabled) {
>>                 zswap_invalidate_entry(tree, entry);
>>
>> --
>> b4 0.10.1
>>