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Date: Wed, 27 Dec 2023 14:32:50 +0800
From: Chengming Zhou <zhouchengming@...edance.com>
To: Barry Song <21cnbao@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Seth Jennings <sjenning@...hat.com>, Johannes Weiner <hannes@...xchg.org>,
Vitaly Wool <vitaly.wool@...sulko.com>, Nhat Pham <nphamcs@...il.com>,
Chris Li <chriscli@...gle.com>, Yosry Ahmed <yosryahmed@...gle.com>,
Dan Streetman <ddstreet@...e.org>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, Chris Li <chrisl@...nel.org>
Subject: Re: [PATCH v4 2/6] mm/zswap: reuse dstmem when decompress
On 2023/12/27 09:24, Barry Song wrote:
> On Wed, Dec 27, 2023 at 4:56 AM Chengming Zhou
> <zhouchengming@...edance.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 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@...il.com>
>> Reviewed-by: Yosry Ahmed <yosryahmed@...gle.com>
>> Acked-by: Chris Li <chrisl@...nel.org> (Google)
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---
>> 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
>>
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