linux-kernel - Re: [PATCH 3/7] mm/zswap: reuse dstmem when decompress

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Message-ID: <73c126cd-5a49-4040-96e2-d416da616158@bytedance.com>
Date:   Wed, 13 Dec 2023 10:41:17 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     Vitaly Wool <vitaly.wool@...sulko.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Seth Jennings <sjenning@...hat.com>,
        Dan Streetman <ddstreet@...e.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yosry Ahmed <yosryahmed@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/7] mm/zswap: reuse dstmem when decompress

On 2023/12/13 06:58, Nhat Pham wrote:
> On Wed, Dec 6, 2023 at 1:46 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 mutex.
> 
> Ah this sounds like a good idea. We have to grab that mutex anyway -
> might as well use the memory slot that is protected by that mutex.
> 
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
>> ---
>>  mm/zswap.c | 29 +++++++++--------------------
>>  1 file changed, 9 insertions(+), 20 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index a6b4859a0164..d93a7b58b5af 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -1758,9 +1758,9 @@ 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));
>> @@ -1782,27 +1782,18 @@ 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;
> 
> We're moving handle (un)mapping and the memory copying inside the
> critical section protected by the mutex. Seems fine to me -
> zswap_store() already did this IIUC.
> 
>>                 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);
>> @@ -1813,15 +1804,13 @@ 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:
> 
> So it reduces the chance of zswap_load() failure due to unable to
> memory allocation failure? Nice!
> 

Yes, no need to allocate temporary memory anymore :)

>> +
>>         spin_lock(&tree->lock);
>>         if (ret && zswap_exclusive_loads_enabled) {
>>                 zswap_invalidate_entry(tree, entry);
>>
>> --
>> b4 0.10.1
> 
> Reviewed-by: Nhat Pham <nphamcs@...il.com>

Thanks for review!