Re: [PATCH v4 1/6] mm/zswap: change dstmem size to one page

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2023/12/27 09:07, Barry Song wrote:
> On Wed, Dec 27, 2023 at 4:55 AM Chengming Zhou
> <zhouchengming@bytedance.com> wrote:
>>
>> Change the dstmem size from 2 * PAGE_SIZE to only one page since
>> we only need at most one page when compress, and the "dlen" is also
>> PAGE_SIZE in acomp_request_set_params(). If the output size > PAGE_SIZE
>> we don't wanna store the output in zswap anyway.
>>
>> So change it to one page, and delete the stale comment.
>>
>> There is no any history about the reason why we needed 2 pages, it has
>> been 2 * PAGE_SIZE since the time zswap was first merged.
> 
> i remember there was an over-compression case,  that means the compressed
> data can be bigger than the source data. the similar thing is also done in zram
> drivers/block/zram/zcomp.c

Right, there is a buffer overflow report[1] that I just +to you.

I think over-compression is all right, but buffer overflow is not acceptable,
so we should fix any buffer overflow problem IMHO. Anyway, 2 pages maybe
overflowed too, just with smaller probability, right?

Thanks.

> 
> int zcomp_compress(struct zcomp_strm *zstrm,
>                 const void *src, unsigned int *dst_len)
> {
>         /*
>          * Our dst memory (zstrm->buffer) is always `2 * PAGE_SIZE' sized
>          * because sometimes we can endup having a bigger compressed data
>          * due to various reasons: for example compression algorithms tend
>          * to add some padding to the compressed buffer. Speaking of padding,
>          * comp algorithm `842' pads the compressed length to multiple of 8
>          * and returns -ENOSP when the dst memory is not big enough, which
>          * is not something that ZRAM wants to see. We can handle the
>          * `compressed_size > PAGE_SIZE' case easily in ZRAM, but when we
>          * receive -ERRNO from the compressing backend we can't help it
>          * anymore. To make `842' happy we need to tell the exact size of
>          * the dst buffer, zram_drv will take care of the fact that
>          * compressed buffer is too big.
>          */
>         *dst_len = PAGE_SIZE * 2;
> 
>         return crypto_comp_compress(zstrm->tfm,
>                         src, PAGE_SIZE,
>                         zstrm->buffer, dst_len);
> }
> 
> 
>>
>> According to Yosry and Nhat, one potential reason is that we used to
>> store a zswap header containing the swap entry in the compressed page
>> for writeback purposes, but we don't do that anymore.
>>
>> This patch works good in kernel build testing even when the input data
>> doesn't compress at all (i.e. dlen == PAGE_SIZE), which we can see
>> from the bpftrace tool:
>>
>> bpftrace -e 'k:zpool_malloc {@[(uint32)arg1==4096]=count()}'
>> @[1]: 2
>> @[0]: 12011430
>>
>> Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
>> Reviewed-by: Nhat Pham <nphamcs@gmail.com>
>> Acked-by: Chris Li <chrisl@kernel.org> (Google)
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
>> ---
>>  mm/zswap.c | 5 ++---
>>  1 file changed, 2 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/zswap.c b/mm/zswap.c
>> index 7ee54a3d8281..976f278aa507 100644
>> --- a/mm/zswap.c
>> +++ b/mm/zswap.c
>> @@ -707,7 +707,7 @@ static int zswap_dstmem_prepare(unsigned int cpu)
>>         struct mutex *mutex;
>>         u8 *dst;
>>
>> -       dst = kmalloc_node(PAGE_SIZE * 2, GFP_KERNEL, cpu_to_node(cpu));
>> +       dst = kmalloc_node(PAGE_SIZE, GFP_KERNEL, cpu_to_node(cpu));
>>         if (!dst)
>>                 return -ENOMEM;
>>
>> @@ -1662,8 +1662,7 @@ bool zswap_store(struct folio *folio)
>>         sg_init_table(&input, 1);
>>         sg_set_page(&input, page, PAGE_SIZE, 0);
>>
>> -       /* zswap_dstmem is of size (PAGE_SIZE * 2). Reflect same in sg_list */
>> -       sg_init_one(&output, dst, PAGE_SIZE * 2);
>> +       sg_init_one(&output, dst, PAGE_SIZE);
>>         acomp_request_set_params(acomp_ctx->req, &input, &output, PAGE_SIZE, dlen);
>>         /*
>>          * it maybe looks a little bit silly that we send an asynchronous request,
>>
>> --
>> b4 0.10.1
>>
> 
> Thanks
> Barry