linux-kernel - Re: [PATCH] erofs: remove unneeded icur field from struct z_erofs_decompress

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Date:   Mon, 17 Apr 2023 15:00:25 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     Yue Hu <zbestahu@...il.com>, xiang@...nel.org, chao@...nel.org,
        linux-erofs@...ts.ozlabs.org
Cc:     jefflexu@...ux.alibaba.com, linux-kernel@...r.kernel.org,
        huyue2@...lpad.com, zhangwen@...lpad.com
Subject: Re: [PATCH] erofs: remove unneeded icur field from struct
 z_erofs_decompress_frontend



On 2023/4/17 14:52, Gao Xiang wrote:
> 
> 
> On 2023/4/17 14:41, Yue Hu wrote:
>> From: Yue Hu <huyue2@...lpad.com>
>>
>> The icur field is only used in z_erofs_try_inplace_io(). Let's just use
>> a local variable instead. And no need to check if the pcluster is inline
>> when setting icur since inline page cannot be used for inplace I/O.

Where do we check if the pcluster is inline?

>>
>> Signed-off-by: Yue Hu <huyue2@...lpad.com>
> 
> Nope, it's a behavior change.
> Other users could feed more inplace I/O pages before I/O submission
> to reduce memory consumption, it's common when applying stress model
> in low memory scenarios.

Oh, I misread it.  I think it can be done in this way although
each following users will now rescan the whole array all the time.

Reviewed-by: Gao Xiang <hsiangkao@...ux.alibaba.com>

> 
> Thanks,
> Gao Xiang
> 
>> ---
>>   fs/erofs/zdata.c | 13 +++++--------
>>   1 file changed, 5 insertions(+), 8 deletions(-)
>>
>> diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
>> index f759152feffa..f8bf2b227942 100644
>> --- a/fs/erofs/zdata.c
>> +++ b/fs/erofs/zdata.c
>> @@ -554,9 +554,6 @@ struct z_erofs_decompress_frontend {
>>       /* used for applying cache strategy on the fly */
>>       bool backmost;
>>       erofs_off_t headoffset;
>> -
>> -    /* a pointer used to pick up inplace I/O pages */
>> -    unsigned int icur;
>>   };
>>   #define DECOMPRESS_FRONTEND_INIT(__i) { \
>> @@ -707,11 +704,13 @@ static bool z_erofs_try_inplace_io(struct z_erofs_decompress_frontend *fe,
>>                      struct z_erofs_bvec *bvec)
>>   {
>>       struct z_erofs_pcluster *const pcl = fe->pcl;
>> +    /* file-backed online pages are traversed in reverse order */

Although please help refine the comment below:

	/* scan & fill inplace I/O pages in the reverse order */

Thanks,
Gao Xiang

>> +    unsigned int icur = pcl->pclusterpages;
>> -    while (fe->icur > 0) {
>> -        if (!cmpxchg(&pcl->compressed_bvecs[--fe->icur].page,
>> +    while (icur > 0) {
>> +        if (!cmpxchg(&pcl->compressed_bvecs[--icur].page,
>>                    NULL, bvec->page)) {
>> -            pcl->compressed_bvecs[fe->icur] = *bvec;
>> +            pcl->compressed_bvecs[icur] = *bvec;
>>               return true;
>>           }
>>       }
>> @@ -877,8 +876,6 @@ static int z_erofs_collector_begin(struct z_erofs_decompress_frontend *fe)
>>       }
>>       z_erofs_bvec_iter_begin(&fe->biter, &fe->pcl->bvset,
>>                   Z_EROFS_INLINE_BVECS, fe->pcl->vcnt);
>> -    /* since file-backed online pages are traversed in reverse order */
>> -    fe->icur = z_erofs_pclusterpages(fe->pcl);
>>       return 0;
>>   }