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Subject: Re: [f2fs-dev] Question about STEP_DECOMPRESS_NOWQ
To:     Daeho Jeong <daeho43@gmail.com>, <linux-kernel@vger.kernel.org>,
        <linux-f2fs-devel@lists.sourceforge.net>, <kernel-team@android.com>
References: <CACOAw_wHONVJL438_Akvo3wZewJeVKiT_8LqW0EXULxAscpGjA@mail.gmail.com>
From:   Chao Yu <yuchao0@huawei.com>
Message-ID: <220b1cb4-9c2e-c9db-5590-6dcd43b9aaed@huawei.com>
Date:   Wed, 9 Sep 2020 15:13:45 +0800
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Hi Daeho,

On 2020/9/9 12:43, Daeho Jeong wrote:
> Hi Chao,
> 
> I have a question about the below flag for decompression.
> 
> STEP_DECOMPRESS_NOWQ,           /* handle normal cluster data inplace */
> 
> According to the comment, you added this for using inplace

I added this for the condition that: in compressed inode, partial clusters are
written as normal (non-compressed) one due to bad compress ratio, during reading
data in normal cluster, we don't need to queue them in workqueue, instead, we
can just handle them in IRQ context (end_io()).

That says, if all pages in bio are non-compressed, we tag step w/ STEP_DECOMPRESS_NOWQ,
once there is at least one page is compressed, we change step to
STEP_DECOMPRESS.

> decompression but inplace decompression mode is not being activated

'inplace decompression' means decompress in
a. reader thread or
b. IRQ context ?

Thanks

> now, since we are setting STEP_DECOMPRESS right after bio_add_page().
> 
>                  if (bio_add_page(bio, page, blocksize, 0) < blocksize)
>                          goto submit_and_realloc;
> 
>                  /* tag STEP_DECOMPRESS to handle IO in wq */
>                  ctx = bio->bi_private;
>                  if (!(ctx->enabled_steps & (1 << STEP_DECOMPRESS)))
>                          ctx->enabled_steps |= 1 << STEP_DECOMPRESS;
> 
> Did you mean to deactivate inplace decompression mode on purpose,
> since it had an issue? I am a little bit confused about this.
> 
> 
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