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Date:   Thu, 31 Oct 2019 01:22:44 +0800
From:   Gao Xiang <hsiangkao@aol.com>
To:     Chao Yu <yuchao0@huawei.com>, Jaegeuk Kim <jaegeuk@kernel.org>,
        linux-kernel@vger.kernel.org,
        linux-f2fs-devel@lists.sourceforge.net,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <willy@infradead.org>,
        Fengguang Wu <fengguang.wu@intel.com>
Subject: Re: [f2fs-dev] [PATCH 2/2] f2fs: support data compression
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Hi Eric,

(add some mm folks...)

On Wed, Oct 30, 2019 at 09:50:56AM -0700, Eric Biggers wrote:

<snip>

> > >>>
> > >>> It isn't really appropriate to create fake pagecache pages like this.  Did you
> > >>> consider changing f2fs to use fscrypt_decrypt_block_inplace() instead?
> > >>
> > >> We need to store i_crypto_info and iv index somewhere, in order to pass them to
> > >> fscrypt_decrypt_block_inplace(), where did you suggest to store them?
> > >>
> > > 
> > > The same place where the pages are stored.
> > 
> > Still we need allocate space for those fields, any strong reason to do so?
> > 
> 
> page->mapping set implies that the page is a pagecache page.  Faking it could
> cause problems with code elsewhere.

Not very related with this patch. Faking page->mapping was used in zsmalloc before
nonLRU migration (see material [1]) and use in erofs now (page->mapping to indicate
nonLRU short lifetime temporary page type, page->private is used for per-page information),
as far as I know, NonLRU page without PAGE_MAPPING_MOVABLE set is safe for most mm code.

On the other hands, I think NULL page->mapping will waste such field in precious
page structure... And we can not get such page type directly only by a NULL --
a truncated file page or just allocated page or some type internal temporary pages...

So I have some proposal is to use page->mapping to indicate specific page type for
such nonLRU pages (by some common convention, e.g. some real structure, rather than
just zero out to waste 8 bytes, it's also natural to indicate some page type by
its `mapping' naming )... Since my English is not very well, I delay it util now...

[1] https://elixir.bootlin.com/linux/v3.18.140/source/mm/zsmalloc.c#L379
    https://lore.kernel.org/linux-mm/1459321935-3655-7-git-send-email-minchan@kernel.org
    and some not very related topic: https://lwn.net/Articles/752564/

Thanks,
Gao Xiang