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Date:   Tue, 19 May 2020 08:16:32 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     Gao Xiang <hsiangkao@gmx.com>
Cc:     Guoqing Jiang <guoqing.jiang@cloud.ionos.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, david@fromorbit.com,
        hch@infradead.org
Subject: Re: [PATCH 10/10] mm/migrate.c: call detach_page_private to cleanup
 code
Message-ID: <20200519151632.GX16070@bombadil.infradead.org>
References: <20200517214718.468-1-guoqing.jiang@cloud.ionos.com>
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 <20200518221235.1fa32c38e5766113f78e3f0d@linux-foundation.org>
 <aade5d75-c9e9-4021-6eb7-174a921a7958@cloud.ionos.com>
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On Tue, May 19, 2020 at 06:06:19PM +0800, Gao Xiang wrote:
> In addition, I found some limitation of new {attach,detach}_page_private
> helper (that is why I was interested in this series at that time [1] [2],
> but I gave up finally) since many patterns (not all) in EROFS are
> 
> io_submit (origin, page locked):
> attach_page_private(page);
> ...
> put_page(page);
> 
> end_io (page locked):
> SetPageUptodate(page);
> unlock_page(page);
> 
> since the page is always locked, so io_submit could be simplified as
> set_page_private(page, ...);
> SetPagePrivate(page);
> , which can save both one temporary get_page(page) and one
> put_page(page) since it could be regarded as safe with page locked.

It's fine to use page private like this without incrementing the refcount,
and I can't find any problematic cases in EROFS like those fixed by commit
8e47a457321ca1a74ad194ab5dcbca764bc70731

So I think the new helpers are not for you, and that's fine.  They'll be
useful for other filesystems which are using page_private differently
from the way that you do.