linux-kernel - Re: [PATCH 10/10] mm/migrate.c: call detach_page

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Message-ID: <20200519151632.GX16070@bombadil.infradead.org>
Date:   Tue, 19 May 2020 08:16:32 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Gao Xiang <hsiangkao@....com>
Cc:     Guoqing Jiang <guoqing.jiang@...ud.ionos.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, david@...morbit.com,
        hch@...radead.org
Subject: Re: [PATCH 10/10] mm/migrate.c: call detach_page_private to cleanup
 code

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.