linux-kernel - Re: [PATCH v2] erofs: use kmap_local_page() only for erofs

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Message-ID: <Y08asdeoz5yOAefN@B-P7TQMD6M-0146.lan>
Date:   Wed, 19 Oct 2022 05:29:21 +0800
From:   Gao Xiang <hsiangkao@...ux.alibaba.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     linux-erofs@...ts.ozlabs.org, Chao Yu <chao@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, ira.weiny@...el.com
Subject: Re: [PATCH v2] erofs: use kmap_local_page() only for erofs_bread()

Hi Fabio,

On Tue, Oct 18, 2022 at 09:18:49PM +0200, Fabio M. De Francesco wrote:
> On Tuesday, October 18, 2022 12:53:13 PM CEST Gao Xiang wrote:
> > Convert all mapped erofs_bread() users to use kmap_local_page()
> > instead of kmap() or kmap_atomic().
> > 
> > Signed-off-by: Gao Xiang <hsiangkao@...ux.alibaba.com>
> > ---
> >  fs/erofs/data.c     | 8 ++------
> >  fs/erofs/internal.h | 3 +--
> >  fs/erofs/xattr.c    | 8 ++++----
> >  fs/erofs/zmap.c     | 4 ++--
> >  4 files changed, 9 insertions(+), 14 deletions(-)
> > 
> 
> I just realized that you know the code of fs/erofs very well. I saw a Gao 
> Xiang in MAINTAINERS, although having a different email address.
> 
> Therefore, I'm sure that everybody can trust that you checked everything is 
> needed to assure the safety of the conversions.
> 
> However, an extended commit message would have prevented me to send you the 
> previous email with all those questions / objections.

Thanks for your suggestion.  Yeah, this conversion looks trivial [since most
paths for erofs_bread() don't have more restriction in principle so we can
just disable migration.  One of what I need to care is nested kmap() usage,
some unmap/remap order cannot be simply converted to kmap_local() but I think
it's not the case for erofs_bread().  Actually EROFS has one of such nested
kmap() usage, but I don't really care its performance on HIGHMEM platforms,
so I think kmap() is still somewhat useful compared to kmap_local() from this
point of view], but in order to make it all work properly, I will try to do
stress test with 32-bit platform later.  Since it targets for the next cycle
6.2, I will do a full stress test in the next following weeks.

Thanks,
Gao Xiang

> 
> Thanks,
> 
> Fabio
>