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Date:   Wed, 19 Oct 2022 11:06:58 +0800
From:   Gao Xiang <hsiangkao@linux.alibaba.com>
To:     Ira Weiny <ira.weiny@intel.com>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        linux-erofs@lists.ozlabs.org, Chao Yu <chao@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] erofs: use kmap_local_page() only for erofs_bread()
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Hi Ira,

On Tue, Oct 18, 2022 at 07:51:55PM -0700, Ira Weiny wrote:
> On Wed, Oct 19, 2022 at 07:36:55AM +0800, Gao Xiang wrote:
> > On Wed, Oct 19, 2022 at 01:21:27AM +0200, Fabio M. De Francesco wrote:
> > > On Tuesday, October 18, 2022 11:29:21 PM CEST Gao Xiang wrote:
> > 
> > ...
> 
> [snip]
> 
> > > 
> > > In Btrfs I solved (thanks to David S.' advice) by mapping only one of two 
> > > pages, only the one coming from the page cache. 
> > > 
> > > The other page didn't need the use of kmap_local_page() because it was 
> > > allocated in the filesystem with "alloc_page(GFP_NOFS)". GFP_NOFS won't ever 
> > > allocate from ZONE_HIGHMEM, therefore a direct page_address() could avoid the 
> > > mapping and the nesting issues.
> > > 
> > > Did you check if you may solve the same way? 
> > 
> > That is not simple.  Currently we have compressed pages and decompressed
> > pages (page cache or others), and they can be unmapped when either data
> > is all consumed, so compressed pages can be unmapped first, or
> > decompressed pages can be unmapped first.  That quite depends on which
> > pages goes first.
> > 
> > I think such usage is a quite common pattern for decoder or encoder,
> > you could take a look at z_erofs_lzma_decompress() in
> > fs/erofs/decompressor_lzma.c.  So kmap() is still useful for such cases
> > since I don't really care the HIGHMEM performance but correctness, but
> > other alternative could churn/complex the map/unmap/remap pattern.
> > 
> 
> When you say kmap() is still useful is this because of the map/unmap ordering
> restrictions or because the address is required in different threads?

... mainly due to map/unmap ordering restriction. I think
the decompressor here could still be a simple dependency.  I'm not
sure if there are more complicated cases (like multiple
decoding/encoding sources into target pages) though..

Thanks,
Gao Xiang

> 
> Ira