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Date:   Tue, 18 Oct 2022 19:51:55 -0700
From:   Ira Weiny <ira.weiny@intel.com>
To:     Gao Xiang <hsiangkao@linux.alibaba.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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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?

Ira