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Date: Mon, 29 Apr 2024 21:26:26 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Steven French <Steven.French@microsoft.com>
Cc: Kairui Song <kasong@tencent.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Huang, Ying" <ying.huang@intel.com>, Chris Li <chrisl@kernel.org>,
	Barry Song <v-songbaohua@oppo.com>,
	Ryan Roberts <ryan.roberts@arm.com>, Neil Brown <neilb@suse.de>,
	Minchan Kim <minchan@kernel.org>, Hugh Dickins <hughd@google.com>,
	David Hildenbrand <david@redhat.com>,
	Yosry Ahmed <yosryahmed@google.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Namjae Jeon <linkinjeon@kernel.org>,
	"Paulo Alcantara (SUSE)" <pc@manguebit.com>,
	Shyam Prasad <Shyam.Prasad@microsoft.com>,
	Bharath S M <bharathsm@microsoft.com>
Subject: Re: [EXTERNAL] [PATCH v3 05/12] cifs: drop usage of page_file_offset
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On Mon, Apr 29, 2024 at 08:19:31PM +0000, Steven French wrote:
> Wouldn't this make it harder to fix the regression when swap file support was temporarily removed from cifs.ko (due to the folio migration)?   I was hoping to come back to fixing swapfile support for cifs.ko in 6.10-rc (which used to pass the various xfstests for this but code got removed with folios/netfs changes).

It was neither the folio conversion nor the netfs conversion which
removed the claim of swap support from cifs, but NeilBrown's
introduction of ->swap_rw.  In commit e1209d3a7a67 he claims that

    Only two filesystems set SWP_FS_OPS:
    - cifs sets the flag, but ->direct_IO always fails so swap cannot work.
    - nfs sets the flag, but ->direct_IO calls generic_write_checks()
      which has failed on swap files for several releases.

As I recall the xfstests only checked that swapon/swapoff works; they
don't actually test that writing to swap and reading back from it work.