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From: Steven French <Steven.French@microsoft.com>
To: Matthew Wilcox <willy@infradead.org>
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
Thread-Topic: [EXTERNAL] [PATCH v3 05/12] cifs: drop usage of page_file_offset
Thread-Index: AQHammhzJrsE0Xg8qUSGz+RQruipXLF/r7BQgAACgwCAAGNMwA==
Date: Tue, 30 Apr 2024 02:23:03 +0000
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Makes sense - I will try to look at this fixing the swapon and reading from=
 swap over smb3.1.1 mounts in the next few weeks, but if you have a good ex=
ample of sample code (from one of the other FS that does this well) that wo=
uld help.

-----Original Message-----
From: Matthew Wilcox <willy@infradead.org>=20
Sent: Monday, April 29, 2024 3:26 PM
To: Steven French <Steven.French@microsoft.com>
Cc: Kairui Song <kasong@tencent.com>; linux-mm@kvack.org; Andrew Morton <ak=
pm@linux-foundation.org>; Huang, Ying <ying.huang@intel.com>; Chris Li <chr=
isl@kernel.org>; Barry Song <v-songbaohua@oppo.com>; Ryan Roberts <ryan.rob=
erts@arm.com>; Neil Brown <neilb@suse.de>; Minchan Kim <minchan@kernel.org>=
; Hugh Dickins <hughd@google.com>; David Hildenbrand <david@redhat.com>; Yo=
sry Ahmed <yosryahmed@google.com>; linux-fsdevel@vger.kernel.org; linux-ker=
nel@vger.kernel.org; Namjae Jeon <linkinjeon@kernel.org>; Paulo Alcantara (=
SUSE) <pc@manguebit.com>; Shyam Prasad <Shyam.Prasad@microsoft.com>; Bharat=
h S M <bharathsm@microsoft.com>
Subject: Re: [EXTERNAL] [PATCH v3 05/12] cifs: drop usage of page_file_offs=
et

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 (whi=
ch used to pass the various xfstests for this but code got removed with fol=
ios/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.