linux-kernel - Re: [EXTERNAL] [PATCH v3 05/12] cifs: drop usage of page_file

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Date: Mon, 29 Apr 2024 21:26:26 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Steven French <Steven.French@...rosoft.com>
Cc: Kairui Song <kasong@...cent.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Huang, Ying" <ying.huang@...el.com>, Chris Li <chrisl@...nel.org>,
	Barry Song <v-songbaohua@...o.com>,
	Ryan Roberts <ryan.roberts@....com>, Neil Brown <neilb@...e.de>,
	Minchan Kim <minchan@...nel.org>, Hugh Dickins <hughd@...gle.com>,
	David Hildenbrand <david@...hat.com>,
	Yosry Ahmed <yosryahmed@...gle.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Namjae Jeon <linkinjeon@...nel.org>,
	"Paulo Alcantara (SUSE)" <pc@...guebit.com>,
	Shyam Prasad <Shyam.Prasad@...rosoft.com>,
	Bharath S M <bharathsm@...rosoft.com>
Subject: Re: [EXTERNAL] [PATCH v3 05/12] cifs: drop usage of page_file_offset

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.