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From:   David Howells <dhowells@redhat.com>
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To:     Matthew Wilcox <willy@infradead.org>
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        Jens Axboe <axboe@kernel.dk>,
        "Darrick J. Wong" <djwong@kernel.org>, linux-block@vger.kernel.org,
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Subject: Re: [PATCH v3 9/9] mm: Remove swap BIO paths and only use DIO paths
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Date:   Sat, 25 Sep 2021 16:36:42 +0100
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Matthew Wilcox <willy@infradead.org> wrote:

> On Fri, Sep 24, 2021 at 06:19:23PM +0100, David Howells wrote:
> > Delete the BIO-generating swap read/write paths and always use ->swap_=
rw().
> > This puts the mapping layer in the filesystem.
> =

> Is SWP_FS_OPS now unused after this patch?

Ummm.  Interesting question - it's only used in swap_set_page_dirty():

int swap_set_page_dirty(struct page *page)
{
	struct swap_info_struct *sis =3D page_swap_info(page);

	if (data_race(sis->flags & SWP_FS_OPS)) {
		struct address_space *mapping =3D sis->swap_file->f_mapping;

		VM_BUG_ON_PAGE(!PageSwapCache(page), page);
		return mapping->a_ops->set_page_dirty(page);
	} else {
		return __set_page_dirty_no_writeback(page);
	}
}


> Also, do we still need ->swap_activate and ->swap_deactivate?

f2fs does quite a lot of work in its ->swap_activate(), as does btrfs.  I'=
m
not sure how necessary it is.  cifs looks like it intends to use it, but i=
t's
not fully implemented yet.  zonefs and nfs do some checking, including hol=
e
checking in nfs's case.  nfs also does some setting up for the sunrpc
transport.

btrfs, cifs, f2fs and nfs all supply ->swap_deactivate() to undo the effec=
ts
of the activation.

David