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Date:   Sat, 25 Sep 2021 18:09:46 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     David Howells <dhowells@redhat.com>
Cc:     hch@lst.de, trond.myklebust@primarydata.com,
        Jens Axboe <axboe@kernel.dk>,
        "Darrick J. Wong" <djwong@kernel.org>, linux-block@vger.kernel.org,
        linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-mm@kvack.org, darrick.wong@oracle.com,
        viro@zeniv.linux.org.uk, jlayton@kernel.org,
        torvalds@linux-foundation.org, linux-nfs@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 9/9] mm: Remove swap BIO paths and only use DIO paths
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On Sat, Sep 25, 2021 at 04:36:42PM +0100, David Howells wrote:
> 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 = page_swap_info(page);
> 
> 	if (data_race(sis->flags & SWP_FS_OPS)) {
> 		struct address_space *mapping = 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);
> 	}
> }

I suspect that's no longer necessary.  NFS was the only filesystem
using SWP_FS_OPS and ...

fs/nfs/file.c:  .set_page_dirty = __set_page_dirty_nobuffers,

so it's not like NFS does anything special to reserve memory to write
back swap pages.

> > 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 it's
> not fully implemented yet.  zonefs and nfs do some checking, including hole
> 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 effects
> of the activation.

Right ... so my question really is, now that we're doing I/O through
aops->direct_IO (or ->swap_rw), do those magic things need to be done?
After all, open(O_DIRECT) doesn't do these same magic things.  They're
really there to allow the direct-to-BIO path to work, and you're removing
that here.