Re: [PATCH v3 9/9] mm: Remove swap BIO paths and only use DIO paths

From: Damien Le Moal <damien.lemoal@opensource.wdc.com>
To: Matthew Wilcox <willy@infradead.org>,
	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
Date: Mon, 27 Sep 2021 08:08:53 +0900	[thread overview]
Message-ID: <5fde9167-6f8b-c698-f34d-d7fafd4219f7@opensource.wdc.com> (raw)
In-Reply-To: <YU9X2o74+aZP4iWV@casper.infradead.org>

On 2021/09/26 2:09, Matthew Wilcox wrote:
> 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.

For zonefs, ->swap_activate() checks that the user is not trying to use a
sequential write only file for swap. Swap cannot work on these files as there
are no guarantees that the writes will be sequential.

-- 
Damien Le Moal
Western Digital Research