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

[Damien Le Moal]

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@xxxxxxxxxxxxx> 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