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

Matthew Wilcox
2021-09-25T17:10:51Z

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.