Re: [PATCH] mm: Free unused swap cache page in write protection fault handler

[huang ying]

On Wed, Jan 13, 2021 at 11:12 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Wed, Jan 13, 2021 at 11:08:56AM +0800, huang ying wrote:
> > On Wed, Jan 13, 2021 at 10:47 AM Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > >
> > > On Tue, Jan 12, 2021 at 6:43 PM Huang Ying <ying.huang@xxxxxxxxx> wrote:
> > > >
> > > > So in this patch, at the end of wp_page_copy(), the old unused swap
> > > > cache page will be tried to be freed.
> > >
> > > I'd much rather free it later when needed, rather than when you're in
> > > a COW section.
> >
> > Unused swap cache isn't unused file cache.  Nobody can reuse them
> > directly before freeing them firstly.  It will make COW a little
> > faster via keeping them.  But I think the overhead to free them isn't
> > high.  While keeping them in system will confuse users (users will
> > expect file cache to use free memory, but not expect unused swap cache
> > to use much free memory), make the swap space more fragmented, and add
> > system overall overhead (scanning LRU list, etc.).
>
> Couldn't we just move it to the tail of the LRU list so it's reclaimed
> first?  Or is locking going to be a problem here?

Yes.  That's a way to reduce the disturbance to the page reclaiming.
For LRU lock contention, is it sufficient to use another pagevec?

Best Regards,
Huang, Ying