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

On Wed, Jan 13, 2021 at 11:12 AM Matthew Wilcox [off-list ref] wrote:

On Wed, Jan 13, 2021 at 11:08:56AM +0800, huang ying wrote:

quoted

On Wed, Jan 13, 2021 at 10:47 AM Linus Torvalds
[off-list ref] wrote:

quoted

On Tue, Jan 12, 2021 at 6:43 PM Huang Ying [off-list ref] wrote:

quoted

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