Re: [PATCH 1/2] shmem: drop lru_add_drain_all from shmem_wait_for_pins

[Hugh Dickins <hughd@google.com>]

On Fri, 3 Nov 2017, Michal Hocko wrote:
> On Fri 03-11-17 00:46:18, Hugh Dickins wrote:
> > 
> > NAK.  shmem_wait_for_pins() is waiting for temporary pins on the pages
> > to go away, and using lru_add_drain_all() in the usual way, to lower
> > the refcount of pages temporarily pinned in a pagevec somewhere.  Page
> > count is touched by draining pagevecs: I'm surprised to see you say
> > that it isn't - or have pagevec page references been eliminated by
> > a recent commit that I missed?
> 
> I must be missing something here. __pagevec_lru_add_fn merely about
> moving the page into the appropriate LRU list, pagevec_move_tail only
> rotates, lru_deactivate_file_fn moves from active to inactive LRUs,
> lru_lazyfree_fn moves from anon to file LRUs and activate_page_drain
> just moves to the active list. None of those operations touch the page
> count AFAICS. So I would agree that some pages might be pinned outside
> of the LRU (lru_add_pvec) and thus unreclaimable but does this really
> matter. Or what else I am missing?

Line 213 of mm/swap.c?  Where pagevec_lru_move_fn() calls release_pages()
to release the extra references (which each page came in with when added).
Think about it, the mayhem that would follow from a page being freed while
on pagevec: of course it must hold a reference.  The only surprise is that
the extra reference is not needed while on LRU: one can think of PageLRU
as an extension of the page count.

Hugh