Re: [PATCH v21 07/19] mm: page_idle_get_page() does not need lru_lock

[Johannes Weiner]

On Thu, Nov 05, 2020 at 04:55:37PM +0800, Alex Shi wrote:
> From: Hugh Dickins <hughd@xxxxxxxxxx>
> 
> It is necessary for page_idle_get_page() to recheck PageLRU() after
> get_page_unless_zero(), but holding lru_lock around that serves no
> useful purpose, and adds to lru_lock contention: delete it.
> 
> See https://lore.kernel.org/lkml/20150504031722.GA2768@blaptop for the
> discussion that led to lru_lock there; but __page_set_anon_rmap() now
> uses WRITE_ONCE(), and I see no other risk in page_idle_clear_pte_refs()
> using rmap_walk() (beyond the risk of racing PageAnon->PageKsm, mostly
> but not entirely prevented by page_count() check in ksm.c's
> write_protect_page(): that risk being shared with page_referenced() and
> not helped by lru_lock).
> 
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>