The releasing process of the non-shared anonymous folio mapped solely by
an exiting process may go through two flows: 1) the anonymous folio is
firstly is swaped-out into swapspace and transformed into a swp_entry in
shrink_folio_list; 2) then the swp_entry is released in the process
exiting flow.  This will result in the high cpu load of releasing a
non-shared anonymous folio mapped solely by an exiting process.

When the low system memory and the exiting process exist at the same time,
it will be likely to happen, because the non-shared anonymous folio mapped
solely by an exiting process may be reclaimed by shrink_folio_list.

This patch is that shrink skips the non-shared anonymous folio solely
mapped by an exting process and this folio is only released directly in
the process exiting flow, which will save swap-out time and alleviate the
load of the process exiting.

Barry provided some effectiveness testing in [1].  "I observed that
this patch effectively skipped 6114 folios (either 4KB or 64KB mTHP),
potentially reducing the swap-out by up to 92MB (97,300,480 bytes)
during the process exit.  The working set size is 256MB."

Link: https://lkml.kernel.org/r/20240710083641.546-1-justinjiang@vivo.com
Link: https://lore.kernel.org/linux-mm/20240710033212.36497-1-21cnbao@gmail.com/ [1]
Signed-off-by: Zhiguo Jiang <justinjiang@vivo.com>
Acked-by: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>