The main benefit of THPs are that they can be mapped at the pmd level,
increasing the likelihood of TLB hit and spending less cycles in page
table walks.  pte-mapped hugepages - that is - hugepage-aligned compound
pages of order HPAGE_PMD_ORDER mapped by ptes - although being contiguous
in physical memory, don't have this advantage.  In fact, one could argue
they are detrimental to system performance overall since they occupy a
precious hugepage-aligned/sized region of physical memory that could
otherwise be used more effectively.  Additionally, pte-mapped hugepages
can be the cheapest memory to collapse for khugepaged since no new
hugepage allocation or copying of memory contents is necessary - we only
need to update the mapping page tables.

In the anonymous collapse path, we are able to collapse pte-mapped
hugepages (albeit, perhaps suboptimally), but the file/shmem path makes no
effort when compound pages (of any order) are encountered.

Identify pte-mapped hugepages in the file/shmem collapse path.  The
final step of which makes a racy check of the value of the pmd to
ensure it maps a pte table.  This should be fine, since races that
result in false-positive (i.e.  attempt collapse even though we
shouldn't) will fail later in collapse_pte_mapped_thp() once we
actually lock mmap_lock and reinspect the pmd value.  Races that result
in false-negatives (i.e.  where we decide to not attempt collapse, but
should have) shouldn't be an issue, since in the worst case, we do
nothing - which is what we've done up to this point.  We make a similar
check in retract_page_tables().  If we do think we've found a
pte-mapped hugepgae in khugepaged context, attempt to update page
tables mapping this hugepage.

Note that these collapses still count towards the
/sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed counter,
and if the pte-mapped hugepage was also mapped into multiple process'
address spaces, could be incremented for each page table update.  Since we
increment the counter when a pte-mapped hugepage is successfully added to
the list of to-collapse pte-mapped THPs, it's possible that we never
actually update the page table either.  This is different from how
file/shmem pages_collapsed accounting works today where only a successful
page cache update is counted (it's also possible here that no page tables
are actually changed).  Though it incurs some slop, this is preferred to
either not accounting for the event at all, or plumbing through data in
struct mm_slot on whether to account for the collapse or not.

Also note that work still needs to be done to support arbitrary compound
pages, and that this should all be converted to using folios.

[shy828301@gmail.com: Spelling mistake, update comment, and add Documentation]
  Link: https://lore.kernel.org/linux-mm/CAHbLzkpHwZxFzjfX9nxVoRhzup8WMjMfyL6Xiq8mZ9M-N3ombw@mail.gmail.com/
Link: https://lkml.kernel.org/r/20220907144521.3115321-3-zokeefe@google.com
Link: https://lkml.kernel.org/r/20220922224046.1143204-3-zokeefe@google.comSigned-off-by: Zach O'Keefe <zokeefe@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Chris Kennelly <ckennelly@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: James Houghton <jthoughton@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rongwei Wang <rongwei.wang@linux.alibaba.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>