mm/khugepaged: retract_page_tables() without mmap or vma lock

Simplify shmem and file THP collapse's retract_page_tables(), and relax
its locking: to improve its success rate and to lessen impact on others.

Instead of its MADV_COLLAPSE case doing set_huge_pmd() at target_addr of
target_mm, leave that part of the work to madvise_collapse() calling
collapse_pte_mapped_thp() afterwards: just adjust collapse_file()'s result
code to arrange for that.  That spares retract_page_tables() four
arguments; and since it will be successful in retracting all of the page
tables expected of it, no need to track and return a result code itself.

It needs i_mmap_lock_read(mapping) for traversing the vma interval tree,
but it does not need i_mmap_lock_write() for that: page_vma_mapped_walk()
allows for pte_offset_map_lock() etc to fail, and uses pmd_lock() for
THPs.  retract_page_tables() just needs to use those same spinlocks to
exclude it briefly, while transitioning pmd from page table to none: so
restore its use of pmd_lock() inside of which pte lock is nested.

Users of pte_offset_map_lock() etc all now allow for them to fail: so
retract_page_tables() now has no use for mmap_write_trylock() or
vma_try_start_write().  In common with rmap and page_vma_mapped_walk(), it
does not even need the mmap_read_lock().

But those users do expect the page table to remain a good page table,
until they unlock and rcu_read_unlock(): so the page table cannot be freed
immediately, but rather by the recently added pte_free_defer().

Use the (usually a no-op) pmdp_get_lockless_sync() to send an interrupt
when PAE, and pmdp_collapse_flush() did not already do so: to make sure
that the start,pmdp_get_lockless(),end sequence in __pte_offset_map()
cannot pick up a pmd entry with mismatched pmd_low and pmd_high.

retract_page_tables() can be enhanced to replace_page_tables(), which
inserts the final huge pmd without mmap lock: going through an invalid
state instead of pmd_none() followed by fault.  But that enhancement does
raise some more questions: leave it until a later release.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Hugh Dickins <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Alistair Popple <[email protected]>
Cc: Aneesh Kumar K.V <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Claudio Imbrenda <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Huang, Ying <[email protected]>
Cc: Ira Weiny <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Pavel Tatashin <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Qi Zheng <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: Russell King <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Song Liu <[email protected]>
Cc: Steven Price <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Thomas Hellström <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Vishal Moola (Oracle) <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: Zack Rusin <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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