[PATCH] move_one_page() atomicity fix

move_one_page() is awkward.  It grabs an atomic_kmap of the source pte
(because it needs to know if there's really a page there) and then it needs
to allocate a pte for the dest.  But it cannot allocate the dest pte while
holding the src's atomic kmap.

So it performs this little dance peeking at pagetables to predict if
alloc_one_pte_map() might need to perform a pte page allocation.

When I wrote this code I made it conditional on CONFIG_HIGHPTE.  But that was
bogus: even in the !CONFIG_HIGHPTE case, get_one_pte_map_nested() will run
atomic_kmap() against the pte page, which disables preemption.

Net effect: with CONFIG_HIGHMEM && !CONFIG_HIGHPTE we can end up performing a
GFP_KERNEL pte page allocation while preemption is disabled.  It triggers a
might_sleep() warning and indeed is buggy.

So the patch removes the conditionality: even in the !CONFIG_HIGHPTE case we
still do the pagetable peek and drop the kmap if necessary.

(Arguably, we shouldn't be performing the atomic_kmap() at all if
!CONFIG_HIGHPTE: all it does is a pointless preemption disable).

(Arguably, kmap_atomic() should not be disabling preemption if the target
page is not highmem.  But we're doing it anyway at present for consistency
(ie: debug coverage) and because the filemap.c pagecache copying functions
rely on kmap_atomic() disabling do_no_page() for all pages: see
do_no_page()'s use of in_atomic()).

mm/mremap.c

@@ -56,7 +56,6 @@ static pte_t *get_one_pte_map_nested(struct mm_struct *mm, unsigned long addr)
 	return pte;
 }
 
-#ifdef CONFIG_HIGHPTE	/* Save a few cycles on the sane machines */
 static inline int page_table_present(struct mm_struct *mm, unsigned long addr)
 {
 	pgd_t *pgd;
@@ -68,9 +67,6 @@ static inline int page_table_present(struct mm_struct *mm, unsigned long addr)
 	pmd = pmd_offset(pgd, addr);
 	return pmd_present(*pmd);
 }
-#else
-#define page_table_present(mm, addr)	(1)
-#endif
 
 static inline pte_t *alloc_one_pte_map(struct mm_struct *mm, unsigned long addr)
 {