mm/gup: update comment explaining why gup_fast() disables IRQs

The current comment in gup_fast() talks about "IPIs that come from THPs
splitting", which is outdated and refers to the old THP splitting
implementation that was removed in commit ad0bed2 ("thp: drop all
split_huge_page()-related code"), which landed in v4.5.  Before then, THP
splitting involved a pmdp_splitting_flush(), which sent an IPI to
serialize against gup_fast().

Nowadays, we use tlb_remove_table_sync_one() to send IPIs that serialize
against gup_fast(); this is used, for example, in THP *collapsing* to stop
gup_fast() walks of a page table before depositing it.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jann Horn <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Kirill A. Shuemov <[email protected]>
Cc: Peter Xu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: thejh

mm/gup.c

@@ -3299,7 +3299,7 @@ static unsigned long gup_fast(unsigned long start, unsigned long end,
 	 * include/asm-generic/tlb.h for more details.
 	 *
 	 * We do not adopt an rcu_read_lock() here as we also want to block IPIs
-	 * that come from THPs splitting.
+	 * that come from callers of tlb_remove_table_sync_one().
 	 */
 	local_irq_save(flags);
 	gup_fast_pgd_range(start, end, gup_flags, pages, &nr_pinned);