KernelThreadSanitizer (ktsan) has shown that the down_read_trylock() of
mmap_sem in try_to_unmap_one() (when going to set PageMlocked on a page
found mapped in a VM_LOCKED vma) is ineffective against races with
exit_mmap()'s munlock_vma_pages_all(), because mmap_sem is not held when
tearing down an mm.

But that's okay, those races are benign; and although we've believed for
years in that ugly down_read_trylock(), it's unsuitable for the job, and
frustrates the good intention of setting PageMlocked when it fails.

It just doesn't matter if here we read vm_flags an instant before or after
a racing mlock() or munlock() or exit_mmap() sets or clears VM_LOCKED: the
syscalls (or exit) work their way up the address space (taking pt locks
after updating vm_flags) to establish the final state.

We do still need to be careful never to mark a page Mlocked (hence
unevictable) by any race that will not be corrected shortly after.  The
page lock protects from many of the races, but not all (a page is not
necessarily locked when it's unmapped).  But the pte lock we just dropped
is good to cover the rest (and serializes even with
munlock_vma_pages_all(), so no special barriers required): now hold on to
the pte lock while calling mlock_vma_page().  Is that lock ordering safe?
Yes, that's how follow_page_pte() calls it, and how page_remove_rmap()
calls the complementary clear_page_mlock().

This fixes the following case (though not a case which anyone has
complained of), which mmap_sem did not: truncation's preliminary
unmap_mapping_range() is supposed to remove even the anonymous COWs of
filecache pages, and that might race with try_to_unmap_one() on a
VM_LOCKED vma, so that mlock_vma_page() sets PageMlocked just after
zap_pte_range() unmaps the page, causing "Bad page state (mlocked)" when
freed.  The pte lock protects against this.

You could say that it also protects against the more ordinary case, racing
with the preliminary unmapping of a filecache page itself: but in our
current tree, that's independently protected by i_mmap_rwsem; and that
race would be why "Bad page state (mlocked)" was seen before commit
48ec833b

 ("Revert mm/memory.c: share the i_mmap_rwsem").

Vlastimil Babka points out another race which this patch protects against.
 try_to_unmap_one() might reach its mlock_vma_page() TestSetPageMlocked a
moment after munlock_vma_pages_all() did its Phase 1 TestClearPageMlocked:
leaving PageMlocked and unevictable when it should be evictable.  mmap_sem
is ineffective because exit_mmap() does not hold it; page lock ineffective
because __munlock_pagevec() only takes it afterwards, in Phase 2; pte lock
is effective because __munlock_pagevec_fill() takes it to get the page,
after VM_LOCKED was cleared from vm_flags, so visible to try_to_unmap_one.

Kirill Shutemov points out that if the compiler chooses to implement a
"vma->vm_flags &= VM_WHATEVER" or "vma->vm_flags |= VM_WHATEVER" operation
with an intermediate store of unrelated bits set, since I'm here foregoing
its usual protection by mmap_sem, try_to_unmap_one() might catch sight of
a spurious VM_LOCKED in vm_flags, and make the wrong decision.  This does
not appear to be an immediate problem, but we may want to define vm_flags
accessors in future, to guard against such a possibility.

While we're here, make a related optimization in try_to_munmap_one(): if
it's doing TTU_MUNLOCK, then there's no point at all in descending the
page tables and getting the pt lock, unless the vma is VM_LOCKED.  Yes,
that can change racily, but it can change racily even without the
optimization: it's not critical.  Far better not to waste time here.

Stopped short of separating try_to_munlock_one() from try_to_munmap_one()
on this occasion, but that's probably the sensible next step - with a
rename, given that try_to_munlock()'s business is to try to set Mlocked.

Updated the unevictable-lru Documentation, to remove its reference to mmap
semaphore, but found a few more updates needed in just that area.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Rik van Riel <riel@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>