mm: don't return 0 too early from find_get_pages()

Callers of find_get_pages(), or its wrapper pagevec_lookup() - notably
truncate_inode_pages_range() - stop looking further when it returns 0.

But if an interrupt comes just after its radix_tree_gang_lookup_slot(),
especially if we have preemptible RCU enabled, isn't it conceivable that
all 14 pages returned could be removed from the page cache by
shrink_page_list(), before find_get_pages() gets to process them?  So
causing it to return 0 although there may be plenty more pages beyond.

Make find_get_pages() and find_get_pages_tag() check for this unlikely
case, and restart should it occur; but callers of find_get_pages_contig()
have no such expectation, it's okay for that to return 0 early.

I have not seen this in practice, just worried by the possibility.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Nick Piggin <npiggin@kernel.dk>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Salman Qazi <sqazi@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index a293181..f807afd 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -885,6 +885,13 @@ repeat:
                pages[ret] = page;
                ret++;
        }
+
+       /*
+        * If all entries were removed before we could secure them,
+        * try again, because callers stop trying once 0 is returned.
+        */
+       if (unlikely(!ret && nr_found))
+               goto restart;
        rcu_read_unlock();
        return ret;
 }
@@ -1004,6 +1011,13 @@ repeat:
                pages[ret] = page;
                ret++;
        }
+
+       /*
+        * If all entries were removed before we could secure them,
+        * try again, because callers stop trying once 0 is returned.
+        */
+       if (unlikely(!ret && nr_found))
+               goto restart;
        rcu_read_unlock();
 
        if (ret)