mm/gup: check every subpage of a compound page during isolation

When pages are isolated in check_and_migrate_movable_pages() we skip
compound number of pages at a time.  However, as Jason noted, it is not
necessary correct that pages[i] corresponds to the pages that we
skipped.  This is because it is possible that the addresses in this
range had split_huge_pmd()/split_huge_pud(), and these functions do not
update the compound page metadata.

The problem can be reproduced if something like this occurs:

1. User faulted huge pages.
2. split_huge_pmd() was called for some reason
3. User has unmapped some sub-pages in the range
4. User tries to longterm pin the addresses.

The resulting pages[i] might end-up having pages which are not compound
size page aligned.

Link: https://lkml.kernel.org/r/20210215161349.246722-3-pasha.tatashin@soleen.com
Fixes: d0c9f2992205 ("mm/gup: speed up check_and_migrate_cma_pages() on huge page")
Signed-off-by: Pavel Tatashin <pasha.tatashin@soleen.com>
Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sashal@kernel.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Tyler Hicks <tyhicks@linux.microsoft.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/gup.c b/mm/gup.c
index b63e2c0df3c758500f68cc391b462432828cf618..16015718c0df06a65645a1a4c7e6ecf991743138 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1609,26 +1609,23 @@ static long check_and_migrate_cma_pages(struct mm_struct *mm,
                                        unsigned int gup_flags)
 {
        unsigned long i;
-       unsigned long step;
        bool drain_allow = true;
        bool migrate_allow = true;
        LIST_HEAD(cma_page_list);
        long ret = nr_pages;
+       struct page *prev_head, *head;
        struct migration_target_control mtc = {
                .nid = NUMA_NO_NODE,
                .gfp_mask = GFP_USER | __GFP_NOWARN,
        };
 
 check_again:
-       for (i = 0; i < nr_pages;) {
-
-               struct page *head = compound_head(pages[i]);
-
-               /*
-                * gup may start from a tail page. Advance step by the left
-                * part.
-                */
-               step = compound_nr(head) - (pages[i] - head);
+       prev_head = NULL;
+       for (i = 0; i < nr_pages; i++) {
+               head = compound_head(pages[i]);
+               if (head == prev_head)
+                       continue;
+               prev_head = head;
                /*
                 * If we get a page from the CMA zone, since we are going to
                 * be pinning these entries, we might as well move them out
@@ -1652,8 +1649,6 @@ check_again:
                                }
                        }
                }
-
-               i += step;
        }
 
        if (!list_empty(&cma_page_list)) {