mm,thp,shm: limit gfp mask to no more than specified

Matthew Wilcox pointed out that the i915 driver opportunistically
allocates tmpfs memory, but will happily reclaim some of its pool if no
memory is available.

Make sure the gfp mask used to opportunistically allocate a THP is always
at least as restrictive as the original gfp mask.

Link: https://lkml.kernel.org/r/20201124194925.623931-3-riel@surriel.com
Signed-off-by: Rik van Riel <riel@surriel.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Xu Yu <xuyu@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index 8414f12ac05cf206084c88ad216df59a3e5a8da0..de261cfbf9871bd06a1e5eeb9b34a727d7250961 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1503,6 +1503,26 @@ static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
        return page;
 }
 
+/*
+ * Make sure huge_gfp is always more limited than limit_gfp.
+ * Some of the flags set permissions, while others set limitations.
+ */
+static gfp_t limit_gfp_mask(gfp_t huge_gfp, gfp_t limit_gfp)
+{
+       gfp_t allowflags = __GFP_IO | __GFP_FS | __GFP_RECLAIM;
+       gfp_t denyflags = __GFP_NOWARN | __GFP_NORETRY;
+       gfp_t result = huge_gfp & ~allowflags;
+
+       /*
+        * Minimize the result gfp by taking the union with the deny flags,
+        * and the intersection of the allow flags.
+        */
+       result |= (limit_gfp & denyflags);
+       result |= (huge_gfp & limit_gfp) & allowflags;
+
+       return result;
+}
+
 static struct page *shmem_alloc_hugepage(gfp_t gfp,
                struct shmem_inode_info *info, pgoff_t index)
 {
@@ -1862,6 +1882,7 @@ repeat:
 
 alloc_huge:
        huge_gfp = vma_thp_gfp_mask(vma);
+       huge_gfp = limit_gfp_mask(huge_gfp, gfp);
        page = shmem_alloc_and_acct_page(huge_gfp, inode, index, true);
        if (IS_ERR(page)) {
 alloc_nohuge: