hugetlb: allow faults to be handled under the VMA lock

Hugetlb can now safely handle faults under the VMA lock, so allow it to do
so.

This patch may cause ltp hugemmap10 to "fail".  Hugemmap10 tests hugetlb
counters, and expects the counters to remain unchanged on failure to
handle a fault.

In hugetlb_no_page(), vmf_anon_prepare() may bailout with no anon_vma
under the VMA lock after allocating a folio for the hugepage.  In
free_huge_folio(), this folio is completely freed on bailout iff there is
a surplus of hugetlb pages.  This will remove a folio off the freelist and
decrement the number of hugepages while ltp expects these counters to
remain unchanged on failure.

Originally this could only happen due to OOM failures, but now it may also
occur after we allocate a hugetlb folio without a suitable anon_vma under
the VMA lock.  This should only happen for the first freshly allocated
hugepage in this vma.

Link: https://lkml.kernel.org/r/20240221234732.187629-6-vishal.moola@gmail.com
Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5e2e4612d458fcf4ddd9807e952a7db4b2c29477..c3d68671cbae87fecde1f04a6befb9f3286b63a0 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -6379,12 +6379,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
                 */
        };
 
-       /* TODO: Handle faults under the VMA lock */
-       if (flags & FAULT_FLAG_VMA_LOCK) {
-               vma_end_read(vma);
-               return VM_FAULT_RETRY;
-       }
-
        /*
         * Serialize hugepage allocation and instantiation, so that we don't
         * get spurious allocation failures if two CPUs race to instantiate