mm/hmm: comment on VM_FAULT_RETRY semantics in handle_mm_fault

The magic dropping of mmap_sem when handle_mm_fault returns VM_FAULT_RETRY
is rather subtile.  Add a comment explaining it.

Link: https://lore.kernel.org/r/20190724065258.16603-8-hch@lst.de
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
[hch: wrote a changelog]
Signed-off-by: Christoph Hellwig <hch@lst.de>

diff --git a/mm/hmm.c b/mm/hmm.c
index 16b6731..54b3a41 100644 (file)
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -301,8 +301,10 @@ static int hmm_vma_do_fault(struct mm_walk *walk, unsigned long addr,
        flags |= hmm_vma_walk->block ? 0 : FAULT_FLAG_ALLOW_RETRY;
        flags |= write_fault ? FAULT_FLAG_WRITE : 0;
        ret = handle_mm_fault(vma, addr, flags);
-       if (ret & VM_FAULT_RETRY)
+       if (ret & VM_FAULT_RETRY) {
+               /* Note, handle_mm_fault did up_read(&mm->mmap_sem)) */
                return -EAGAIN;
+       }
        if (ret & VM_FAULT_ERROR) {
                *pfn = range->values[HMM_PFN_ERROR];
                return -EFAULT;