mm/hmm: remove pgmap checking for devmap pages

The checking boils down to some racy check if the pagemap is still
available or not. Instead of checking this, rely entirely on the
notifiers, if a pagemap is destroyed then all pages that belong to it must
be removed from the tables and the notifiers triggered.

Link: https://lore.kernel.org/r/20200327200021.29372-2-jgg@ziepe.ca
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

diff --git a/mm/hmm.c b/mm/hmm.c
index a491d9aaafe45d91f0138b8152f44cb2c50e429a..3a2610e0713329bdc032bc68020e45e5b7425ec8 100644 (file)
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -28,7 +28,6 @@
 
 struct hmm_vma_walk {
        struct hmm_range        *range;
-       struct dev_pagemap      *pgmap;
        unsigned long           last;
        unsigned int            flags;
 };
@@ -196,19 +195,8 @@ static int hmm_vma_handle_pmd(struct mm_walk *walk, unsigned long addr,
                return hmm_vma_fault(addr, end, fault, write_fault, walk);
 
        pfn = pmd_pfn(pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
-       for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++) {
-               if (pmd_devmap(pmd)) {
-                       hmm_vma_walk->pgmap = get_dev_pagemap(pfn,
-                                             hmm_vma_walk->pgmap);
-                       if (unlikely(!hmm_vma_walk->pgmap))
-                               return -EBUSY;
-               }
+       for (i = 0; addr < end; addr += PAGE_SIZE, i++, pfn++)
                pfns[i] = hmm_device_entry_from_pfn(range, pfn) | cpu_flags;
-       }
-       if (hmm_vma_walk->pgmap) {
-               put_dev_pagemap(hmm_vma_walk->pgmap);
-               hmm_vma_walk->pgmap = NULL;
-       }
        hmm_vma_walk->last = end;
        return 0;
 }
@@ -300,15 +288,6 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
        if (fault || write_fault)
                goto fault;
 
-       if (pte_devmap(pte)) {
-               hmm_vma_walk->pgmap = get_dev_pagemap(pte_pfn(pte),
-                                             hmm_vma_walk->pgmap);
-               if (unlikely(!hmm_vma_walk->pgmap)) {
-                       pte_unmap(ptep);
-                       return -EBUSY;
-               }
-       }
-
        /*
         * Since each architecture defines a struct page for the zero page, just
         * fall through and treat it like a normal page.
@@ -328,10 +307,6 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
        return 0;
 
 fault:
-       if (hmm_vma_walk->pgmap) {
-               put_dev_pagemap(hmm_vma_walk->pgmap);
-               hmm_vma_walk->pgmap = NULL;
-       }
        pte_unmap(ptep);
        /* Fault any virtual address we were asked to fault */
        return hmm_vma_fault(addr, end, fault, write_fault, walk);
@@ -418,16 +393,6 @@ again:
                        return r;
                }
        }
-       if (hmm_vma_walk->pgmap) {
-               /*
-                * We do put_dev_pagemap() here and not in hmm_vma_handle_pte()
-                * so that we can leverage get_dev_pagemap() optimization which
-                * will not re-take a reference on a pgmap if we already have
-                * one.
-                */
-               put_dev_pagemap(hmm_vma_walk->pgmap);
-               hmm_vma_walk->pgmap = NULL;
-       }
        pte_unmap(ptep - 1);
 
        hmm_vma_walk->last = addr;
@@ -491,20 +456,9 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
                }
 
                pfn = pud_pfn(pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);
-               for (i = 0; i < npages; ++i, ++pfn) {
-                       hmm_vma_walk->pgmap = get_dev_pagemap(pfn,
-                                             hmm_vma_walk->pgmap);
-                       if (unlikely(!hmm_vma_walk->pgmap)) {
-                               ret = -EBUSY;
-                               goto out_unlock;
-                       }
+               for (i = 0; i < npages; ++i, ++pfn)
                        pfns[i] = hmm_device_entry_from_pfn(range, pfn) |
                                  cpu_flags;
-               }
-               if (hmm_vma_walk->pgmap) {
-                       put_dev_pagemap(hmm_vma_walk->pgmap);
-                       hmm_vma_walk->pgmap = NULL;
-               }
                hmm_vma_walk->last = end;
                goto out_unlock;
        }