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Re: hugetlb BUILD REGRESSION in linux-next20221121 — Semantic Matching Tool

Re: hugetlb BUILD REGRESSION in linux-next20221121

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On 11/28/22 11:40 PM, Rasmus Villemoes wrote:
On 29/11/2022 05.57, Dan Carpenter wrote:
On Mon, Nov 28, 2022 at 02:34:54PM -0800, Sidhartha Kumar wrote:
Hello,

One of my patches in linux-next was flagged as a Unverified Error/Warning
with the following warning[1]:

mm/hugetlb.c:2073 alloc_pool_huge_page() error: uninitialized symbol
'folio'.

The relevant change is:

-    struct page *page;
+    struct folio *folio;
      int nr_nodes, node;
      gfp_t gfp_mask = htlb_alloc_mask(h) | __GFP_THISNODE;

      for_each_node_mask_to_alloc(h, nr_nodes, node, nodes_allowed) {
-        page = alloc_fresh_huge_page(h, gfp_mask, node, nodes_allowed,
-                        node_alloc_noretry);
-        if (page)
+        folio = alloc_fresh_hugetlb_folio(h, gfp_mask, node,
+                    nodes_allowed, node_alloc_noretry);
+        if (folio)
              break;
      }

-    if (!page)
+    if (!folio)

It looks like I can initialize folio to NULL to avoid this error but I'm not
sure how this would cause a regression as previously the page variable was
unitialized as well. Please let me know if I am missing something in my
patch or if this should be ignored.
Both the original and the new code trigger a Smatch warning.  I don't
know why the kbuild-bot marks this as a new warning.  Possibly that's
because the variable name has changed?  The kbuild-bot is run by Intel.

The problem is obviously that Smatch doesn't know that we always enter
the loop.  There are hack arounds that I could do for this, but
sometimes we dont' actually enter the loop so changing this will hide
bugs...  I am conflicted on this.
It seems that a reasonably clean way to deal with this, since there's
not a lot of code following the loop, is to simply move that code into
the "if (page/folio)" branch. I.e. change the tail to


    if (page) {
      free_huge_page(page); /* free it into the hugepage allocator */
      return 1;
    }
  } // this is the end of loop
  return 0;

or however it looks after the above change.

Thanks for the suggestion, this looks like a good way to avoid the smatch warning and I'll add it to the next version of my patch.

Thanks.

Sidhartha Kumar


Rasmus




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