mm/folio: avoid special handling for order value 0 in folio_set_order

folio_set_order(folio, 0) is used in kernel at two places
__destroy_compound_gigantic_folio and __prep_compound_gigantic_folio.
Currently, It is called to clear out the folio->_folio_nr_pages and
folio->_folio_order.

For __destroy_compound_gigantic_folio:
In past, folio_set_order(folio, 0) was needed because page->mapping used
to overlap with _folio_nr_pages and _folio_order. So if these fields were
left uncleared during freeing gigantic hugepages, they were causing
"BUG: bad page state" due to non-zero page->mapping. Now, After
Commit a01f43901cfb ("hugetlb: be sure to free demoted CMA pages to
CMA") page->mapping has explicitly been cleared out for tail pages. Also,
_folio_order and _folio_nr_pages no longer overlaps with page->mapping.

So, folio_set_order(folio, 0) can be removed from freeing gigantic
folio path (__destroy_compound_gigantic_folio).

Another place, folio_set_order(folio, 0) is called inside
__prep_compound_gigantic_folio during error path. Here,
folio_set_order(folio, 0) can also be removed if we move
folio_set_order(folio, order) after for loop.

The patch also moves _folio_set_head call in __prep_compound_gigantic_folio()
such that we avoid clearing them in the error path.

Also, as Mike pointed out:
"It would actually be better to move the calls _folio_set_head and
folio_set_order in __prep_compound_gigantic_folio() as suggested here. Why?
In the current code, the ref count on the 'head page' is still 1 (or more)
while those calls are made. So, someone could take a speculative ref on the
page BEFORE the tail pages are set up."

This way, folio_set_order(folio, 0) is no more needed. And it will also
helps removing the confusion of folio order being set to 0 (as _folio_order
field is part of first tail page).

Testing: I have run LTP tests, which all passes. and also I have written
the test in LTP which tests the bug caused by compound_nr and page->mapping
overlapping.

https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/mem/hugetlb/hugemmap/hugemmap32.c

Running on older kernel ( < 5.10-rc7) with the above bug this fails while
on newer kernel and, also with this patch it passes.

Link: https://lkml.kernel.org/r/20230609162907.111756-1-tsahu@linux.ibm.com
Signed-off-by: Tarun Sahu <tsahu@linux.ibm.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index dfa412d..de96cd9 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -1489,7 +1489,6 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
                        set_page_refcounted(p);
        }
 
-       folio_set_order(folio, 0);
        __folio_clear_head(folio);
 }
 
@@ -1951,9 +1950,6 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
        struct page *p;
 
        __folio_clear_reserved(folio);
-       __folio_set_head(folio);
-       /* we rely on prep_new_hugetlb_folio to set the destructor */
-       folio_set_order(folio, order);
        for (i = 0; i < nr_pages; i++) {
                p = folio_page(folio, i);
 
@@ -1999,6 +1995,9 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
                if (i != 0)
                        set_compound_head(p, &folio->page);
        }
+       __folio_set_head(folio);
+       /* we rely on prep_new_hugetlb_folio to set the destructor */
+       folio_set_order(folio, order);
        atomic_set(&folio->_entire_mapcount, -1);
        atomic_set(&folio->_nr_pages_mapped, 0);
        atomic_set(&folio->_pincount, 0);
@@ -2017,8 +2016,6 @@ out_error:
                p = folio_page(folio, j);
                __ClearPageReserved(p);
        }
-       folio_set_order(folio, 0);
-       __folio_clear_head(folio);
        return false;
 }
 

diff --git a/mm/internal.h b/mm/internal.h
index f45f5eb..faf0508 100644 (file)
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -438,16 +438,12 @@ int split_free_page(struct page *free_page,
  */
 static inline void folio_set_order(struct folio *folio, unsigned int order)
 {
-       if (WARN_ON_ONCE(!folio_test_large(folio)))
+       if (WARN_ON_ONCE(!order || !folio_test_large(folio)))
                return;
 
        folio->_folio_order = order;
 #ifdef CONFIG_64BIT
-       /*
-        * When hugetlb dissolves a folio, we need to clear the tail
-        * page, rather than setting nr_pages to 1.
-        */
-       folio->_folio_nr_pages = order ? 1U << order : 0;
+       folio->_folio_nr_pages = 1U << order;
 #endif
 }