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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 a01f43901c ("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>
This commit is contained in:
Tarun Sahu 2023-06-09 21:59:07 +05:30 committed by Andrew Morton
parent be5e015d10
commit e3b7bf972d
2 changed files with 5 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

9
mm/hugetlb.c
View File

@ -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;
}

8
mm/internal.h
View File

@ -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
}