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author | Tarun Sahu <tsahu@linux.ibm.com> | 2023-06-09 21:59:07 +0530 |
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committer | Andrew Morton <akpm@linux-foundation.org> | 2023-06-19 16:19:11 -0700 |
commit | e3b7bf972d632288ccad95b116628e3141be676e (patch) | |
tree | ed0e3c29b62dc1ea8c089d73cdbaf6d4b71849ef /mm/hugetlb.c | |
parent | be5e015d107d5336f298b74ea5a4f0b1773bc6f9 (diff) | |
download | lwn-e3b7bf972d632288ccad95b116628e3141be676e.tar.gz lwn-e3b7bf972d632288ccad95b116628e3141be676e.zip |
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>
Diffstat (limited to 'mm/hugetlb.c')
-rw-r--r-- | mm/hugetlb.c | 9 |
1 files changed, 3 insertions, 6 deletions
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index dfa412d8cb30..de96cd936b12 100644 --- 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; } |