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mm, futex: fix shared futex pgoff on shmem huge page 

[ Upstream commit fe19bd3dae3d15d2fbfdb3de8839a6ea0fe94264 ]

If more than one futex is placed on a shmem huge page, it can happen
that waking the second wakes the first instead, and leaves the second
waiting: the key's shared.pgoff is wrong.

When 3.11 commit 13d60f4b6a ("futex: Take hugepages into account when
generating futex_key"), the only shared huge pages came from hugetlbfs,
and the code added to deal with its exceptional page->index was put into
hugetlb source.  Then that was missed when 4.8 added shmem huge pages.

page_to_pgoff() is what others use for this nowadays: except that, as
currently written, it gives the right answer on hugetlbfs head, but
nonsense on hugetlbfs tails.  Fix that by calling hugetlbfs-specific
hugetlb_basepage_index() on PageHuge tails as well as on head.

Yes, it's unconventional to declare hugetlb_basepage_index() there in
pagemap.h, rather than in hugetlb.h; but I do not expect anything but
page_to_pgoff() ever to need it.

[akpm@linux-foundation.org: give hugetlb_basepage_index() prototype the correct scope]

Link: https://lkml.kernel.org/r/b17d946b-d09-326e-b42a-52884c36df32@google.com
Fixes: 800d8c63b2 ("shmem: add huge pages support")
Reported-by: Neel Natu <neelnatu@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Zhang Yi <wetpzy@gmail.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Note on stable backport: leave redundant #include <linux/hugetlb.h>
in kernel/futex.c, to avoid conflict over the header files included.

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Hugh Dickins 2021-06-24 18:39:52 -07:00 committed by Sasha Levin
parent a33b70d625
commit 61168eafe0
4 changed files with 9 additions and 27 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

16
include/linux/hugetlb.h
View File

@ -469,17 +469,6 @@ static inline int hstate_index(struct hstate *h)
return h - hstates;
}
pgoff_t __basepage_index(struct page *page);
/* Return page->index in PAGE_SIZE units */
static inline pgoff_t basepage_index(struct page *page)
{
if (!PageCompound(page))
return page->index;
return __basepage_index(page);
}
extern int dissolve_free_huge_page(struct page *page);
extern int dissolve_free_huge_pages(unsigned long start_pfn,
unsigned long end_pfn);
@ -695,11 +684,6 @@ static inline int hstate_index(struct hstate *h)
return 0;
}
static inline pgoff_t basepage_index(struct page *page)
{
return page->index;
}
static inline int dissolve_free_huge_page(struct page *page)
{
return 0;

13
include/linux/pagemap.h
View File

@ -397,7 +397,7 @@ static inline struct page *read_mapping_page(struct address_space *mapping,
}
/*
* Get index of the page with in radix-tree
* Get index of the page within radix-tree (but not for hugetlb pages).
* (TODO: remove once hugetlb pages will have ->index in PAGE_SIZE)
*/
static inline pgoff_t page_to_index(struct page *page)
@ -416,15 +416,16 @@ static inline pgoff_t page_to_index(struct page *page)
return pgoff;
}
extern pgoff_t hugetlb_basepage_index(struct page *page);
/*
* Get the offset in PAGE_SIZE.
* (TODO: hugepage should have ->index in PAGE_SIZE)
* Get the offset in PAGE_SIZE (even for hugetlb pages).
* (TODO: hugetlb pages should have ->index in PAGE_SIZE)
*/
static inline pgoff_t page_to_pgoff(struct page *page)
{
if (unlikely(PageHeadHuge(page)))
return page->index << compound_order(page);
if (unlikely(PageHuge(page)))
return hugetlb_basepage_index(page);
return page_to_index(page);
}

2
kernel/futex.c
View File

@ -737,7 +737,7 @@ again:
key->both.offset |= FUT_OFF_INODE; /* inode-based key */
key->shared.i_seq = get_inode_sequence_number(inode);
key->shared.pgoff = basepage_index(tail);
key->shared.pgoff = page_to_pgoff(tail);
rcu_read_unlock();
}

5
mm/hugetlb.c
View File

@ -1461,15 +1461,12 @@ int PageHeadHuge(struct page *page_head)
return get_compound_page_dtor(page_head) == free_huge_page;
}
pgoff_t __basepage_index(struct page *page)
pgoff_t hugetlb_basepage_index(struct page *page)
{
struct page *page_head = compound_head(page);
pgoff_t index = page_index(page_head);
unsigned long compound_idx;
if (!PageHuge(page_head))
return page_index(page);
if (compound_order(page_head) >= MAX_ORDER)
compound_idx = page_to_pfn(page) - page_to_pfn(page_head);
else