Commit feda5c39 authored by Hugh Dickins's avatar Hugh Dickins Committed by Andrew Morton
Browse files

mm/pgtable: delete pmd_trans_unstable() and friends 

Delete pmd_trans_unstable, pmd_none_or_trans_huge_or_clear_bad() and
pmd_devmap_trans_unstable(), all now unused.

With mixed feelings, delete all the comments on pmd_trans_unstable(). 
That was very good documentation of a subtle state, and this series does
not even eliminate that state: but rather, normalizes and extends it,
asking pte_offset_map[_lock]() callers to anticipate failure, without
regard for whether mmap_read_lock() or mmap_write_lock() is held.

Retain pud_trans_unstable(), which has one use in __handle_mm_fault(), but
delete its equivalent pud_none_or_trans_huge_or_dev_or_clear_bad().  While
there, move the default arch_needs_pgtable_deposit() definition up near
where pgtable_trans_huge_deposit() and withdraw() are declared.

Link: https://lkml.kernel.org/r/5abdab3-3136-b42e-274d-9c6281bfb79@google.com


Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Song Liu <song@kernel.org>
Cc: Steven Price <steven.price@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Zack Rusin <zackr@vmware.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent c7ad0880

include/linux/pgtable.h

+7 −96
Original line number Diff line number Diff line
@@ -599,6 +599,10 @@ extern void pgtable_trans_huge_deposit(struct mm_struct *mm, pmd_t *pmdp, 
extern pgtable_t pgtable_trans_huge_withdraw(struct mm_struct *mm, pmd_t *pmdp);

#endif



#ifndef arch_needs_pgtable_deposit

#define arch_needs_pgtable_deposit() (false)

#endif



#ifdef CONFIG_TRANSPARENT_HUGEPAGE

/*

 * This is an implementation of pmdp_establish() that is only suitable for an
@@ -1300,9 +1304,10 @@ static inline int pud_trans_huge(pud_t pud) 
}

#endif



/* See pmd_none_or_trans_huge_or_clear_bad for discussion. */

static inline int pud_none_or_trans_huge_or_dev_or_clear_bad(pud_t *pud)

static inline int pud_trans_unstable(pud_t *pud)

{

#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && \

	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)

	pud_t pudval = READ_ONCE(*pud);



	if (pud_none(pudval) || pud_trans_huge(pudval) || pud_devmap(pudval))
@@ -1311,102 +1316,8 @@ static inline int pud_none_or_trans_huge_or_dev_or_clear_bad(pud_t *pud) 
		pud_clear_bad(pud);

		return 1;

	}

	return 0;

}



/* See pmd_trans_unstable for discussion. */

static inline int pud_trans_unstable(pud_t *pud)

{

#if defined(CONFIG_TRANSPARENT_HUGEPAGE) &&			\

	defined(CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD)

	return pud_none_or_trans_huge_or_dev_or_clear_bad(pud);

#else

	return 0;

#endif

}



#ifndef arch_needs_pgtable_deposit

#define arch_needs_pgtable_deposit() (false)

#endif

/*

 * This function is meant to be used by sites walking pagetables with

 * the mmap_lock held in read mode to protect against MADV_DONTNEED and

 * transhuge page faults. MADV_DONTNEED can convert a transhuge pmd

 * into a null pmd and the transhuge page fault can convert a null pmd

 * into an hugepmd or into a regular pmd (if the hugepage allocation

 * fails). While holding the mmap_lock in read mode the pmd becomes

 * stable and stops changing under us only if it's not null and not a

 * transhuge pmd. When those races occurs and this function makes a

 * difference vs the standard pmd_none_or_clear_bad, the result is

 * undefined so behaving like if the pmd was none is safe (because it

 * can return none anyway). The compiler level barrier() is critically

 * important to compute the two checks atomically on the same pmdval.

 *

 * For 32bit kernels with a 64bit large pmd_t this automatically takes

 * care of reading the pmd atomically to avoid SMP race conditions

 * against pmd_populate() when the mmap_lock is hold for reading by the

 * caller (a special atomic read not done by "gcc" as in the generic

 * version above, is also needed when THP is disabled because the page

 * fault can populate the pmd from under us).

 */

static inline int pmd_none_or_trans_huge_or_clear_bad(pmd_t *pmd)

{

	pmd_t pmdval = pmdp_get_lockless(pmd);

	/*

	 * !pmd_present() checks for pmd migration entries

	 *

	 * The complete check uses is_pmd_migration_entry() in linux/swapops.h

	 * But using that requires moving current function and pmd_trans_unstable()

	 * to linux/swapops.h to resolve dependency, which is too much code move.

	 *

	 * !pmd_present() is equivalent to is_pmd_migration_entry() currently,

	 * because !pmd_present() pages can only be under migration not swapped

	 * out.

	 *

	 * pmd_none() is preserved for future condition checks on pmd migration

	 * entries and not confusing with this function name, although it is

	 * redundant with !pmd_present().

	 */

	if (pmd_none(pmdval) || pmd_trans_huge(pmdval) ||

		(IS_ENABLED(CONFIG_ARCH_ENABLE_THP_MIGRATION) && !pmd_present(pmdval)))

		return 1;

	if (unlikely(pmd_bad(pmdval))) {

		pmd_clear_bad(pmd);

		return 1;

	}

	return 0;

}



/*

 * This is a noop if Transparent Hugepage Support is not built into

 * the kernel. Otherwise it is equivalent to

 * pmd_none_or_trans_huge_or_clear_bad(), and shall only be called in

 * places that already verified the pmd is not none and they want to

 * walk ptes while holding the mmap sem in read mode (write mode don't

 * need this). If THP is not enabled, the pmd can't go away under the

 * code even if MADV_DONTNEED runs, but if THP is enabled we need to

 * run a pmd_trans_unstable before walking the ptes after

 * split_huge_pmd returns (because it may have run when the pmd become

 * null, but then a page fault can map in a THP and not a regular page).

 */

static inline int pmd_trans_unstable(pmd_t *pmd)

{

#ifdef CONFIG_TRANSPARENT_HUGEPAGE

	return pmd_none_or_trans_huge_or_clear_bad(pmd);

#else

	return 0;

#endif

}



/*

 * the ordering of these checks is important for pmds with _page_devmap set.

 * if we check pmd_trans_unstable() first we will trip the bad_pmd() check

 * inside of pmd_none_or_trans_huge_or_clear_bad(). this will end up correctly

 * returning 1 but not before it spams dmesg with the pmd_clear_bad() output.

 */

static inline int pmd_devmap_trans_unstable(pmd_t *pmd)

{

	return pmd_devmap(*pmd) || pmd_trans_unstable(pmd);

}



#ifndef CONFIG_NUMA_BALANCING

mm/khugepaged.c

+0 −4
Original line number Diff line number Diff line
@@ -944,10 +944,6 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address, 
	return SCAN_SUCCEED;

}



/*

 * See pmd_trans_unstable() for how the result may change out from

 * underneath us, even if we hold mmap_lock in read.

 */

static int find_pmd_or_thp_or_none(struct mm_struct *mm,

				   unsigned long address,

				   pmd_t **pmd)