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Date: Mon, 06 Feb 2017 07:02:41 -0600
From: "Zi Yan" <zi.yan@...t.com>
To: "Naoya Horiguchi" <n-horiguchi@...jp.nec.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>,
"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"minchan@...nel.org" <minchan@...nel.org>,
"vbabka@...e.cz" <vbabka@...e.cz>,
"mgorman@...hsingularity.net" <mgorman@...hsingularity.net>,
"khandual@...ux.vnet.ibm.com" <khandual@...ux.vnet.ibm.com>,
"Zi Yan" <ziy@...dia.com>
Subject: Re: [PATCH v3 03/14] mm: use pmd lock instead of racy checks in
zap_pmd_range()
On 6 Feb 2017, at 1:43, Naoya Horiguchi wrote:
> On Sun, Feb 05, 2017 at 11:12:41AM -0500, Zi Yan wrote:
>> From: Zi Yan <ziy@...dia.com>
>>
>> Originally, zap_pmd_range() checks pmd value without taking pmd lock.
>> This can cause pmd_protnone entry not being freed.
>>
>> Because there are two steps in changing a pmd entry to a pmd_protnone
>> entry. First, the pmd entry is cleared to a pmd_none entry, then,
>> the pmd_none entry is changed into a pmd_protnone entry.
>> The racy check, even with barrier, might only see the pmd_none entry
>> in zap_pmd_range(), thus, the mapping is neither split nor zapped.
>>
>> Later, in free_pmd_range(), pmd_none_or_clear() will see the
>> pmd_protnone entry and clear it as a pmd_bad entry. Furthermore,
>> since the pmd_protnone entry is not properly freed, the corresponding
>> deposited pte page table is not freed either.
>>
>> This causes memory leak or kernel crashing, if VM_BUG_ON() is enabled.
>>
>> This patch relies on __split_huge_pmd_locked() and
>> __zap_huge_pmd_locked().
>>
>> Signed-off-by: Zi Yan <zi.yan@...rutgers.edu>
>> ---
>> mm/memory.c | 24 +++++++++++-------------
>> 1 file changed, 11 insertions(+), 13 deletions(-)
>>
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 3929b015faf7..7cfdd5208ef5 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -1233,33 +1233,31 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
>> struct zap_details *details)
>> {
>> pmd_t *pmd;
>> + spinlock_t *ptl;
>> unsigned long next;
>>
>> pmd = pmd_offset(pud, addr);
>> + ptl = pmd_lock(vma->vm_mm, pmd);
>
> If USE_SPLIT_PMD_PTLOCKS is true, pmd_lock() returns different ptl for
> each pmd. The following code runs over pmds within [addr, end) with
> a single ptl (of the first pmd,) so I suspect this locking really works.
> Maybe pmd_lock() should be called inside while loop?
According to include/linux/mm.h, pmd_lockptr() first gets the page the pmd is in,
using mask = ~(PTRS_PER_PMD * sizeof(pmd_t) -1) = 0xfffffffffffff000 and virt_to_page().
Then, ptlock_ptr() gets spinlock_t either from page->ptl (split case) or
mm->page_table_lock (not split case).
It seems to me that all PMDs in one page table page share a single spinlock. Let me know
if I misunderstand any code.
But your suggestion can avoid holding the pmd lock for long without cond_sched(),
I can move the spinlock inside the loop.
Thanks.
diff --git a/mm/memory.c b/mm/memory.c
index 5299b261c4b4..ff61d45eaea7 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1260,31 +1260,34 @@ static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
struct zap_details *details)
{
pmd_t *pmd;
- spinlock_t *ptl;
+ spinlock_t *ptl = NULL;
unsigned long next;
pmd = pmd_offset(pud, addr);
- ptl = pmd_lock(vma->vm_mm, pmd);
do {
+ ptl = pmd_lock(vma->vm_mm, pmd);
next = pmd_addr_end(addr, end);
if (is_swap_pmd(*pmd) || pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
if (next - addr != HPAGE_PMD_SIZE) {
VM_BUG_ON_VMA(vma_is_anonymous(vma) &&
!rwsem_is_locked(&tlb->mm->mmap_sem), vma);
__split_huge_pmd_locked(vma, pmd, addr, false);
- } else if (__zap_huge_pmd_locked(tlb, vma, pmd, addr))
- continue;
+ } else if (__zap_huge_pmd_locked(tlb, vma, pmd, addr)) {
+ spin_unlock(ptl);
+ goto next;
+ }
/* fall through */
}
- if (pmd_none_or_clear_bad(pmd))
- continue;
+ if (pmd_none_or_clear_bad(pmd)) {
+ spin_unlock(ptl);
+ goto next;
+ }
spin_unlock(ptl);
next = zap_pte_range(tlb, vma, pmd, addr, next, details);
+next:
cond_resched();
- spin_lock(ptl);
} while (pmd++, addr = next, addr != end);
- spin_unlock(ptl);
return addr;
}
>
> Thanks,
> Naoya Horiguchi
>
>> do {
>> next = pmd_addr_end(addr, end);
>> if (pmd_trans_huge(*pmd) || pmd_devmap(*pmd)) {
>> if (next - addr != HPAGE_PMD_SIZE) {
>> VM_BUG_ON_VMA(vma_is_anonymous(vma) &&
>> !rwsem_is_locked(&tlb->mm->mmap_sem), vma);
>> - __split_huge_pmd(vma, pmd, addr, false, NULL);
>> - } else if (zap_huge_pmd(tlb, vma, pmd, addr))
>> - goto next;
>> + __split_huge_pmd_locked(vma, pmd, addr, false);
>> + } else if (__zap_huge_pmd_locked(tlb, vma, pmd, addr))
>> + continue;
>> /* fall through */
>> }
>> - /*
>> - * Here there can be other concurrent MADV_DONTNEED or
>> - * trans huge page faults running, and if the pmd is
>> - * none or trans huge it can change under us. This is
>> - * because MADV_DONTNEED holds the mmap_sem in read
>> - * mode.
>> - */
>> - if (pmd_none_or_trans_huge_or_clear_bad(pmd))
>> - goto next;
>> +
>> + if (pmd_none_or_clear_bad(pmd))
>> + continue;
>> + spin_unlock(ptl);
>> next = zap_pte_range(tlb, vma, pmd, addr, next, details);
>> -next:
>> cond_resched();
>> + spin_lock(ptl);
>> } while (pmd++, addr = next, addr != end);
>> + spin_unlock(ptl);
>>
>> return addr;
>> }
>> --
>> 2.11.0
>>
--
Best Regards
Yan Zi
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