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// Copyright (c) 2009-2010 Satoshi Nakamoto
// Copyright (c) 2009-2015 The Bitcoin Core developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.
#ifndef BITCOIN_SUPPORT_PAGELOCKER_H
#define BITCOIN_SUPPORT_PAGELOCKER_H
#include "support/cleanse.h"
#include <map>
#include <boost/thread/mutex.hpp>
#include <boost/thread/once.hpp>
/**
* Thread-safe class to keep track of locked (ie, non-swappable) memory pages.
*
* Memory locks do not stack, that is, pages which have been locked several times by calls to mlock()
* will be unlocked by a single call to munlock(). This can result in keying material ending up in swap when
* those functions are used naively. This class simulates stacking memory locks by keeping a counter per page.
*
* @note By using a map from each page base address to lock count, this class is optimized for
* small objects that span up to a few pages, mostly smaller than a page. To support large allocations,
* something like an interval tree would be the preferred data structure.
*/
template <class Locker>
class LockedPageManagerBase
{
public:
LockedPageManagerBase(size_t _page_size) : page_size(_page_size)
{
// Determine bitmask for extracting page from address
assert(!(_page_size & (_page_size - 1))); // size must be power of two
page_mask = ~(_page_size - 1);
}
~LockedPageManagerBase()
{
}
// For all pages in affected range, increase lock count
void LockRange(void* p, size_t size)
{
boost::mutex::scoped_lock lock(mutex);
if (!size)
return;
const size_t base_addr = reinterpret_cast<size_t>(p);
const size_t start_page = base_addr & page_mask;
const size_t end_page = (base_addr + size - 1) & page_mask;
for (size_t page = start_page; page <= end_page; page += page_size) {
Histogram::iterator it = histogram.find(page);
if (it == histogram.end()) // Newly locked page
{
locker.Lock(reinterpret_cast<void*>(page), page_size);
histogram.insert(std::make_pair(page, 1));
} else // Page was already locked; increase counter
{
it->second += 1;
}
}
}
// For all pages in affected range, decrease lock count
void UnlockRange(void* p, size_t size)
{
boost::mutex::scoped_lock lock(mutex);
if (!size)
return;
const size_t base_addr = reinterpret_cast<size_t>(p);
const size_t start_page = base_addr & page_mask;
const size_t end_page = (base_addr + size - 1) & page_mask;
for (size_t page = start_page; page <= end_page; page += page_size) {
Histogram::iterator it = histogram.find(page);
assert(it != histogram.end()); // Cannot unlock an area that was not locked
// Decrease counter for page, when it is zero, the page will be unlocked
it->second -= 1;
if (it->second == 0) // Nothing on the page anymore that keeps it locked
{
// Unlock page and remove the count from histogram
locker.Unlock(reinterpret_cast<void*>(page), page_size);
histogram.erase(it);
}
}
}
// Get number of locked pages for diagnostics
int GetLockedPageCount()
{
boost::mutex::scoped_lock lock(mutex);
return histogram.size();
}
private:
Locker locker;
boost::mutex mutex;
size_t page_size, page_mask;
// map of page base address to lock count
typedef std::map<size_t, int> Histogram;
Histogram histogram;
};
/**
* OS-dependent memory page locking/unlocking.
* Defined as policy class to make stubbing for test possible.
*/
class MemoryPageLocker
{
public:
/** Lock memory pages.
* addr and len must be a multiple of the system page size
*/
bool Lock(const void* addr, size_t len);
/** Unlock memory pages.
* addr and len must be a multiple of the system page size
*/
bool Unlock(const void* addr, size_t len);
};
/**
* Singleton class to keep track of locked (ie, non-swappable) memory pages, for use in
* std::allocator templates.
*
* Some implementations of the STL allocate memory in some constructors (i.e., see
* MSVC's vector<T> implementation where it allocates 1 byte of memory in the allocator.)
* Due to the unpredictable order of static initializers, we have to make sure the
* LockedPageManager instance exists before any other STL-based objects that use
* secure_allocator are created. So instead of having LockedPageManager also be
* static-initialized, it is created on demand.
*/
class LockedPageManager : public LockedPageManagerBase<MemoryPageLocker>
{
public:
static LockedPageManager& Instance()
{
boost::call_once(LockedPageManager::CreateInstance, LockedPageManager::init_flag);
return *LockedPageManager::_instance;
}
private:
LockedPageManager();
static void CreateInstance()
{
// Using a local static instance guarantees that the object is initialized
// when it's first needed and also deinitialized after all objects that use
// it are done with it. I can think of one unlikely scenario where we may
// have a static deinitialization order/problem, but the check in
// LockedPageManagerBase's destructor helps us detect if that ever happens.
static LockedPageManager instance;
LockedPageManager::_instance = &instance;
}
static LockedPageManager* _instance;
static boost::once_flag init_flag;
};
#endif // BITCOIN_SUPPORT_PAGELOCKER_H
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