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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
// --------------------------------------------------------------------------------
// LazyCOW.h
//
//
// Provides support for "lazy copy-on-write" pages.
//
// NGEN images contain a large amount of writable data. At runtime, we typically only actually write to a small portion of this data.
// When we write to a writable page in an image, the OS must create a process-local copy of that page, so that other proceses
// cannot see the written data. To prevent this copy from failing, the OS pre-commits space in the pagefile for all writable
// pages when the image is loaded. Thus we get charged for every writable page, even if we only write to a few of them.
//
// FEATURE_LAZY_COW_PAGES enables "lazy copy-on-write." We mark the pages in the image file as read-only, and thus the OS
// does not pre-commit pagefile for these pages. At runtime, prior to writing to any page, we update it to be writable.
// This may fail, and thus is not appropriate for scenarios where strong reliability guarantees must be met. But for
// devices with small memory this is still worth it.
//
// --------------------------------------------------------------------------------
#ifndef LAZY_COW_H
#define LAZY_COW_H
#ifdef FEATURE_LAZY_COW_PAGES
#ifdef _WIN64 // due to the way we track pages, we cannot currently support 64-bit.
#error FEATURE_LAZY_COW_PAGES is only supported on 32-bit platforms.
#endif
class PEDecoder;
// If hModule is a native image, establishes copy-on-write tracking for the image.
// FreeLazyCOWPages must be called immediately before the module is unloaded.
void AllocateLazyCOWPages(PEDecoder * pImage);
// If hModule is a native image, disestablishes copy-on-write tracking for the image.
// The image must be immediately unloaded following this call.
void FreeLazyCOWPages(PEDecoder * pImage);
bool IsInReadOnlyLazyCOWPage(void* p);
// Forces the page(s) covered by the given address range to be made writable,
// if they are being tracked as copy-on-write pages. Otherwise does nothing.
// Returns false if we could not allocate the necessary memory.
bool EnsureWritablePagesNoThrow(void* p, size_t len);
// Version for executable pages
bool EnsureWritableExecutablePagesNoThrow(void* p, size_t len);
// Throwing version of EnsureWritablePagesNoThrow
void EnsureWritablePages(void* p, size_t len);
// Version for executable pages
void EnsureWritableExecutablePages(void* p, size_t len);
#else //FEATURE_LAZY_COW_PAGES
inline bool EnsureWritablePagesNoThrow(void* p, size_t len)
{
return true;
}
inline bool EnsureWritableExecutablePagesNoThrow(void* p, size_t len)
{
return true;
}
inline void EnsureWritablePages(void* p, size_t len)
{
}
inline void EnsureWritableExecutablePages(void* p, size_t len)
{
}
#endif //FEATURE_LAZY_COW_PAGES
// Typed version of EnsureWritable. Returns p, so this can be inserted in expressions.
// Ignores any failure to allocate. In typical cases this means that the write will AV.
// In the CLR that's OK; we handle the AV, try EnsureWritable(void*,size_t), and
// fail-fast when it fails.
template<typename T>
inline T* EnsureWritablePages(T* p)
{
EnsureWritablePages(p, sizeof(T));
return p;
}
template<typename T>
inline T* EnsureWritableExecutablePages(T* p)
{
EnsureWritableExecutablePages(p, sizeof(T));
return p;
}
#endif // LAZY_COW_H
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