Purgeable Memory Allocations for Linux

I saw (part of) a video, OS hacking: Purgeable memory, by Andreas Kling who’s writing an operating system called Serenity and recording videos his progress. In the video he implements purgeable memory as found on some Apple platforms by adding special support in the kernel. A process tells the kernel that a particular range of memory isn’t important, and so the kernel can reclaim if it the system is under memory pressure — the memory is purgeable.

Linux has a mechanism like this, madvise(2), that allows processes to provide hints to the kernel on how memory is expected to be used. The flag of interest is MADV_FREE:

The application no longer requires the pages in the range specified by addr and len. The kernel can thus free these pages, but the freeing could be delayed until memory pressure occurs. For each of the pages that has been marked to be freed but has not yet been freed, the free operation will be canceled if the caller writes into the page.

So, given this, I built a proof of concept / toy on top of MADV_FREE that provides this functionality for Linux:

https://github.com/skeeto/purgeable

It allocates anonymous pages using mmap(2). When the allocation is “unlocked” — i.e. the process isn’t actively using it — its pages are marked with MADV_FREE so that the kernel can reclaim them at any time. To lock the allocation so that the process can safely make use of them, the MADV_FREE is canceled. This is all a little trickier than it sounds, and that’s the subject of this article.

Note: There’s also MADV_DONTNEED which seems like it would fit the bill, but it’s implemented incorrectly in Linux. It immediately frees the pages, and so it’s useless for implementing purgeable memory.

Purgeable API

Before diving into the implementation, here’s the API. It’s just four functions with no structure definitions. The pointer used by the API is the memory allocation itself. All the bookkeeping associated with that pointer is hidden away, out of sight from the API’s consumer. The full documentation is in purgeable.h.

void *purgeable_alloc(size_t);
void  purgeable_unlock(void *);
void *purgeable_lock(void *);
void  purgeable_free(void *);

The semantics are much like a C++ weak_ptr in that locking both validates that the allocation is still available and creates a “strong” reference to it that prevents it from being purged. Though unlike a weak reference, the allocation is stickier. It will remain until the system is actually under pressure, not just when the garbage collector happens to run or the last strong reference is gone.

Here’s how it might be used to, say, store decoded PNG data that can decompressed again if needed:

uint32_t *texture = 0;
struct png *png = png_load("texture.png");
if (!png) die();

/* ... */

for (;;) {
    if (!texture) {
        texture = purgeable_alloc(png->width * png->height * 4);
        if (!texture) die();
        png_decode_rgba(png, texture);
    } else if (!purgeable_lock(texture)) {
        purgeable_free(texture);
        texture = 0;
        continue;
    }
    glTexImage2D(
        GL_TEXTURE_2D, 0,
        GL_RGBA, png->width, png->height, 0,
        GL_RGBA, GL_UNSIGNED_BYTE, texture
    );
    purgeable_unlock(texture);
    break;
}

Memory is allocated in a locked state since it’s very likely to be immediately filled with data. The application should unlock it before moving on with other tasks. The purgeable memory must always be freed using purgeable_free(), even if purgeable_lock() failed. This not only frees the bookkeeping, but also releases the now-zero pages and the mapping itself. Originally I had purgeable_lock() free the purgeable memory on failure, but I felt this was clearer. There’s no technical reason it couldn’t, though.

Purgeable Implementation

The main challenge is that the kernel doesn’t necessarily treat the MADV_FREE range contiguously. It might reclaim just some pages, and do so in an arbitrary order. In order to lock the region, each page must be handled individually. Per the man page quoted above, reversing MADV_FREE requires a write to each page — to either trigger a page fault or set a dirty bit.

The only way to tell if a page has been purged is to check if it’s been filled with zeros. That’s easy if we’re sure a particular byte in the page should be zero, but, since this is a library, the caller might just store anything on these pages.

So here’s my solution: To unlock a page, look at the first byte on the page. Remember whether or not it’s zero. If it’s zero, write a 1 into that byte. Once this has been done for all pages, use madvise(2) to mark them all MADV_FREE.

With this approach, the library only needs to track one bit of information per page regardless of the page’s contents. Assuming 4kB pages, each 32kB of allocation has 1 byte of overhead (amortized) — or ~0.003% overhead. Not too bad!

Locking purgeable memory is a little trickier. Again, each page must be visited in turn, and if any page was purged, then the whole allocation is considered lost. If the first byte was non-zero when unlocking, the library checks that it’s still non-zero. If the first byte was zero when unlocking, then it prepares to write a zero back into that byte, which must currently be non-zero.

In either case, the MADV_FREE needs to be canceled using a write, so the library does an atomic compare-and-swap (CAS) to write the correct byte into the page, even if it’s the same value in the non-zero case. The atomic CAS is essential because it ensures the page wasn’t purged between the check and the write, as both are done together, atomically. If every page has the expected first byte, and every CAS succeeded, then the purgeable memory has been successfully locked.

As an optimization, the library could consider more than just the first byte, and look at, say, the first long int on each page. The library does less work when the page contains a non-zero value, and the chance of an arbitrary 8-byte value being zero is much lower. However, I wanted to avoid potential aliasing issues, especially if this library were to be embedded, so I passed on the idea.

Bookkeeping

The bookkeeping data is stored just before the buffer returned as the purgeable memory, and it’s never marked with MADV_FREE. Assuming 4kB pages, for each 128MB of purgeable memory the library allocates one extra anonymous page to track it. The number of pages in the allocation is stored just before the purgeable memory as a size_t, and the rest is the per-page bit table described above.

size_t *p = purgeable_alloc(1<<14);
size_t numpages = p[-1];

So the library can immediately find it starting from the purgeable memory address. Here’s an illustration:

      ,--- p
      |
      v
----------------------------------------------
|...Z|    |    |    |    |    |    |    |    |
----------------------------------------------
 ^  ^
 |  |
 |  `--- size_t numpages
 |
 `--- bit table

The downside is that buffer underflows in the application would easily trample the numpages value because it’s located immediately adjacent. It would be safer to move it to the beginning of the first page before the purgeable memory, but this would have made bit table access more complicated. While the region is locked, the contents of the bit table don’t matter, so it won’t be damaged by an underflow. Another idea: put a checksum alongside numpages. It could just be a simple integer hash.

This makes for a really slick API since the consumer doesn’t need to track anything more than a single pointer, the address of the purgeable memory allocation itself.

Worth using?

I’m not quite sure how often I’d actually use purgeable memory in real programs, especially in software intended to be portable. Each operating system needs its own implementation, and this library is not portable since it relies on interfaces and behaviors specific to Linux.

It also has a not-so-unlikely pathological case: Imagine a program that makes two purgeable memory allocation, and they’re large enough that one always evicts the other. The program would thrash back and forth fighting itself as it used each allocation. Detecting this situation might be difficult, especially as the number of purgeable memory allocations increases.

Regardless, it’s another tool for my software toolbelt.

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null program

Chris Wellons

(PGP)