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Subject[RFC][PATCH] mm: madvise: MADV_POPULATE for quick pre-faulting
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I've been doing some testing involving large amounts of
page cache. It's quite painful to get hundreds of GB
of page cache mapped in, especially when I am trying to
do it in parallel threads. This is true even when the
page cache is already allocated and I only need to map
it in. The test:

1. take 160 16MB files
2. clone 160 threads, mmap the 16MB files, and either
a. walk through the file touching each page
b. run MADV_POPULATE on the file
3. MADV_DONTNEED on the mmap()'d area

160 threads/processes:
faulting | MADV_POPULATE
Threads: 698 | 102239 (146x speedup)
Proceeses: 154247 | 297518 (1.9x speedup)

single threaded:
faulting | MADV_POPULATE
1908 | 3710 (1.9x speedup)

To fix the thread suckage, this patch just walks the
VMAs and maps all the pages in. Since it does a
bunch of them in one go, it amortizes the cost of
acquiring the mmap_sem across all of those pages.

FAQ:

Why do threads suck so much?

Bouncing the mmap_sem cacheline around, plus anything
else that we write to during a fault. We do one page,
move the cachelines to another CPU, do one more page,
etc...

Does MADV_DONTNEED work for this?

No. It brings the pages in to the page cache, but
does not map them the way it is implemented at the
moment. I guess we'd be within our rights to make
it behave like MADV_POPULATE if we want though.



---

linux.git-davehans/include/uapi/asm-generic/mman-common.h | 1
linux.git-davehans/mm/madvise.c | 40 +++++++++++++-
2 files changed, 40 insertions(+), 1 deletion(-)

diff -puN include/uapi/asm-generic/mman-common.h~madv_populate include/uapi/asm-generic/mman-common.h
--- linux.git/include/uapi/asm-generic/mman-common.h~madv_populate 2013-06-27 15:22:35.651854196 -0700
+++ linux.git-davehans/include/uapi/asm-generic/mman-common.h 2013-06-27 15:22:35.656854418 -0700
@@ -51,6 +51,7 @@
#define MADV_DONTDUMP 16 /* Explicity exclude from the core dump,
overrides the coredump filter bits */
#define MADV_DODUMP 17 /* Clear the MADV_NODUMP flag */
+#define MADV_POPULATE 18 /* Fill in mapping like faults would */

/* compatibility flags */
#define MAP_FILE 0
diff -puN mm/madvise.c~madv_populate mm/madvise.c
--- linux.git/mm/madvise.c~madv_populate 2013-06-27 15:22:35.652854240 -0700
+++ linux.git-davehans/mm/madvise.c 2013-06-27 15:22:35.656854418 -0700
@@ -19,6 +19,7 @@
#include <linux/blkdev.h>
#include <linux/swap.h>
#include <linux/swapops.h>
+#include "internal.h"

/*
* Any behaviour which results in changes to the vma->vm_flags needs to
@@ -31,6 +32,7 @@ static int madvise_need_mmap_write(int b
case MADV_REMOVE:
case MADV_WILLNEED:
case MADV_DONTNEED:
+ case MADV_POPULATE:
return 0;
default:
/* be safe, default to 1. list exceptions explicitly */
@@ -252,6 +254,39 @@ static long madvise_willneed(struct vm_a
}

/*
+ * Do not just populate the page cache (WILLNEED), also map the pages.
+ */
+static long madvise_populate(struct vm_area_struct * vma,
+ struct vm_area_struct ** prev,
+ unsigned long start, unsigned long end)
+{
+ struct file *file = vma->vm_file;
+ int locked = 1;
+ int ret;
+
+ if (file && file->f_mapping->a_ops->get_xip_mem) {
+ /* no bad return value, but ignore advice */
+ return 0;
+ }
+
+ ret = __mlock_vma_pages_range(vma, start, end, &locked);
+ /*
+ * Make sure that out down_read() matches (read vs.
+ * write) what we did in sys_madvise.
+ */
+ BUG_ON(madvise_need_mmap_write(MADV_POPULATE));
+ if (!locked) {
+ down_read(&current->mm->mmap_sem);
+ /* tell sys_madvise we drop mmap_sem: */
+ *prev = NULL;
+ } else {
+ *prev = vma;
+ }
+
+ return ret;
+}
+
+/*
* Application no longer needs these pages. If the pages are dirty,
* it's OK to just throw them away. The app will be more careful about
* data it wants to keep. Be sure to free swap resources too. The
@@ -378,6 +413,8 @@ madvise_vma(struct vm_area_struct *vma,
return madvise_remove(vma, prev, start, end);
case MADV_WILLNEED:
return madvise_willneed(vma, prev, start, end);
+ case MADV_POPULATE:
+ return madvise_populate(vma, prev, start, end);
case MADV_DONTNEED:
return madvise_dontneed(vma, prev, start, end);
default:
@@ -407,6 +444,7 @@ madvise_behavior_valid(int behavior)
#endif
case MADV_DONTDUMP:
case MADV_DODUMP:
+ case MADV_POPULATE:
return 1;

default:
@@ -536,7 +574,7 @@ SYSCALL_DEFINE3(madvise, unsigned long,
goto out;
if (prev)
vma = prev->vm_next;
- else /* madvise_remove dropped mmap_sem */
+ else /* madvise_remove/populate dropped mmap_sem */
vma = find_vma(current->mm, start);
}
out:
_

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