[PATCH RFC 0/4] Optimize mremap during mutual alignment within PMD

[PATCH RFC 0/4] Optimize mremap during mutual alignment within PMD

[Joel Fernandes (Google)]

Hello,

I am posting this as an RFC for any feedback. I have tested them suitably and I
am continuing to test them.

These patches optimizes the start addresses in move_page_tables(). It addresses a
warning [1] that occurs due to a downward, overlapping move on a mutually-aligned
offset within a PMD during exec. By initiating the copy process at the PMD
level when such alignment is present, we can prevent this warning and speed up
the copying process at the same time. Linus Torvalds suggested this idea.

Please check the individual patches for more details.

thanks,

 - Joel

[1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@dhcp22.suse.cz/

Joel Fernandes (Google) (4):
mm/mremap: Optimize the start addresses in move_page_tables()
selftests: mm: Fix failure case when new remap region was not found
selftests: mm: Add a test for mutually aligned moves > PMD size
selftests: mm: Add a test for remapping to area immediately after
existing mapping

mm/mremap.c                              | 49 +++++++++++++++++
tools/testing/selftests/mm/mremap_test.c | 69 +++++++++++++++++++++---
2 files changed, 112 insertions(+), 6 deletions(-)

--
2.40.1.606.ga4b1b128d6-goog

[PATCH RFC 1/4] mm/mremap: Optimize the start addresses in move_page_tables()

[Joel Fernandes (Google)]

Recently, we see reports [1] of a warning that triggers due to
move_page_tables() doing a downward and overlapping move on a
mutually-aligned offset within a PMD. By mutual alignment, I
mean the source and destination addresses of the mremap are at
the same offset within a PMD.

This mutual alignment along with the fact that the move is downward is
sufficient to cause a warning related to having an allocated PMD that
does not have PTEs in it.

This warning will only trigger when there is mutual alignment in the
move operation. A solution, as suggested by Linus Torvalds [2], is to
initiate the copy process at the PMD level whenever such alignment is
present. Implementing this approach will not only prevent the warning
from being triggered, but it will also optimize the operation as this
method should enhance the speed of the copy process whenever there's a
possibility to start copying at the PMD level.

Some more points:
a. The optimization can be done only when both the source and
destination of the mremap do not have anything mapped below it up to a
PMD boundary. I add support to detect that.

b. #1 is not a problem for the call to move_page_tables() from exec.c as
nothing is expected to be mapped below the source/destination. However,
for non-overlapping mutually aligned moves as triggered by mremap(2),
I added support for checking such cases.

c. I currently only optimize for PMD moves, in the future I/we can build
on this work and do PUD moves as well if there is a need for this. But I
want to take it one step at a time.

[1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@dhcp22.suse.cz/
[2] https://lore.kernel.org/all/CAHk-=whd7msp8reJPfeGNyt0LiySMT0egExx3TVZSX3Ok6X=9g@mail.gmail.com/

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 mm/mremap.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/mm/mremap.c b/mm/mremap.c
index 411a85682b58..d7ee3b20e170 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -478,6 +478,51 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma,
 	return moved;
 }
 
+/*
+ * A helper to check if a previous mapping exists. Required for
+ * move_page_tables() and realign_addr() to determine if a previous mapping
+ * exists before we can do realignment optimizations.
+ */
+static bool check_addr_in_prev(struct vm_area_struct *vma, unsigned long addr,
+			       unsigned long mask)
+{
+	int addr_masked = addr & mask;
+	struct vm_area_struct *prev = NULL, *cur = NULL;
+
+	/* If the masked address is within vma, there is no prev mapping of concern. */
+	if (vma->vm_start <= addr_masked)
+		return false;
+
+	/*
+	 * Attempt to find vma before prev that contains the address.
+	 * On any issue, assume the address is within a previous mapping.
+	 * @mmap write lock is held here, so the lookup is safe.
+	 */
+	cur = find_vma_prev(vma->vm_mm, vma->vm_start, &prev);
+	if (!cur || cur != vma || !prev)
+		return true;
+
+	/* The masked address fell within a previous mapping. */
+	if (prev->vm_end > addr_masked)
+		return true;
+
+	return false;
+}
+
+/* Opportunistically realign to specified boundary for faster copy. */
+static void realign_addr(unsigned long *old_addr, struct vm_area_struct *old_vma,
+			 unsigned long *new_addr, struct vm_area_struct *new_vma,
+			 unsigned long mask)
+{
+	if ((*old_addr & ~mask) &&
+	    (*old_addr & ~mask) == (*new_addr & ~mask) &&
+	    !check_addr_in_prev(old_vma, *old_addr, mask) &&
+	    !check_addr_in_prev(new_vma, *new_addr, mask)) {
+		*old_addr = *old_addr & mask;
+		*new_addr = *new_addr & mask;
+	}
+}
+
 unsigned long move_page_tables(struct vm_area_struct *vma,
 		unsigned long old_addr, struct vm_area_struct *new_vma,
 		unsigned long new_addr, unsigned long len,
@@ -493,6 +538,10 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
 
 	old_end = old_addr + len;
 
+	/* If possible, realign addresses to PMD boundary for faster copy. */
+	if (len >= PMD_SIZE)
+		realign_addr(&old_addr, vma, &new_addr, new_vma, PMD_MASK);
+
 	if (is_vm_hugetlb_page(vma))
 		return move_hugetlb_page_tables(vma, new_vma, old_addr,
 						new_addr, len);
-- 
2.40.1.606.ga4b1b128d6-goog

[PATCH RFC 2/4] selftests: mm: Fix failure case when new remap region was not found

[Joel Fernandes (Google)]

When a valid remap region could not be found, the source mapping is not
cleaned up. Fix the goto statement such that the clean up happens.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 tools/testing/selftests/mm/mremap_test.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/mremap_test.c b/tools/testing/selftests/mm/mremap_test.c
index 5c3773de9f0f..6822d657f589 100644
--- a/tools/testing/selftests/mm/mremap_test.c
+++ b/tools/testing/selftests/mm/mremap_test.c
@@ -316,7 +316,7 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 		if (addr + c.dest_alignment < addr) {
 			ksft_print_msg("Couldn't find a valid region to remap to\n");
 			ret = -1;
-			goto out;
+			goto clean_up_src;
 		}
 		addr += c.dest_alignment;
 	}
-- 
2.40.1.606.ga4b1b128d6-goog

[PATCH RFC 3/4] selftests: mm: Add a test for mutually aligned moves > PMD size

[Joel Fernandes (Google)]

This patch adds a test case to check if a PMD-alignment optimization
successfully happens.

I add support to make sure there is some room before the source mapping,
otherwise the optimization to trigger PMD-aligned move will be disabled
as the kernel will detect that a mapping before the source exists and
such optimization becomes impossible.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 tools/testing/selftests/mm/mremap_test.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/mremap_test.c b/tools/testing/selftests/mm/mremap_test.c
index 6822d657f589..6304eb0947a3 100644
--- a/tools/testing/selftests/mm/mremap_test.c
+++ b/tools/testing/selftests/mm/mremap_test.c
@@ -44,6 +44,7 @@ enum {
 	_1MB = 1ULL << 20,
 	_2MB = 2ULL << 20,
 	_4MB = 4ULL << 20,
+	_5MB = 5ULL << 20,
 	_1GB = 1ULL << 30,
 	_2GB = 2ULL << 30,
 	PMD = _2MB,
@@ -235,6 +236,11 @@ static void *get_source_mapping(struct config c)
 	unsigned long long mmap_min_addr;
 
 	mmap_min_addr = get_mmap_min_addr();
+	/*
+	 * For some tests, we need to not have any mappings below the
+	 * source mapping. Add some headroom to mmap_min_addr for this.
+	 */
+	mmap_min_addr += 10 * _4MB;
 
 retry:
 	addr += c.src_alignment;
@@ -434,7 +440,7 @@ static int parse_args(int argc, char **argv, unsigned int *threshold_mb,
 	return 0;
 }
 
-#define MAX_TEST 13
+#define MAX_TEST 14
 #define MAX_PERF_TEST 3
 int main(int argc, char **argv)
 {
@@ -500,6 +506,10 @@ int main(int argc, char **argv)
 	test_cases[12] = MAKE_TEST(PUD, PUD, _2GB, NON_OVERLAPPING, EXPECT_SUCCESS,
 				   "2GB mremap - Source PUD-aligned, Destination PUD-aligned");
 
+	/* Src and Dest addr 1MB aligned. 5MB mremap. */
+	test_cases[13] = MAKE_TEST(_1MB, _1MB, _5MB, NON_OVERLAPPING, EXPECT_SUCCESS,
+				  "5MB mremap - Source 1MB-aligned, Destination 1MB-aligned");
+
 	perf_test_cases[0] =  MAKE_TEST(page_size, page_size, _1GB, NON_OVERLAPPING, EXPECT_SUCCESS,
 					"1GB mremap - Source PTE-aligned, Destination PTE-aligned");
 	/*
-- 
2.40.1.606.ga4b1b128d6-goog

[PATCH RFC 4/4] selftests: mm: Add a test for remapping to area immediately after existing mapping

[Joel Fernandes (Google)]

This patch adds support for verifying that we correctly handle the
situation where something is already mapped before the destination of the remap.

Any realignment of destination address and PMD-copy will destroy that
existing mapping. In such cases, we need to avoid doing the optimization.

To test this, we map an area called the preamble before the remap
region. Then we verify after the mremap operation that this region did not get
corrupted.

Putting some prints in the kernel, I verified that we optimize
correctly in different situations:

Optimize when there is alignment and no previous mapping (this is tested
by previous patch).
<prints>
check_addr_in_prev(old_vma->vm_start=2900000, old_addr=2900000, mask=-2097152): 0
check_addr_in_prev(new_vma->vm_start=2f00000, new_addr=2f00000, mask=-2097152): 0
=== Starting move_page_tables ===
Doing PUD move for 2800000 -> 2e00000 of extent=200000 <-- Optimization
Doing PUD move for 2a00000 -> 3000000 of extent=200000
Doing PUD move for 2c00000 -> 3200000 of extent=200000
</prints>

Don't optimize when there is alignment but there is previous mapping
(this is tested by this patch).
Notice that check_addr_in_prev() returns 1 for the destination mapping
as we detected there is something there.
<prints>
check_addr_in_prev(old_vma->vm_start=2900000, old_addr=2900000, mask=-2097152): 0
check_addr_in_prev(new_vma->vm_start=5700000, new_addr=5700000, mask=-2097152): 1
=== Starting move_page_tables ===
Doing move_ptes for 2900000 -> 5700000 of extent=100000 <-- Unoptimized
Doing PUD move for 2a00000 -> 5800000 of extent=200000
Doing PUD move for 2c00000 -> 5a00000 of extent=200000
</prints>

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 tools/testing/selftests/mm/mremap_test.c | 57 +++++++++++++++++++++---
 1 file changed, 52 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/mm/mremap_test.c b/tools/testing/selftests/mm/mremap_test.c
index 6304eb0947a3..d7366074e2a8 100644
--- a/tools/testing/selftests/mm/mremap_test.c
+++ b/tools/testing/selftests/mm/mremap_test.c
@@ -29,6 +29,7 @@ struct config {
 	unsigned long long dest_alignment;
 	unsigned long long region_size;
 	int overlapping;
+	int dest_preamble_size;
 };
 
 struct test {
@@ -283,7 +284,7 @@ static void *get_source_mapping(struct config c)
 static long long remap_region(struct config c, unsigned int threshold_mb,
 			      char pattern_seed)
 {
-	void *addr, *src_addr, *dest_addr;
+	void *addr, *src_addr, *dest_addr, *dest_preamble_addr;
 	unsigned long long i;
 	struct timespec t_start = {0, 0}, t_end = {0, 0};
 	long long  start_ns, end_ns, align_mask, ret, offset;
@@ -300,7 +301,7 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 		goto out;
 	}
 
-	/* Set byte pattern */
+	/* Set byte pattern for source block. */
 	srand(pattern_seed);
 	for (i = 0; i < threshold; i++)
 		memset((char *) src_addr + i, (char) rand(), 1);
@@ -312,6 +313,9 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 	addr = (void *) (((unsigned long long) src_addr + c.region_size
 			  + offset) & align_mask);
 
+	/* Remap after the destination block preamble. */
+	addr += c.dest_preamble_size;
+
 	/* See comment in get_source_mapping() */
 	if (!((unsigned long long) addr & c.dest_alignment))
 		addr = (void *) ((unsigned long long) addr | c.dest_alignment);
@@ -327,6 +331,24 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 		addr += c.dest_alignment;
 	}
 
+	if (c.dest_preamble_size) {
+		dest_preamble_addr = mmap((void *) addr - c.dest_preamble_size, c.dest_preamble_size,
+					  PROT_READ | PROT_WRITE,
+					  MAP_FIXED_NOREPLACE | MAP_ANONYMOUS | MAP_SHARED,
+							-1, 0);
+		if (dest_preamble_addr == MAP_FAILED) {
+			ksft_print_msg("Failed to map dest preamble region: %s\n",
+					strerror(errno));
+			ret = -1;
+			goto clean_up_src;
+		}
+
+		/* Set byte pattern for the dest preamble block. */
+		srand(pattern_seed);
+		for (i = 0; i < c.dest_preamble_size; i++)
+			memset((char *) dest_preamble_addr + i, (char) rand(), 1);
+	}
+
 	clock_gettime(CLOCK_MONOTONIC, &t_start);
 	dest_addr = mremap(src_addr, c.region_size, c.region_size,
 					  MREMAP_MAYMOVE|MREMAP_FIXED, (char *) addr);
@@ -335,7 +357,7 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 	if (dest_addr == MAP_FAILED) {
 		ksft_print_msg("mremap failed: %s\n", strerror(errno));
 		ret = -1;
-		goto clean_up_src;
+		goto clean_up_dest_preamble;
 	}
 
 	/* Verify byte pattern after remapping */
@@ -353,6 +375,23 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
 		}
 	}
 
+	/* Verify the dest preamble byte pattern after remapping */
+	if (c.dest_preamble_size) {
+		srand(pattern_seed);
+		for (i = 0; i < c.dest_preamble_size; i++) {
+			char c = (char) rand();
+
+			if (((char *) dest_preamble_addr)[i] != c) {
+				ksft_print_msg("Preamble data after remap doesn't match at offset %d\n",
+					       i);
+				ksft_print_msg("Expected: %#x\t Got: %#x\n", c & 0xff,
+					       ((char *) dest_preamble_addr)[i] & 0xff);
+				ret = -1;
+				goto clean_up_dest;
+			}
+		}
+	}
+
 	start_ns = t_start.tv_sec * NS_PER_SEC + t_start.tv_nsec;
 	end_ns = t_end.tv_sec * NS_PER_SEC + t_end.tv_nsec;
 	ret = end_ns - start_ns;
@@ -365,6 +404,9 @@ static long long remap_region(struct config c, unsigned int threshold_mb,
  */
 clean_up_dest:
 	munmap(dest_addr, c.region_size);
+clean_up_dest_preamble:
+	if (c.dest_preamble_size && dest_preamble_addr)
+		munmap(dest_preamble_addr, c.dest_preamble_size);
 clean_up_src:
 	munmap(src_addr, c.region_size);
 out:
@@ -440,7 +482,7 @@ static int parse_args(int argc, char **argv, unsigned int *threshold_mb,
 	return 0;
 }
 
-#define MAX_TEST 14
+#define MAX_TEST 15
 #define MAX_PERF_TEST 3
 int main(int argc, char **argv)
 {
@@ -449,7 +491,7 @@ int main(int argc, char **argv)
 	unsigned int threshold_mb = VALIDATION_DEFAULT_THRESHOLD;
 	unsigned int pattern_seed;
 	int num_expand_tests = 2;
-	struct test test_cases[MAX_TEST];
+	struct test test_cases[MAX_TEST] = {};
 	struct test perf_test_cases[MAX_PERF_TEST];
 	int page_size;
 	time_t t;
@@ -510,6 +552,11 @@ int main(int argc, char **argv)
 	test_cases[13] = MAKE_TEST(_1MB, _1MB, _5MB, NON_OVERLAPPING, EXPECT_SUCCESS,
 				  "5MB mremap - Source 1MB-aligned, Destination 1MB-aligned");
 
+	/* Src and Dest addr 1MB aligned. 5MB mremap. */
+	test_cases[14] = MAKE_TEST(_1MB, _1MB, _5MB, NON_OVERLAPPING, EXPECT_SUCCESS,
+				  "5MB mremap - Source 1MB-aligned, Dest 1MB-aligned with 40MB Preamble");
+	test_cases[14].config.dest_preamble_size = 10 * _4MB;
+
 	perf_test_cases[0] =  MAKE_TEST(page_size, page_size, _1GB, NON_OVERLAPPING, EXPECT_SUCCESS,
 					"1GB mremap - Source PTE-aligned, Destination PTE-aligned");
 	/*
-- 
2.40.1.606.ga4b1b128d6-goog

Re: [PATCH RFC 1/4] mm/mremap: Optimize the start addresses in move_page_tables()

[Linus Torvalds]

On Wed, May 17, 2023 at 7:18 PM Joel Fernandes (Google)
<joel@joelfernandes.org> wrote:
>
> This warning will only trigger when there is mutual alignment in the
> move operation. A solution, as suggested by Linus Torvalds [2], is to
> initiate the copy process at the PMD level whenever such alignment is
> present.

So this patch is actually simpler than I thought it would be.

But I'm a bit nervous about it. In particular, it ends doing

        old_end = old_addr + len;
        ... expand old_addr/new_addr down to the pmd boundary ..
        return len + old_addr - old_end;        /* how much done */

doesn't that return value end up being nonsensical now?

In particular, I think it can return a *negative* value, because of
how old_addr was moved down, and the "now much done" might indeed be
"negative" in the sense that it failed the move even "before" the
original starting point.

And that negative value then ends up being a large positive one as an
"unsigned long", of course.

So I get the feeling that it wants something like

        if (old_addr + len < old_end)
                return 0;

there at the end.

But maybe there is something in there that guarantees that that case
never happens. I didn't think too deeply about it, I just felt this
looked odd.

               Linus

Re: [PATCH RFC 1/4] mm/mremap: Optimize the start addresses in move_page_tables()

[Joel Fernandes]

On Wed, May 17, 2023 at 09:12:07PM -0700, Linus Torvalds wrote:
> On Wed, May 17, 2023 at 7:18 PM Joel Fernandes (Google)
> <joel@joelfernandes.org> wrote:
> >
> > This warning will only trigger when there is mutual alignment in the
> > move operation. A solution, as suggested by Linus Torvalds [2], is to
> > initiate the copy process at the PMD level whenever such alignment is
> > present.
> 
> So this patch is actually simpler than I thought it would be.
> 
> But I'm a bit nervous about it. In particular, it ends doing
> 
>         old_end = old_addr + len;
>         ... expand old_addr/new_addr down to the pmd boundary ..
>         return len + old_addr - old_end;        /* how much done */
> 
> doesn't that return value end up being nonsensical now?

Aargh, Sorry to miss that. Yes, it ends up being bogus in the case where the
loop broke out early due to failure (but only on the first PMD move failure
AFAICS). In the success case (or failures after the first PMD move), it does
not matter because old_addr is updated to what it was without the
optimization.

> In particular, I think it can return a *negative* value, because of
> how old_addr was moved down, and the "now much done" might indeed be
> "negative" in the sense that it failed the move even "before" the
> original starting point.
> 
> And that negative value then ends up being a large positive one as an
> "unsigned long", of course.
> 
> So I get the feeling that it wants something like
> 
>         if (old_addr + len < old_end)
>                 return 0;

I think that will fix it (thanks!). The main thing I think is to not mess up
the second call to move_page_tables() in mremap where it tries to move the
half-moved stuff back:

  move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,  true);

There moved_len comes from the return value of the first call to
move_page_tables().

If we realigned, and then the first PMD alloc failed, moved_len might be
negative as you pointed. If the first PMD move passed, then there is no
issue as I mentioned above.

I will fix this in the next revision and also add a test case for this, I am
wondering how to test it without some kind of error-injection to make the
first PMD copy fail. In any case, I will try to hack my local kernel to test
that.

thanks,

 - Joel


> there at the end.
> 
> But maybe there is something in there that guarantees that that case
> never happens. I didn't think too deeply about it, I just felt this
> looked odd.
> 
>                Linus