Hello,
Here is v2 of the mremap start address optimization / fix for exec warning.
v1->v2:
1. Trigger the optimization for mremaps smaller than a PMD. I tested by tracing
that it works correctly.
2. Fix issue with bogus return value found by Linus if we broke out of the
above loop for the first PMD itself.
Description of patches:
These patches optimizes the start addresses in move_page_tables() and tests the
changes. 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%[email protected]/
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 | 56 +++++++++++++++++++
tools/testing/selftests/mm/mremap_test.c | 69 +++++++++++++++++++++---
2 files changed, 119 insertions(+), 6 deletions(-)
--
2.40.1.698.g37aff9b760-goog
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) <[email protected]>
---
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;
}
--
2.40.1.698.g37aff9b760-goog
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%[email protected]/
[2] https://lore.kernel.org/all/CAHk-=whd7msp8reJPfeGNyt0LiySMT0egExx3TVZSX3Ok6X=9g@mail.gmail.com/
Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Joel Fernandes (Google) <[email protected]>
---
mm/mremap.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 56 insertions(+)
diff --git a/mm/mremap.c b/mm/mremap.c
index 411a85682b58..be81eb3a80a8 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 - (old_addr & ~PMD_MASK))
+ 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);
@@ -565,6 +614,13 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
mmu_notifier_invalidate_range_end(&range);
+ /*
+ * Prevent negative return values when {old,new}_addr was realigned
+ * but we broke out of the above loop for the first PMD itself.
+ */
+ if (len + old_addr < old_end)
+ return 0;
+
return len + old_addr - old_end; /* how much done */
}
--
2.40.1.698.g37aff9b760-goog