Now that GUP no longer requires VM_FAULT_WRITE, break_ksm() is the sole
remaining user of VM_FAULT_WRITE. As we also want to stop triggering a
fake write fault and instead use FAULT_FLAG_UNSHARE -- similar to
GUP-triggered unsharing when taking a R/O pin on a shared anonymous page
(including KSM pages), let's stop relying on VM_FAULT_WRITE.
Let's rework break_ksm() to not rely on the return value of
handle_mm_fault() anymore to figure out whether COW-breaking was
successful. Simply perform another follow_page() lookup to verify the
result.
While this makes break_ksm() slightly less efficient, we can simplify
handle_mm_fault() a little and easily switch to FAULT_FLAG_UNSHARE
without introducing similar KSM-specific behavior for
FAULT_FLAG_UNSHARE.
In my setup (AMD Ryzen 9 3900X), running the KSM selftest to test
unmerge performance on 2 GiB (taskset 0x8 ./ksm_tests -D -s 2048), this
results in a performance degradation of ~4% -- 5% (old: ~5250 MiB/s,
new: ~5010 MiB/s).
I don't think that we particularly care about that performance drop when
unmerging. If it ever turns out to be an actual performance issue, we can
think about a better alternative for FAULT_FLAG_UNSHARE -- let's just keep
it simple for now.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---
mm/ksm.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)
diff --git a/mm/ksm.c b/mm/ksm.c
index 0cd2f4b62334..e8d987fb379e 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -473,26 +473,27 @@ static int break_ksm(struct vm_area_struct *vma, unsigned long addr)
vm_fault_t ret = 0;
do {
+ bool ksm_page = false;
+
cond_resched();
page = follow_page(vma, addr,
FOLL_GET | FOLL_MIGRATION | FOLL_REMOTE);
if (IS_ERR_OR_NULL(page))
break;
if (PageKsm(page))
- ret = handle_mm_fault(vma, addr,
- FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
- NULL);
- else
- ret = VM_FAULT_WRITE;
+ ksm_page = true;
put_page(page);
- } while (!(ret & (VM_FAULT_WRITE | VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
+
+ if (!ksm_page)
+ return 0;
+ ret = handle_mm_fault(vma, addr,
+ FAULT_FLAG_WRITE | FAULT_FLAG_REMOTE,
+ NULL);
+ } while (!(ret & (VM_FAULT_SIGBUS | VM_FAULT_SIGSEGV | VM_FAULT_OOM)));
/*
- * We must loop because handle_mm_fault() may back out if there's
- * any difficulty e.g. if pte accessed bit gets updated concurrently.
- *
- * VM_FAULT_WRITE is what we have been hoping for: it indicates that
- * COW has been broken, even if the vma does not permit VM_WRITE;
- * but note that a concurrent fault might break PageKsm for us.
+ * We must loop until we no longer find a KSM page because
+ * handle_mm_fault() may back out if there's any difficulty e.g. if
+ * pte accessed bit gets updated concurrently.
*
* VM_FAULT_SIGBUS could occur if we race with truncation of the
* backing file, which also invalidates anonymous pages: that's
--
2.37.3
