Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared memory types

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Am 30.05.22 um 18:00 schrieb Peter Xu:
On Mon, May 30, 2022 at 11:52:54AM -0400, Peter Xu wrote:
On Mon, May 30, 2022 at 11:35:10AM +0200, Christian Borntraeger wrote:


Am 29.05.22 um 22:33 schrieb Heiko Carstens:
[...]

Guess the patch below on top of your patch is what we want.
Just for clarification: if gmap is not NULL then the process is a kvm
process. So, depending on the workload, this optimization makes sense.

diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
index 4608cc962ecf..e1d40ca341b7 100644
--- a/arch/s390/mm/fault.c
+++ b/arch/s390/mm/fault.c
@@ -436,12 +436,11 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
   	/* The fault is fully completed (including releasing mmap lock) */
   	if (fault & VM_FAULT_COMPLETED) {
-		/*
-		 * Gmap will need the mmap lock again, so retake it.  TODO:
-		 * only conditionally take the lock when CONFIG_PGSTE set.
-		 */
-		mmap_read_lock(mm);
-		goto out_gmap;
+		if (gmap) {
+			mmap_read_lock(mm);
+			goto out_gmap;
+		}
+		goto out;

Hmm, right after I replied I found "goto out" could be problematic, since
all s390 callers of do_exception() will assume it an error condition (side
note: "goto out_gmap" contains one step to clear "fault" to 0).  I'll
replace this with "return 0" instead if it looks good to both of you.

I'll wait for a confirmation before reposting.  Thanks,

Yes, that sounds good and thank you for double checking.



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