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[PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory

Started byDominik Dingel <dingel@linux.vnet.ibm.com>
First post2015-11-19 01:00 +0100
Last post2015-11-19 12:00 +0100
Articles 4 — 3 participants

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  [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory Dominik Dingel <dingel@linux.vnet.ibm.com> - 2015-11-19 01:00 +0100
    Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in  guest memory Martin Schwidefsky <schwidefsky@de.ibm.com> - 2015-11-19 09:20 +0100
      Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in  guest memory Christian Borntraeger <borntraeger@de.ibm.com> - 2015-11-19 09:30 +0100
        Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in  guest memory Dominik Dingel <dingel@linux.vnet.ibm.com> - 2015-11-19 12:00 +0100

#1272689 — [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory

FromDominik Dingel <dingel@linux.vnet.ibm.com>
Date2015-11-19 01:00 +0100
Subject[PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory
Message-ID<qwkNQ-1An-5@gated-at.bofh.it>
The userfaultfd does need FAULT_FLAG_ALLOW_RETRY to not return
VM_FAULT_SIGBUS.  So we improve the gmap code to handle one
VM_FAULT_RETRY.

Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
---
 arch/s390/mm/pgtable.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
index 54ef3bc..8a0025d 100644
--- a/arch/s390/mm/pgtable.c
+++ b/arch/s390/mm/pgtable.c
@@ -577,15 +577,22 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
 	       unsigned int fault_flags)
 {
 	unsigned long vmaddr;
-	int rc;
+	int rc, fault;
 
+	fault_flags |= FAULT_FLAG_ALLOW_RETRY;
+retry:
 	down_read(&gmap->mm->mmap_sem);
 	vmaddr = __gmap_translate(gmap, gaddr);
 	if (IS_ERR_VALUE(vmaddr)) {
 		rc = vmaddr;
 		goto out_up;
 	}
-	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags)) {
+	fault = fixup_user_fault(current, gmap->mm, vmaddr, fault_flags);
+	if (fault & VM_FAULT_RETRY) {
+		fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
+		fault_flags |= FAULT_FLAG_TRIED;
+		goto retry;
+	} else if (fault) {
 		rc = -EFAULT;
 		goto out_up;
 	}
@@ -717,10 +724,13 @@ int gmap_ipte_notify(struct gmap *gmap, unsigned long gaddr, unsigned long len)
 	spinlock_t *ptl;
 	pte_t *ptep, entry;
 	pgste_t pgste;
+	int fault, fault_flags;
 	int rc = 0;
 
+	fault_flags = FAULT_FLAG_WRITE | FAULT_FLAG_ALLOW_RETRY;
 	if ((gaddr & ~PAGE_MASK) || (len & ~PAGE_MASK))
 		return -EINVAL;
+retry:
 	down_read(&gmap->mm->mmap_sem);
 	while (len) {
 		/* Convert gmap address and connect the page tables */
@@ -730,7 +740,12 @@ int gmap_ipte_notify(struct gmap *gmap, unsigned long gaddr, unsigned long len)
 			break;
 		}
 		/* Get the page mapped */
-		if (fixup_user_fault(current, gmap->mm, addr, FAULT_FLAG_WRITE)) {
+		fault = fixup_user_fault(current, gmap->mm, addr, fault_flags);
+		if (fault & VM_FAULT_RETRY) {
+			fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
+			fault_flags |= FAULT_FLAG_TRIED;
+			goto retry;
+		} else if (fault) {
 			rc = -EFAULT;
 			break;
 		}
@@ -794,7 +809,9 @@ int set_guest_storage_key(struct mm_struct *mm, unsigned long addr,
 	spinlock_t *ptl;
 	pgste_t old, new;
 	pte_t *ptep;
+	int fault, fault_flags;
 
+	fault_flags = FAULT_FLAG_WRITE | FAULT_FLAG_ALLOW_RETRY;
 	down_read(&mm->mmap_sem);
 retry:
 	ptep = get_locked_pte(mm, addr, &ptl);
@@ -805,10 +822,13 @@ retry:
 	if (!(pte_val(*ptep) & _PAGE_INVALID) &&
 	     (pte_val(*ptep) & _PAGE_PROTECT)) {
 		pte_unmap_unlock(ptep, ptl);
-		if (fixup_user_fault(current, mm, addr, FAULT_FLAG_WRITE)) {
+		fault = fixup_user_fault(current, mm, addr, fault_flags);
+		if (fault && !(fault & VM_FAULT_RETRY)) {
 			up_read(&mm->mmap_sem);
 			return -EFAULT;
 		}
+		fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
+		fault_flags |= FAULT_FLAG_TRIED;
 		goto retry;
 	}
 
-- 
2.3.9

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#1272917 — Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory

FromMartin Schwidefsky <schwidefsky@de.ibm.com>
Date2015-11-19 09:20 +0100
SubjectRe: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory
Message-ID<qwsBJ-6MQ-11@gated-at.bofh.it>
In reply to#1272689
On Thu, 19 Nov 2015 00:49:58 +0100
Dominik Dingel <dingel@linux.vnet.ibm.com> wrote:

> The userfaultfd does need FAULT_FLAG_ALLOW_RETRY to not return
> VM_FAULT_SIGBUS.  So we improve the gmap code to handle one
> VM_FAULT_RETRY.
> 
> Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
> ---
>  arch/s390/mm/pgtable.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
> index 54ef3bc..8a0025d 100644
> --- a/arch/s390/mm/pgtable.c
> +++ b/arch/s390/mm/pgtable.c
> @@ -577,15 +577,22 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
>  	       unsigned int fault_flags)
>  {
>  	unsigned long vmaddr;
> -	int rc;
> +	int rc, fault;
> 
> +	fault_flags |= FAULT_FLAG_ALLOW_RETRY;
> +retry:
>  	down_read(&gmap->mm->mmap_sem);
>  	vmaddr = __gmap_translate(gmap, gaddr);
>  	if (IS_ERR_VALUE(vmaddr)) {
>  		rc = vmaddr;
>  		goto out_up;
>  	}
> -	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags)) {
> +	fault = fixup_user_fault(current, gmap->mm, vmaddr, fault_flags);
> +	if (fault & VM_FAULT_RETRY) {
> +		fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
> +		fault_flags |= FAULT_FLAG_TRIED;
> +		goto retry;
> +	} else if (fault) {
>  		rc = -EFAULT;
>  		goto out_up;
>  	}

Me thinks that you want to add the retry code into fixup_user_fault itself.
You basically have the same code around the three calls to fixup_user_fault.
Yes, it will be a common code patch but I guess that it will be acceptable
given userfaultfd as a reason.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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#1272926 — Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory

FromChristian Borntraeger <borntraeger@de.ibm.com>
Date2015-11-19 09:30 +0100
SubjectRe: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory
Message-ID<qwsLn-6Qc-7@gated-at.bofh.it>
In reply to#1272917
On 11/19/2015 09:18 AM, Martin Schwidefsky wrote:
> On Thu, 19 Nov 2015 00:49:58 +0100
> Dominik Dingel <dingel@linux.vnet.ibm.com> wrote:
> 
>> The userfaultfd does need FAULT_FLAG_ALLOW_RETRY to not return
>> VM_FAULT_SIGBUS.  So we improve the gmap code to handle one
>> VM_FAULT_RETRY.
>>
>> Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
>> ---
>>  arch/s390/mm/pgtable.c | 28 ++++++++++++++++++++++++----
>>  1 file changed, 24 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
>> index 54ef3bc..8a0025d 100644
>> --- a/arch/s390/mm/pgtable.c
>> +++ b/arch/s390/mm/pgtable.c
>> @@ -577,15 +577,22 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
>>  	       unsigned int fault_flags)
>>  {
>>  	unsigned long vmaddr;
>> -	int rc;
>> +	int rc, fault;
>>
>> +	fault_flags |= FAULT_FLAG_ALLOW_RETRY;
>> +retry:
>>  	down_read(&gmap->mm->mmap_sem);
>>  	vmaddr = __gmap_translate(gmap, gaddr);
>>  	if (IS_ERR_VALUE(vmaddr)) {
>>  		rc = vmaddr;
>>  		goto out_up;
>>  	}
>> -	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags)) {
>> +	fault = fixup_user_fault(current, gmap->mm, vmaddr, fault_flags);
>> +	if (fault & VM_FAULT_RETRY) {
>> +		fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
>> +		fault_flags |= FAULT_FLAG_TRIED;
>> +		goto retry;
>> +	} else if (fault) {
>>  		rc = -EFAULT;
>>  		goto out_up;
>>  	}
> 
> Me thinks that you want to add the retry code into fixup_user_fault itself.
> You basically have the same code around the three calls to fixup_user_fault.
> Yes, it will be a common code patch but I guess that it will be acceptable
> given userfaultfd as a reason.

That makes a lot of sense. In an earlier discussion (a followup of Jasons
mm: Loosen MADV_NOHUGEPAGE to enable Qemu postcopy on s390) patch.

Andrea suggested the following:

It's probably better to add a fixup_user_fault_unlocked that will work
like get_user_pages_unlocked. I.e. leaves the details of the mmap_sem
locking internally to the function, and will handle VM_FAULT_RETRY
automatically by re-taking the mmap_sem and repeating the
fixup_user_fault after updating the FAULT_FLAG_ALLOW_RETRY to
FAULT_FLAG_TRIED.

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#1273033 — Re: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory

FromDominik Dingel <dingel@linux.vnet.ibm.com>
Date2015-11-19 12:00 +0100
SubjectRe: [PATCH 2/2] s390/mm: allow gmap code to retry on faulting in guest memory
Message-ID<qwv6z-8c4-41@gated-at.bofh.it>
In reply to#1272926
On Thu, 19 Nov 2015 09:25:24 +0100
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 11/19/2015 09:18 AM, Martin Schwidefsky wrote:
> > On Thu, 19 Nov 2015 00:49:58 +0100
> > Dominik Dingel <dingel@linux.vnet.ibm.com> wrote:
> > 
> >> The userfaultfd does need FAULT_FLAG_ALLOW_RETRY to not return
> >> VM_FAULT_SIGBUS.  So we improve the gmap code to handle one
> >> VM_FAULT_RETRY.
> >>
> >> Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
> >> ---
> >>  arch/s390/mm/pgtable.c | 28 ++++++++++++++++++++++++----
> >>  1 file changed, 24 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
> >> index 54ef3bc..8a0025d 100644
> >> --- a/arch/s390/mm/pgtable.c
> >> +++ b/arch/s390/mm/pgtable.c
> >> @@ -577,15 +577,22 @@ int gmap_fault(struct gmap *gmap, unsigned long gaddr,
> >>  	       unsigned int fault_flags)
> >>  {
> >>  	unsigned long vmaddr;
> >> -	int rc;
> >> +	int rc, fault;
> >>
> >> +	fault_flags |= FAULT_FLAG_ALLOW_RETRY;
> >> +retry:
> >>  	down_read(&gmap->mm->mmap_sem);
> >>  	vmaddr = __gmap_translate(gmap, gaddr);
> >>  	if (IS_ERR_VALUE(vmaddr)) {
> >>  		rc = vmaddr;
> >>  		goto out_up;
> >>  	}
> >> -	if (fixup_user_fault(current, gmap->mm, vmaddr, fault_flags)) {
> >> +	fault = fixup_user_fault(current, gmap->mm, vmaddr, fault_flags);
> >> +	if (fault & VM_FAULT_RETRY) {
> >> +		fault_flags &= ~FAULT_FLAG_ALLOW_RETRY;
> >> +		fault_flags |= FAULT_FLAG_TRIED;
> >> +		goto retry;
> >> +	} else if (fault) {
> >>  		rc = -EFAULT;
> >>  		goto out_up;
> >>  	}
> > 
> > Me thinks that you want to add the retry code into fixup_user_fault itself.
> > You basically have the same code around the three calls to fixup_user_fault.
> > Yes, it will be a common code patch but I guess that it will be acceptable
> > given userfaultfd as a reason.
> 
> That makes a lot of sense. In an earlier discussion (a followup of Jasons
> mm: Loosen MADV_NOHUGEPAGE to enable Qemu postcopy on s390) patch.
> 
> Andrea suggested the following:
> 
> It's probably better to add a fixup_user_fault_unlocked that will work
> like get_user_pages_unlocked. I.e. leaves the details of the mmap_sem
> locking internally to the function, and will handle VM_FAULT_RETRY
> automatically by re-taking the mmap_sem and repeating the
> fixup_user_fault after updating the FAULT_FLAG_ALLOW_RETRY to
> FAULT_FLAG_TRIED.

I know, I saw his mail. But within the gmap code we need to take the mmap_sem before calling fixup_user_fault as well as holding it for later on like __gmap_link.

We could introduce a new wrapper arround fixup_user_fault, like:
fixup_user_fault_retry, which would take care of the retry logic, but does not encapsulate the complete mmap_sem logic.

@Kirill would that be acceptable for you as well?

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