[PATCH] fs: Replace kmap{,_atomic}() with kmap_local_page()

[PATCH] fs: Replace kmap{,_atomic}() with kmap_local_page()

[Fabio M. De Francesco]

The use of kmap() and kmap_atomic() are being deprecated in favor of
kmap_local_page().

With kmap_local_page(), the mappings are per thread, CPU local and not
globally visible. Furthermore, the mappings can be acquired from any
context (including interrupts).

Therefore, use kmap_local_page() in exec.c because these mappings are per
thread, CPU local, and not globally visible.

Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
HIGHMEM64GB enabled.

Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---
 fs/exec.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 0989fb8472a1..4a2129c0d422 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -583,11 +583,11 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
 
 				if (kmapped_page) {
 					flush_dcache_page(kmapped_page);
-					kunmap(kmapped_page);
+					kunmap_local(kaddr);
 					put_arg_page(kmapped_page);
 				}
 				kmapped_page = page;
-				kaddr = kmap(kmapped_page);
+				kaddr = kmap_local_page(kmapped_page);
 				kpos = pos & PAGE_MASK;
 				flush_arg_page(bprm, kpos, kmapped_page);
 			}
@@ -601,7 +601,7 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
 out:
 	if (kmapped_page) {
 		flush_dcache_page(kmapped_page);
-		kunmap(kmapped_page);
+		kunmap_local(kaddr);
 		put_arg_page(kmapped_page);
 	}
 	return ret;
@@ -883,11 +883,11 @@ int transfer_args_to_stack(struct linux_binprm *bprm,
 
 	for (index = MAX_ARG_PAGES - 1; index >= stop; index--) {
 		unsigned int offset = index == stop ? bprm->p & ~PAGE_MASK : 0;
-		char *src = kmap(bprm->page[index]) + offset;
+		char *src = kmap_local_page(bprm->page[index]) + offset;
 		sp -= PAGE_SIZE - offset;
 		if (copy_to_user((void *) sp, src, PAGE_SIZE - offset) != 0)
 			ret = -EFAULT;
-		kunmap(bprm->page[index]);
+		kunmap_local(src);
 		if (ret)
 			goto out;
 	}
@@ -1680,13 +1680,13 @@ int remove_arg_zero(struct linux_binprm *bprm)
 			ret = -EFAULT;
 			goto out;
 		}
-		kaddr = kmap_atomic(page);
+		kaddr = kmap_local_page(page);
 
 		for (; offset < PAGE_SIZE && kaddr[offset];
 				offset++, bprm->p++)
 			;
 
-		kunmap_atomic(kaddr);
+		kunmap_local(kaddr);
 		put_arg_page(page);
 	} while (offset == PAGE_SIZE);
 
-- 
2.36.1

Re: [PATCH] fs: Replace kmap{,_atomic}() with kmap_local_page()

[Eric W. Biederman]

"Fabio M. De Francesco" <fmdefrancesco@gmail.com> writes:

> The use of kmap() and kmap_atomic() are being deprecated in favor of
> kmap_local_page().
>
> With kmap_local_page(), the mappings are per thread, CPU local and not
> globally visible. Furthermore, the mappings can be acquired from any
> context (including interrupts).
>
> Therefore, use kmap_local_page() in exec.c because these mappings are per
> thread, CPU local, and not globally visible.
>
> Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
> HIGHMEM64GB enabled.

Can someone please refresh my memory on what is going on.

I remember there were limitations that kmap_atomic had that are hard to
meet so something I think it was kmap_local was invented and created
to be the kmap_atomic replacement.

What are the requirements on kmap_local?  In copy_strings
kmap is called in contexts that can sleep in page faults so any
nearly any requirement except a thread local use is invalidated.

As you have described kmap_local above it does not sound like kmap_local
is safe in this context, but that could just be a problem in description
that my poor memory does is not recalling the necessary details to
correct.

Eric

> Suggested-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> ---
>  fs/exec.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 0989fb8472a1..4a2129c0d422 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -583,11 +583,11 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>  
>  				if (kmapped_page) {
>  					flush_dcache_page(kmapped_page);
> -					kunmap(kmapped_page);
> +					kunmap_local(kaddr);
>  					put_arg_page(kmapped_page);
>  				}
>  				kmapped_page = page;
> -				kaddr = kmap(kmapped_page);
> +				kaddr = kmap_local_page(kmapped_page);
>  				kpos = pos & PAGE_MASK;
>  				flush_arg_page(bprm, kpos, kmapped_page);
>  			}
> @@ -601,7 +601,7 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>  out:
>  	if (kmapped_page) {
>  		flush_dcache_page(kmapped_page);
> -		kunmap(kmapped_page);
> +		kunmap_local(kaddr);
>  		put_arg_page(kmapped_page);
>  	}
>  	return ret;
> @@ -883,11 +883,11 @@ int transfer_args_to_stack(struct linux_binprm *bprm,
>  
>  	for (index = MAX_ARG_PAGES - 1; index >= stop; index--) {
>  		unsigned int offset = index == stop ? bprm->p & ~PAGE_MASK : 0;
> -		char *src = kmap(bprm->page[index]) + offset;
> +		char *src = kmap_local_page(bprm->page[index]) + offset;
>  		sp -= PAGE_SIZE - offset;
>  		if (copy_to_user((void *) sp, src, PAGE_SIZE - offset) != 0)
>  			ret = -EFAULT;
> -		kunmap(bprm->page[index]);
> +		kunmap_local(src);
>  		if (ret)
>  			goto out;
>  	}
> @@ -1680,13 +1680,13 @@ int remove_arg_zero(struct linux_binprm *bprm)
>  			ret = -EFAULT;
>  			goto out;
>  		}
> -		kaddr = kmap_atomic(page);
> +		kaddr = kmap_local_page(page);
>  
>  		for (; offset < PAGE_SIZE && kaddr[offset];
>  				offset++, bprm->p++)
>  			;
>  
> -		kunmap_atomic(kaddr);
> +		kunmap_local(kaddr);
>  		put_arg_page(page);
>  	} while (offset == PAGE_SIZE);

Re: [PATCH] fs: Replace kmap{,_atomic}() with kmap_local_page()

[Fabio M. De Francesco]

On giovedì 30 giugno 2022 19:38:08 CEST Eric W. Biederman wrote:
> "Fabio M. De Francesco" <fmdefrancesco@gmail.com> writes:
> 
> > The use of kmap() and kmap_atomic() are being deprecated in favor of
> > kmap_local_page().
> >
> > With kmap_local_page(), the mappings are per thread, CPU local and not
> > globally visible. Furthermore, the mappings can be acquired from any
> > context (including interrupts).
> >
> > Therefore, use kmap_local_page() in exec.c because these mappings are 
per
> > thread, CPU local, and not globally visible.
> >
> > Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
> > HIGHMEM64GB enabled.
> 
> Can someone please refresh my memory on what is going on.
> 
> I remember there were limitations that kmap_atomic had that are hard to
> meet so something I think it was kmap_local was invented and created
> to be the kmap_atomic replacement.

Please read highmem.rst. I've updated that document weeks ago:
https://docs.kernel.org/vm/highmem.html?highlight=highmem

Currently it contains many more information I can ever place here in order 
to answer your questions.

Believe me, this is not by any means a way to elude your questions. I'm 
pretty sure that by reading that document you'll have a clear vision on 
what is going on :-)

> 
> What are the requirements on kmap_local?  In copy_strings
> kmap is called in contexts that can sleep in page faults

No problems with kmap_local_page() with regard to page faults (again, 
please read the above-mentioned document).

From that document...

"It’s valid to take pagefaults in a local kmap region []".

"Each call of kmap_atomic() in the kernel creates a non-preemptible section 
and disable pagefaults. This could be a source of unwanted latency. 
Therefore users should prefer kmap_local_page() instead of kmap_atomic().".

> so any
> nearly any requirement except a thread local use is invalidated.
> 
> As you have described kmap_local above it does not sound like kmap_local
> is safe in this context,

Sorry, probably I should add that taking page faults is allowed. Would you 
prefer I send a v2 and add this information?

Thanks,

Fabio

> but that could just be a problem in description
> that my poor memory does is not recalling the necessary details to
> correct.
> 
> Eric
> 
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> > ---
> >  fs/exec.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> >
> > diff --git a/fs/exec.c b/fs/exec.c
> > index 0989fb8472a1..4a2129c0d422 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -583,11 +583,11 @@ static int copy_strings(int argc, struct 
user_arg_ptr argv,
> >  
> >  				if (kmapped_page) {
> >  					
flush_dcache_page(kmapped_page);
> > -					kunmap(kmapped_page);
> > +					kunmap_local(kaddr);
> >  					
put_arg_page(kmapped_page);
> >  				}
> >  				kmapped_page = page;
> > -				kaddr = kmap(kmapped_page);
> > +				kaddr = 
kmap_local_page(kmapped_page);
> >  				kpos = pos & PAGE_MASK;
> >  				flush_arg_page(bprm, kpos, 
kmapped_page);
> >  			}
> > @@ -601,7 +601,7 @@ static int copy_strings(int argc, struct 
user_arg_ptr argv,
> >  out:
> >  	if (kmapped_page) {
> >  		flush_dcache_page(kmapped_page);
> > -		kunmap(kmapped_page);
> > +		kunmap_local(kaddr);
> >  		put_arg_page(kmapped_page);
> >  	}
> >  	return ret;
> > @@ -883,11 +883,11 @@ int transfer_args_to_stack(struct linux_binprm 
*bprm,
> >  
> >  	for (index = MAX_ARG_PAGES - 1; index >= stop; index--) {
> >  		unsigned int offset = index == stop ? bprm->p & 
~PAGE_MASK : 0;
> > -		char *src = kmap(bprm->page[index]) + offset;
> > +		char *src = kmap_local_page(bprm->page[index]) + 
offset;
> >  		sp -= PAGE_SIZE - offset;
> >  		if (copy_to_user((void *) sp, src, PAGE_SIZE - offset) 
!= 0)
> >  			ret = -EFAULT;
> > -		kunmap(bprm->page[index]);
> > +		kunmap_local(src);
> >  		if (ret)
> >  			goto out;
> >  	}
> > @@ -1680,13 +1680,13 @@ int remove_arg_zero(struct linux_binprm *bprm)
> >  			ret = -EFAULT;
> >  			goto out;
> >  		}
> > -		kaddr = kmap_atomic(page);
> > +		kaddr = kmap_local_page(page);
> >  
> >  		for (; offset < PAGE_SIZE && kaddr[offset];
> >  				offset++, bprm->p++)
> >  			;
> >  
> > -		kunmap_atomic(kaddr);
> > +		kunmap_local(kaddr);
> >  		put_arg_page(page);
> >  	} while (offset == PAGE_SIZE);
> 

Re: [PATCH] fs: Replace kmap{,_atomic}() with kmap_local_page()

[Ira Weiny]

On Thu, Jun 30, 2022 at 06:35:27PM +0200, Fabio M. De Francesco wrote:
> The use of kmap() and kmap_atomic() are being deprecated in favor of
> kmap_local_page().
> 
> With kmap_local_page(), the mappings are per thread, CPU local and not
> globally visible. Furthermore, the mappings can be acquired from any
> context (including interrupts).
> 
> Therefore, use kmap_local_page() in exec.c because these mappings are per
> thread, CPU local, and not globally visible.
> 
> Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
> HIGHMEM64GB enabled.
> 
> Suggested-by: Ira Weiny <ira.weiny@intel.com>

This looks good but there is a kmap_atomic() in this file which I _think_ can
be converted as well.  But that is good as a separate patch.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
> ---
>  fs/exec.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 0989fb8472a1..4a2129c0d422 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -583,11 +583,11 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>  
>  				if (kmapped_page) {
>  					flush_dcache_page(kmapped_page);
> -					kunmap(kmapped_page);
> +					kunmap_local(kaddr);
>  					put_arg_page(kmapped_page);
>  				}
>  				kmapped_page = page;
> -				kaddr = kmap(kmapped_page);
> +				kaddr = kmap_local_page(kmapped_page);
>  				kpos = pos & PAGE_MASK;
>  				flush_arg_page(bprm, kpos, kmapped_page);
>  			}
> @@ -601,7 +601,7 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
>  out:
>  	if (kmapped_page) {
>  		flush_dcache_page(kmapped_page);
> -		kunmap(kmapped_page);
> +		kunmap_local(kaddr);
>  		put_arg_page(kmapped_page);
>  	}
>  	return ret;
> @@ -883,11 +883,11 @@ int transfer_args_to_stack(struct linux_binprm *bprm,
>  
>  	for (index = MAX_ARG_PAGES - 1; index >= stop; index--) {
>  		unsigned int offset = index == stop ? bprm->p & ~PAGE_MASK : 0;
> -		char *src = kmap(bprm->page[index]) + offset;
> +		char *src = kmap_local_page(bprm->page[index]) + offset;
>  		sp -= PAGE_SIZE - offset;
>  		if (copy_to_user((void *) sp, src, PAGE_SIZE - offset) != 0)
>  			ret = -EFAULT;
> -		kunmap(bprm->page[index]);
> +		kunmap_local(src);
>  		if (ret)
>  			goto out;
>  	}
> @@ -1680,13 +1680,13 @@ int remove_arg_zero(struct linux_binprm *bprm)
>  			ret = -EFAULT;
>  			goto out;
>  		}
> -		kaddr = kmap_atomic(page);
> +		kaddr = kmap_local_page(page);
>  
>  		for (; offset < PAGE_SIZE && kaddr[offset];
>  				offset++, bprm->p++)
>  			;
>  
> -		kunmap_atomic(kaddr);
> +		kunmap_local(kaddr);
>  		put_arg_page(page);
>  	} while (offset == PAGE_SIZE);
>  
> -- 
> 2.36.1
> 

Re: [PATCH] fs: Replace kmap{,_atomic}() with kmap_local_page()

[Fabio M. De Francesco]

On venerdì 8 luglio 2022 22:18:35 CEST Ira Weiny wrote:
> On Thu, Jun 30, 2022 at 06:35:27PM +0200, Fabio M. De Francesco wrote:
> > The use of kmap() and kmap_atomic() are being deprecated in favor of
> > kmap_local_page().
> > 
> > With kmap_local_page(), the mappings are per thread, CPU local and not
> > globally visible. Furthermore, the mappings can be acquired from any
> > context (including interrupts).
> > 
> > Therefore, use kmap_local_page() in exec.c because these mappings are 
per
> > thread, CPU local, and not globally visible.
> > 
> > Tested with xfstests on a QEMU + KVM 32-bits VM booting a kernel with
> > HIGHMEM64GB enabled.
> > 
> > Suggested-by: Ira Weiny <ira.weiny@intel.com>
> 
> This looks good but there is a kmap_atomic() in this file which I _think_ 
can
> be converted as well.  But that is good as a separate patch.
> 
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> 

Thanks for your review!

I didn't notice that kmap_atomic(). I'll send a conversion with a separate 
patch.

Fabio