linux-kernel - Re: [PATCH] mm/memory: Replace kmap() with kmap_local

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Date: Wed, 20 Dec 2023 11:53:34 -0800
From: Ira Weiny <ira.weiny@...el.com>
To: "Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>
CC: Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] mm/memory: Replace kmap() with kmap_local_page()

Fabio M. De Francesco wrote:
> On Monday, 18 December 2023 04:34:13 CET Ira Weiny wrote:
> > Fabio M. De Francesco wrote:
> > 
> > [snip]
> > 
> > > Cc: Ira Weiny <ira.weiny@...el.com>
> > > Signed-off-by: Fabio M. De Francesco
> > > <fabio.maria.de.francesco@...ux.intel.com> ---
> > > 
> > >  mm/memory.c | 5 ++---
> > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 7d9f6b685032..88377a107fbe 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -5852,7 +5852,7 @@ static int __access_remote_vm(struct mm_struct *mm,
> > > unsigned long addr,> 
> > >  			if (bytes > PAGE_SIZE-offset)
> > >  			
> > >  				bytes = PAGE_SIZE-offset;
> > > 
> > > -			maddr = kmap(page);
> > > +			maddr = kmap_local_page(page);
> > > 
> > >  			if (write) {
> > >  			
> > >  				copy_to_user_page(vma, page, addr,
> > >  				
> > >  						  maddr + offset, buf, 
> bytes);
> > > 
> > > @@ -5861,8 +5861,7 @@ static int __access_remote_vm(struct mm_struct *mm,
> > > unsigned long addr,> 
> > >  				copy_from_user_page(vma, page, addr,
> > >  				
> > >  						    buf, maddr + offset, 
> bytes);
> > >  			
> > >  			}
> > > 
> > > -			kunmap(page);
> > > -			put_page(page);
> > > +			unmap_and_put_page(page, maddr);
> > 
> > Does this really have the same functionality?
> > 
> > Ira
> 
> Do you have any specific reasons to say that? 
> 
> The unmap_and_put_page() helper was created by Al Viro (it initially was 
> put_and_unmap_page() and I sent a patch to rename it to the current name). He 
> noticed that we have lots of kunmap_local() followed by put_page(). 
> 
> The current implementation has then been changed (Matthew did it, if I 
> remember correctly).
> 
> My understanding of the current implementation is that unmap_and_put_page() 
> calls folio_release_kmap(), taking as arguments the folio which the page 
> belongs to and the kernel virtual address returned by kmap_local_page().
> 
> folio_release_kmap() calls kunmap_local() and then folio_put(). The last is 
> called on the folio obtained by the unmap_and_put_page() wrapper and, if I'm 
> not wrong, it releases refcounts on folios like put_page() does on pages.

This is where my consternation came from.  I saw the folio_put() and did
not realize that get_page() now calls folio_get().

> 
> Am I missing something?

Nope, I just did not have time to trace code yesterday.

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