Re: [PATCH -mm] vmscan: make mapped executable pages the firstclass citizen

[Minchan Kim]

Hi, Let me have a question. 

On Fri, 8 May 2009 11:02:09 +0800
Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:

> On Thu, May 07, 2009 at 11:10:39PM +0800, Johannes Weiner wrote:
> > On Thu, May 07, 2009 at 08:11:01PM +0800, Wu Fengguang wrote:
> > > Introduce AS_EXEC to mark executables and their linked libraries, and to
> > > protect their referenced active pages from being deactivated.
> > > 
> > > CC: Elladan <elladan@xxxxxxxxxx>
> > > CC: Nick Piggin <npiggin@xxxxxxx>
> > > CC: Johannes Weiner <hannes@xxxxxxxxxxx>
> > > CC: Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx>
> > > CC: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> > > Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Acked-by: Rik van Riel <riel@xxxxxxxxxx>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > > ---
> > >  include/linux/pagemap.h |    1 +
> > >  mm/mmap.c               |    2 ++
> > >  mm/nommu.c              |    2 ++
> > >  mm/vmscan.c             |   35 +++++++++++++++++++++++++++++++++--
> > >  4 files changed, 38 insertions(+), 2 deletions(-)
> > > 
> > > --- linux.orig/include/linux/pagemap.h
> > > +++ linux/include/linux/pagemap.h
> > > @@ -25,6 +25,7 @@ enum mapping_flags {
> > >  #ifdef CONFIG_UNEVICTABLE_LRU
> > >  	AS_UNEVICTABLE	= __GFP_BITS_SHIFT + 3,	/* e.g., ramdisk, SHM_LOCK */
> > >  #endif
> > > +	AS_EXEC		= __GFP_BITS_SHIFT + 4,	/* mapped PROT_EXEC somewhere */
> > >  };
> > >  
> > >  static inline void mapping_set_error(struct address_space *mapping, int error)
> > > --- linux.orig/mm/mmap.c
> > > +++ linux/mm/mmap.c
> > > @@ -1194,6 +1194,8 @@ munmap_back:
> > >  			goto unmap_and_free_vma;
> > >  		if (vm_flags & VM_EXECUTABLE)
> > >  			added_exe_file_vma(mm);
> > > +		if (vm_flags & VM_EXEC)
> > > +			set_bit(AS_EXEC, &file->f_mapping->flags);
> > >  	} else if (vm_flags & VM_SHARED) {
> > >  		error = shmem_zero_setup(vma);
> > >  		if (error)
> > > --- linux.orig/mm/nommu.c
> > > +++ linux/mm/nommu.c
> > > @@ -1224,6 +1224,8 @@ unsigned long do_mmap_pgoff(struct file 
> > >  			added_exe_file_vma(current->mm);
> > >  			vma->vm_mm = current->mm;
> > >  		}
> > > +		if (vm_flags & VM_EXEC)
> > > +			set_bit(AS_EXEC, &file->f_mapping->flags);
> > >  	}
> > 
> > I find it a bit ugly that it applies an attribute of the memory area
> > (per mm) to the page cache mapping (shared).  Because this in turn
> > means that the reference through a non-executable vma might get the
> > pages rotated just because there is/was an executable mmap around.
> 
> Right, the intention was to identify a whole executable/library file,
> eg. /bin/bash or /lib/libc-2.9.so, covering both _text_ and _data_
> sections.

But, your patch is care just text section. 
Do I miss something ?

> > >  	down_write(&nommu_region_sem);
> > > --- linux.orig/mm/vmscan.c
> > > +++ linux/mm/vmscan.c
> > > @@ -1230,6 +1230,7 @@ static void shrink_active_list(unsigned 
> > >  	unsigned long pgmoved;
> > >  	unsigned long pgscanned;
> > >  	LIST_HEAD(l_hold);	/* The pages which were snipped off */
> > > +	LIST_HEAD(l_active);
> > >  	LIST_HEAD(l_inactive);
> > >  	struct page *page;
> > >  	struct pagevec pvec;
> > > @@ -1269,8 +1270,15 @@ static void shrink_active_list(unsigned 
> > >  
> > >  		/* page_referenced clears PageReferenced */
> > >  		if (page_mapping_inuse(page) &&
> > > -		    page_referenced(page, 0, sc->mem_cgroup))
> > > +		    page_referenced(page, 0, sc->mem_cgroup)) {
> > > +			struct address_space *mapping = page_mapping(page);
> > > +
> > >  			pgmoved++;
> > > +			if (mapping && test_bit(AS_EXEC, &mapping->flags)) {
> > > +				list_add(&page->lru, &l_active);
> > > +				continue;
> > > +			}
> > > +		}
> > 
> > Since we walk the VMAs in page_referenced anyway, wouldn't it be
> > better to check if one of them is executable?  This would even work
> > for executable anon pages.  After all, there are applications that cow
> > executable mappings (sbcl and other language environments that use an
> > executable, run-time modified core image come to mind).
> 
> The page_referenced() path will only cover the _text_ section.  But

Why did you said that "The page_referenced() path will only cover the ""_text_"" section" ? 
Could you elaborate please ?

> yeah, the _data_ section is more likely to grow huge in some rare cases.
>
> Thanks,
> Fengguang
> 
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-- 
Kinds Regards
Minchan Kim
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