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SubjectRe: [PATCH -mm] vmscan: make mapped executable pages the first class citizen
Hi, Let me have a question. 

On Fri, 8 May 2009 11:02:09 +0800
Wu Fengguang <fengguang.wu@intel.com> 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@eskimo.com>
> > > CC: Nick Piggin <npiggin@suse.de>
> > > CC: Johannes Weiner <hannes@cmpxchg.org>
> > > CC: Christoph Lameter <cl@linux-foundation.org>
> > > CC: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > > Acked-by: Peter Zijlstra <peterz@infradead.org>
> > > Acked-by: Rik van Riel <riel@redhat.com>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > > 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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