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    SubjectRe: [PATCH -mm] vmscan: make mapped executable pages the first class citizen
    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.

    > > 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
    yeah, the _data_ section is more likely to grow huge in some rare cases.

    Thanks,
    Fengguang


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