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    SubjectRe: [PATCH] After swapout/swapin private dirty mappings become clean
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    On Tuesday 14 September 2010 17:03:59 Richard Guenther wrote:
    > On Tue, 14 Sep 2010, Nikanth Karthikesan wrote:
    > > /proc/$pid/smaps broken: After swapout/swapin private dirty mappings
    > > become clean.
    > >
    > > When a page with private file mapping becomes dirty, the vma will be in
    > > both i_mmap tree and anon_vma list. The /proc/$pid/smaps will account
    > > these pages as dirty and backed by the file.
    > >
    > > But when those dirty pages gets swapped out, and when they are read back
    > > from swap, they would be marked as clean, as it should be, as they are
    > > part of swap cache now.
    > >
    > > But the /proc/$pid/smaps would report the vma as a mapping of a file and
    > > it is clean. The pages are actually in same state i.e., dirty with
    > > respect to file still, but which was once reported as dirty is now being
    > > reported as clean to user-space.
    > >
    > > This confuses tools like gdb which uses this information. Those tools
    > > think that those pages were never modified and it creates problem when
    > > they create dumps.
    > >
    > > The file mapping of the vma also cannot be broken as pages never read
    > > earlier, will still have to come from the file. Just that those dirty
    > > pages have become clean anonymous pages.
    > >
    > > During swaping in, restoring the exact state as dirty file-backed pages
    > > before swapout would be useless, as there in no real bug. Breaking the
    > > vma with only anonymous pages as seperate vmas unnecessary may not be a
    > > good thing as well. So let us just export the information that a
    > > file-backed vma has anonymous dirty pages.
    > >
    > > Export this information in smaps by prepending file-names with "[anon]+",
    > > when some of the pages in a file backed vma become anonymous.
    >
    > For the sake of not breaking existing tools I'd prefer appending
    > " [anon]" instead.
    >

    A filename might have it as suffix!

    > Though a much simpler thing would be to account
    > the clean anon pages as Private_Dirty (with respect to the backing
    > file displayed).

    Agreed.

    > Anonymous vmas in /proc/smaps seem to contain
    > Private_Dirty pages as well. So I still don't understand why this
    > isn't just an accounting bug.
    >

    anonymous pages can be dirty as well. Even these pages now marked as clean
    would be marked as dirty again, if they are re-dirtied.

    Thanks
    Nikanth

    > Thanks,
    > Richard.
    >
    > > Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
    > >
    > > ---
    > >
    > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    > > index 439fc1f..68f9806 100644
    > > --- a/fs/proc/task_mmu.c
    > > +++ b/fs/proc/task_mmu.c
    > > @@ -242,6 +242,8 @@ static void show_map_vma(struct seq_file *m, struct
    > > vm_area_struct *vma) */
    > > if (file) {
    > > pad_len_spaces(m, len);
    > > + if (vma->anon_vma)
    > > + seq_puts(m, "[anon]+");
    > > seq_path(m, &file->f_path, "\n");
    > > } else {
    > > const char *name = arch_vma_name(vma);
    >


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