Re: [PATCH] After swapout/swapin private dirty mappings become clean

Nikanth Karthikesan
2010-09-15T04:35:03Z

On Wednesday 15 September 2010 05:54:31 KOSAKI Motohiro 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.
>
> Why can't gdb check Swap: field in smaps? I think Swap!=0 mean we need dump
> out.
>

Yes. When the page is swapped out it is accounted in "Swap:".

> Am I missing anything?
>

But when it gets swapped in back to memory, it is removed from "Swap:" and
added to "Private_Clean:" instead of "Private_Dirty:".

Thanks
Nikanth

> - kosaki
>
> > Export this information in smaps by prepending file-names with "[anon]+",
> > when some of the pages in a file backed vma become anonymous.
> >
> > 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);
> >
> >
> > --
> > To unsubscribe, send a message with 'unsubscribe linux-mm' in
> > the body to majordomo@kvack.org. For more info on Linux MM,
> > see: http://www.linux-mm.org/ .
> > Don't email: email@kvack.org
>