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

KOSAKI Motohiro
2010-09-15T04:46:42Z

> 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:".

Here is the code.
I think the page will become dirty, again.

--------------------------------------------------------------
int try_to_free_swap(struct page *page)
{
VM_BUG_ON(!PageLocked(page));

if (!PageSwapCache(page))
return 0;
if (PageWriteback(page))
return 0;
if (page_swapcount(page))
return 0;

delete_from_swap_cache(page);
SetPageDirty(page);
return 1;
}