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

Nikanth Karthikesan
2010-09-15T06:26:31Z

On Wednesday 15 September 2010 10:45:28 KOSAKI Motohiro wrote:
> > On Wednesday 15 September 2010 10:16:36 KOSAKI Motohiro wrote:
> > > > 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;
> > > }
> >
> > I think this gets called only when the swap space gets freed.
>
> this is try-to-free-swap-space.
> delete_from_swap_cache() does actual free.
>
> > But when the
> > page is just swapped out and swapped in, and the page is still part of
> > SwapCache, it will be marked as clean, when the I/O read from swap
> > completes.
>
> Because in this case, the swap entry is not freed yet. Then the page is
> still clean and swap field is still !0.
>
> PageSwapCache == the page has backend swap entry == the page may be clean.
> But, When the swap entry is removed, page will become dirty again.
>

Correct.

> As I said, following is incorrect.

No.

> In almost case, swap entry is not
> removed at swap-in. Please grep try_to_free_swap() callers and
>

Correct

> > > > But when it gets swapped in back to memory, it is removed from
> > > > "Swap:"
>

I mean the "Swap:" field in smaps file here, not the swapcache.

Thanks
Nikanth