Re: [PATCH] mm, swap: Fix swapoff with KSM pages

[Vineeth Pillai <vpillai@digitalocean.com>]

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Thanks for letting me know Andrew! I shall include all the fixes in the
next iteration.

Thanks,
Vineeth

On Thu, Dec 27, 2018 at 9:55 PM Andrew Morton <akpm@linux-foundation.org>
wrote:

> On Wed, 26 Dec 2018 13:15:22 +0800 Huang Ying <ying.huang@intel.com>
> wrote:
>
> > KSM pages may be mapped to the multiple VMAs that cannot be reached
> > from one anon_vma.  So during swapin, a new copy of the page need to
> > be generated if a different anon_vma is needed, please refer to
> > comments of ksm_might_need_to_copy() for details.
> >
> > During swapoff, unuse_vma() uses anon_vma (if available) to locate VMA
> > and virtual address mapped to the page, so not all mappings to a
> > swapped out KSM page could be found.  So in try_to_unuse(), even if
> > the swap count of a swap entry isn't zero, the page needs to be
> > deleted from swap cache, so that, in the next round a new page could
> > be allocated and swapin for the other mappings of the swapped out KSM
> > page.
> >
> > But this contradicts with the THP swap support.  Where the THP could
> > be deleted from swap cache only after the swap count of every swap
> > entry in the huge swap cluster backing the THP has reach 0.  So
> > try_to_unuse() is changed in commit e07098294adf ("mm, THP, swap:
> > support to reclaim swap space for THP swapped out") to check that
> > before delete a page from swap cache, but this has broken KSM swapoff
> > too.
> >
> > Fortunately, KSM is for the normal pages only, so the original
> > behavior for KSM pages could be restored easily via checking
> > PageTransCompound().  That is how this patch works.
> >
> > ...
> >
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -2197,7 +2197,8 @@ int try_to_unuse(unsigned int type, bool frontswap,
> >                */
> >               if (PageSwapCache(page) &&
> >                   likely(page_private(page) == entry.val) &&
> > -                 !page_swapped(page))
> > +                 (!PageTransCompound(page) ||
> > +                  !swap_page_trans_huge_swapped(si, entry)))
> >                       delete_from_swap_cache(compound_head(page));
> >
>
> The patch "mm, swap: rid swapoff of quadratic complexity" changes this
> code significantly.  There are a few issues with that patch so I'll
> drop it for now.
>
> Vineeth, please ensure that future versions retain the above fix,
> thanks.
>
>
>

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