Re: [PATCH v5] mm: shrink skip folio mapped by an exiting process

[Barry Song]

On Mon, Jul 8, 2024 at 9:36 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 08.07.24 11:04, Zhiguo Jiang wrote:
> > The releasing process of the non-shared anonymous folio mapped solely by
> > an exiting process may go through two flows: 1) the anonymous folio is
> > firstly is swaped-out into swapspace and transformed into a swp_entry
> > in shrink_folio_list; 2) then the swp_entry is released in the process
> > exiting flow. This will increase the cpu load of releasing a non-shared
> > anonymous folio mapped solely by an exiting process, because the folio
> > go through swap-out and the releasing the swapspace and swp_entry.
> >
> > When system is low memory, it is more likely to occur, because more
> > backend applidatuions will be killed.
> >
> > The modification is that shrink skips the non-shared anonymous folio
> > solely mapped by an exting process and the folio is only released
> > directly in the process exiting flow, which will save swap-out time
> > and alleviate the load of the process exiting.
> >
> > Signed-off-by: Zhiguo Jiang <justinjiang@xxxxxxxx>
> > ---
> >
> > Change log:
> > v4->v5:
> > 1.Modify to skip non-shared anonymous folio only.
> > 2.Update comments for pra->referenced = -1.
> > v3->v4:
> > 1.Modify that the unshared folios mapped only in exiting task are skip.
> > v2->v3:
> > Nothing.
> > v1->v2:
> > 1.The VM_EXITING added in v1 patch is removed, because it will fail
> > to compile in 32-bit system.
> >
> >   mm/rmap.c   | 13 +++++++++++++
> >   mm/vmscan.c |  7 ++++++-
> >   2 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 26806b49a86f..5b5281d71dbb
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -843,6 +843,19 @@ static bool folio_referenced_one(struct folio *folio,
> >       int referenced = 0;
> >       unsigned long start = address, ptes = 0;
> >
> > +     /*
> > +      * Skip the non-shared anonymous folio mapped solely by
> > +      * the single exiting process, and release it directly
> > +      * in the process exiting.
> > +      */
> > +     if ((!atomic_read(&vma->vm_mm->mm_users) ||
> > +             test_bit(MMF_OOM_SKIP, &vma->vm_mm->flags)) &&
> > +             folio_test_anon(folio) && folio_test_swapbacked(folio) &&
> > +             !folio_likely_mapped_shared(folio)) {
>
> I'm currently working on moving all folio_likely_mapped_shared() under
> the PTL, where we are then sure that the folio is actually mapped by
> this process (e.g., no concurrent unmapping poisslbe).
>
> Can we do the same here directly?

Implementing this is challenging because page_vma_mapped_walk() is
responsible for
traversing the page table to acquire and release the PTL. This becomes
particularly
complex with mTHP, as we may need to interrupt the page_vma_mapped_walk
loop at the first PTE.

>
> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry