Re: [RFC PATCH] mm: hold PTL from the first PTE while reclaiming a large folio

[Huang, Ying]

Barry Song <21cnbao@xxxxxxxxx> writes:

> On Tue, Mar 5, 2024 at 8:30 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Barry Song <21cnbao@xxxxxxxxx> writes:
>>
>> > From: Barry Song <v-songbaohua@xxxxxxxx>
>> >
>> > page_vma_mapped_walk() within try_to_unmap_one() races with other
>> > PTEs modification such as break-before-make, while iterating PTEs
>>
>> Sorry, I don't know what is "break-before-make", can you elaborate?
>> IIUC, ptep_modify_prot_start()/ptep_modify_prot_commit() can clear PTE
>> temporarily, which may cause race with page_vma_mapped_walk().  Is that
>> the issue that you try to fix?
>
> we are writing pte to zero(break) before writing a new value(make).

OK.  Is break and make is commonly used terminology in kernel?  If not,
it's better to explain a little (e.g., ptep_get_and_clear() / modify /
set_pte_at()).

> while
> this behavior is within PTL in another thread,  page_vma_mapped_walk()
> of try_to_unmap_one thread won't take PTL till it meets a present PTE.

IIUC, !pte_none() should be enough?

> for example, if another threads are modifying nr_pages PTEs under PTL,
> but we don't hold PTL, we might skip one or two PTEs at the beginning of
> a large folio.
> For a large folio, after try_to_unmap_one(), we may result in PTE0 and PTE1
> untouched but PTE2~nr_pages-1 are set to swap entries.
>
> by holding PTL from PTE0 for large folios, we won't get these intermediate
> values. At the moment we get PTL, other threads have done.

Got it!  Thanks!

--
Best Regards,
Huang, Ying

>>
>> --
>> Best Regards,
>> Huang, Ying
>>
>> > of a large folio, it will only begin to acquire PTL after it gets
>> > a valid(present) PTE. break-before-make intermediately sets PTEs
>> > to pte_none. Thus, a large folio's PTEs might be partially skipped
>> > in try_to_unmap_one().
>> > For example, for an anon folio, after try_to_unmap_one(), we may
>> > have PTE0 present, while PTE1 ~ PTE(nr_pages - 1) are swap entries.
>> > So folio will be still mapped, the folio fails to be reclaimed.
>> > What’s even more worrying is, its PTEs are no longer in a unified
>> > state. This might lead to accident folio_split() afterwards. And
>> > since a part of PTEs are now swap entries, accessing them will
>> > incur page fault - do_swap_page.
>> > It creates both anxiety and more expense. While we can't avoid
>> > userspace's unmap to break up unified PTEs such as CONT-PTE for
>> > a large folio, we can indeed keep away from kernel's breaking up
>> > them due to its code design.
>> > This patch is holding PTL from PTE0, thus, the folio will either
>> > be entirely reclaimed or entirely kept. On the other hand, this
>> > approach doesn't increase PTL contention. Even w/o the patch,
>> > page_vma_mapped_walk() will always get PTL after it sometimes
>> > skips one or two PTEs because intermediate break-before-makes
>> > are short, according to test. Of course, even w/o this patch,
>> > the vast majority of try_to_unmap_one still can get PTL from
>> > PTE0. This patch makes the number 100%.
>> > The other option is that we can give up in try_to_unmap_one
>> > once we find PTE0 is not the first entry we get PTL, we call
>> > page_vma_mapped_walk_done() to end the iteration at this case.
>> > This will keep the unified PTEs while the folio isn't reclaimed.
>> > The result is quite similar with small folios with one PTE -
>> > either entirely reclaimed or entirely kept.
>> > Reclaiming large folios by holding PTL from PTE0 seems a better
>> > option comparing to giving up after detecting PTL begins from
>> > non-PTE0.
>> >
>> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
>> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
>> > ---
>> >  mm/vmscan.c | 11 +++++++++++
>> >  1 file changed, 11 insertions(+)
>> >
>> > diff --git a/mm/vmscan.c b/mm/vmscan.c
>> > index 0b888a2afa58..e4722fbbcd0c 100644
>> > --- a/mm/vmscan.c
>> > +++ b/mm/vmscan.c
>> > @@ -1270,6 +1270,17 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>> >
>> >                       if (folio_test_pmd_mappable(folio))
>> >                               flags |= TTU_SPLIT_HUGE_PMD;
>> > +                     /*
>> > +                      * if page table lock is not held from the first PTE of
>> > +                      * a large folio, some PTEs might be skipped because of
>> > +                      * races with break-before-make, for example, PTEs can
>> > +                      * be pte_none intermediately, thus one or more PTEs
>> > +                      * might be skipped in try_to_unmap_one, we might result
>> > +                      * in a large folio is partially mapped and partially
>> > +                      * unmapped after try_to_unmap
>> > +                      */
>> > +                     if (folio_test_large(folio))
>> > +                             flags |= TTU_SYNC;
>> >
>> >                       try_to_unmap(folio, flags);
>> >                       if (folio_mapped(folio)) {
>
> Thanks
> Barry