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

[Barry Song]

On Tue, Mar 5, 2024 at 8:55 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Barry Song <21cnbao@xxxxxxxxx> writes:
>
> > On Tue, Mar 5, 2024 at 10:15 AM David Hildenbrand <david@redhatcom> wrote:
> >> > But we did "resolve" those bugs by entirely untouching all PTEs if we
> >> > found some PTEs were skipped in try_to_unmap_one [1].
> >> >
> >> > While we find we only get the PTL from 2nd, 3rd but not
> >> > 1st PTE, we entirely give up on try_to_unmap_one, and leave
> >> > all PTEs untouched.
> >> >
> >> > /* we are not starting from head */
> >> > if (!IS_ALIGNED((unsigned long)pvmw.pte, CONT_PTES * sizeof(*pvmw.pte))) {
> >> >                     ret = false;
> >> >                     atomic64_inc(&perf_stat.mapped_walk_start_from_non_head);
> >> >                     set_pte_at(mm, address, pvmw.pte, pteval);
> >> >                     page_vma_mapped_walk_done(&pvmw);
> >> >                     break;
> >> > }
> >> > This will ensure all PTEs still have a unified state such as CONT-PTE
> >> > after try_to_unmap fails.
> >> > I feel this could have some false postive because when racing
> >> > with unmap, 1st PTE might really become pte_none. So explicitly
> >> > holding PTL from 1st PTE seems a better way.
> >>
> >> Can we estimate the "cost" of holding the PTL?
> >>
> >
> > This is just moving PTL acquisition one or two PTE earlier in those corner
> > cases. In normal cases, it doesn't affect when PTL is held.
>
> The mTHP may be mapped at the end of page table.  In that case, the PTL
> will be held longer.  Or am I missing something?

no. this patch doesn't change when we release PTL but change when we
get PTL.

when the original code iterates nr_pages PTEs in a large folio, it will skip
invalid PTEs, when it meets a valid one, it will acquire PTL. so if it gets
intermediate PTE values some other threads are modifying, it might
skip PTE0, or sometimes PTE0 and PTE1 according to my test. but
arriving at PTE2, likely other threads have written a new value, so we
will begin to hold PTL and iterate till the end of the large folio.

The proposal is that we directly get PTL from PTE0, thus we don't get
intermediate values for the head of nr_pages PTEs. this will ensure
a large folio is either completely unmapped or completely mapped.
but not partially mapped and partially unmapped.

>
> --
> Best Regards,
> Huang, Ying
>
>
> > In normal cases, page_vma_mapped_walk will find PTE0 is present, thus hold
> > PTL immediately. in corner cases, page_vma_mapped_walk races with break-
> > before-make, after skipping one or two PTEs whose states are transferring,
> > it will find a present pte then acquire lock.
> >
> >> --
> >> Cheers,
> >>
> >> David / dhildenb
> >
Thanks
Barry