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

[Barry Song]

On Tue, Mar 5, 2024 at 10:11 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 05/03/2024 09:08, Barry Song wrote:
> > On Tue, Mar 5, 2024 at 9:54 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> >>
> >> On 04/03/2024 21:57, Barry Song wrote:
> >>> On Tue, Mar 5, 2024 at 1:21 AM Ryan Roberts <ryan.roberts@armcom> wrote:
> >>>>
> >>>> Hi Barry,
> >>>>
> >>>> On 04/03/2024 10:37, Barry Song wrote:
> >>>>> 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
> >>>>> 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().
> >>>>
> >>>> I just want to check my understanding here - I think the problem occurs for
> >>>> PTE-mapped, PMD-sized folios as well as smaller-than-PMD-size large folios? Now
> >>>> that I've had a look at the code and have a better understanding, I think that
> >>>> must be the case? And therefore this problem exists independently of my work to
> >>>> support swap-out of mTHP? (From your previous report I was under the impression
> >>>> that it only affected mTHP).
> >>>
> >>> I think this affects all large folios with PTEs entries more than 1. but hugeTLB
> >>> is handled as a whole in try_to_unmap_one and its rmap is removed all
> >>> together, i feel hugeTLB doesn't have this problem.
> >>>
> >>>>
> >>>> Its just that the problem is becoming more pronounced because with mTHP,
> >>>> PTE-mapped large folios are much more common?
> >>>
> >>> right. as now large folios become a more common case, and it is my case
> >>> running in millions of phones.
> >>>
> >>> BTW, I feel we can somehow learn from hugeTLB, for example, we can reclaim
> >>> all PTEs all together rather than iterating PTEs one by one. This will improve
> >>> performance. for example, a batched
> >>> set_ptes_to_swap_entries()
> >>> {
> >>> }
> >>> then we only need to loop once for a large folio, right now we are looping
> >>> nr_pages times.
> >>
> >> You still need a pte-pte loop somewhere. In hugetlb's case it's in the arch
> >> implementation. HugeTLB ptes are all a fixed size for a given VMA, which makes
> >> things a bit easier too, whereas in the regular mm, they are now a variable size.
> >>
> >> David and I introduced folio_pte_batch() to help gather batches of ptes, and it
> >> uses the contpte bit to avoid iterating over intermediate ptes. And I'm adding
> >> swap_pte_batch() which does a similar thing for swap entry batching in v4 of my
> >> swap-out series.
> >>
> >> For your set_ptes_to_swap_entries() example, I'm not sure what it would do other
> >> than loop over the PTEs setting an incremented swap entry to each one? How is
> >> that more performant?
> >
> > right now, while (page_vma_mapped_walk(&pvmw)) will loop nr_pages for each
> > PTE, if each PTE, we do lots of checks within the loop.
> >
> > by implementing set_ptes_to_swap_entries(), we can iterate once for
> > page_vma_mapped_walk(), after folio_pte_batch() has confirmed
> > the large folio is completely mapped, we set nr_pages swap entries
> > all together.
> >
> > we are replacing
> >
> > for(i=0;i<nr_pages;i++)     /* page_vma_mapped_walk */
> > {
> >         lots of checks;
> >         clear PTEn
> >         set PTEn to swap
> > }
>
> OK so you are effectively hoisting "lots of checks" out of the loop?

no. page_vma_mapped_walk returns nr_pages times. We are doing
same check each time.  Each time, we do tlbi and set one PTE.

>
> >
> > by
> >
> > if (large folio && folio_pte_batch() == nr_pages)
> >     set_ptes_to_swap_entries().

for this, we do check for one time, and we do much less tlbi.

> >
> >>
> >
> > Thanks,
> > Ryan

Thanks
Barry