linux-kernel - Re: [PATCH 27/31] mm/khugepaged: allow pte_offset_map[

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Date:   Wed, 24 May 2023 14:59:17 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Mike Rapoport <rppt@...nel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Peter Xu <peterx@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Alistair Popple <apopple@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Steven Price <steven.price@....com>,
        SeongJae Park <sj@...nel.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Zack Rusin <zackr@...are.com>, Jason Gunthorpe <jgg@...pe.ca>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Minchan Kim <minchan@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Song Liu <song@...nel.org>,
        Thomas Hellstrom <thomas.hellstrom@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 27/31] mm/khugepaged: allow pte_offset_map[_lock]() to fail

On Tue, May 23, 2023 at 9:44 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> On Mon, 22 May 2023, Yang Shi wrote:
> > On Sun, May 21, 2023 at 10:24 PM Hugh Dickins <hughd@...gle.com> wrote:
> > >
> > > __collapse_huge_page_swapin(): don't drop the map after every pte, it
> > > only has to be dropped by do_swap_page(); give up if pte_offset_map()
> > > fails; trace_mm_collapse_huge_page_swapin() at the end, with result;
> > > fix comment on returned result; fix vmf.pgoff, though it's not used.
> > >
> > > collapse_huge_page(): use pte_offset_map_lock() on the _pmd returned
> > > from clearing; allow failure, but it should be impossible there.
> > > hpage_collapse_scan_pmd() and collapse_pte_mapped_thp() allow for
> > > pte_offset_map_lock() failure.
> > >
> > > Signed-off-by: Hugh Dickins <hughd@...gle.com>
> >
> > Reviewed-by: Yang Shi <shy828301@...il.com>
>
> Thanks.
>
> >
> > A nit below:
> >
> > > ---
> > >  mm/khugepaged.c | 72 +++++++++++++++++++++++++++++++++----------------
> > >  1 file changed, 49 insertions(+), 23 deletions(-)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 732f9ac393fc..49cfa7cdfe93 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> ...
> > > @@ -1029,24 +1040,29 @@ static int __collapse_huge_page_swapin(struct mm_struct *mm,
> > >                  * resulting in later failure.
> > >                  */
> > >                 if (ret & VM_FAULT_RETRY) {
> > > -                       trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
> > >                         /* Likely, but not guaranteed, that page lock failed */
> > > -                       return SCAN_PAGE_LOCK;
> > > +                       result = SCAN_PAGE_LOCK;
> >
> > With per-VMA lock, this may not be true anymore, at least not true
> > until per-VMA lock supports swap fault. It may be better to have a
> > more general failure code, for example, SCAN_FAIL. But anyway you
> > don't have to change it in your patch, I can send a follow-up patch
> > once this series is landed on mm-unstable.
>
> Interesting point (I've not tried to wrap my head around what differences
> per-VMA locking would make to old likelihoods here), and thank you for
> deferring a change on it - appreciated.
>
> Something to beware of, if you do choose to change it: mostly those
> SCAN codes (I'm not a fan of them!) are only for a tracepoint somewhere,
> but madvise_collapse() and madvise_collapse_errno() take some of them
> more seriously than others - I think SCAN_PAGE_LOCK ends up as an
> EAGAIN (rightly), but SCAN_FAIL as an EINVAL (depends).
>
> But maybe there are layers in between which do not propagate the result
> code, I didn't check.  All in all, not something I'd spend time on myself.

Thanks, Hugh. A second look shows do_swap_page() should not return
VM_FAULT_RETRY due to per-VMA lock since it depends on
FAULT_FLAG_VMA_LOCK flag, but it is actually not set in khugepaged
path. Khugepaged just has FAULT_FLAG_ALLOW_RETRY flag set. So we don't
have to change anything.

>
> Hugh