linux-kernel - Re: [syzbot] WARNING in follow_hugetlb

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Date:   Sat, 21 May 2022 11:25:00 -0700
From:   Minchan Kim <minchan@...nel.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        syzbot <syzbot+acf65ca584991f3cc447@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        llvm@...ts.linux.dev, nathan@...nel.org, ndesaulniers@...gle.com,
        syzkaller-bugs@...glegroups.com, trix@...hat.com,
        Matthew Wilcox <willy@...radead.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [syzbot] WARNING in follow_hugetlb_page

On Sat, May 21, 2022 at 06:46:27PM +0200, David Hildenbrand wrote:

< snip >

> >> The general rules are:
> >>
> >> ZONE_MOVABLE: nobody is allowed to place unmovable allocations there; it
> >> could prevent memory offlining/unplug.
> >>
> >> CMA: nobody *but the designated owner* is allowed to place unmovable
> >> memory there; it could prevent the actual owner to allocate contiguous
> >> memory.
> > 
> > I am confused what's the meaning of designated owner and actuall owner
> > in your context.
> 
> designated==actual here. I just wanted to distinguish from someone
> current temporary owner of the page ("allocated it via a movable
> allocation") but the actual designated owner (e.g., hugetlb CMA)
> 
> The page/memory owner terminology is just confusing. Let's rephrase to:
> only the CMA area owner is allowed to place unmovable allocations there.

Yeah, the CMA area owner is much better.

> 
> > 
> > What I thought about the issue based on you explanation:
> > 
> > HugeTLB allocates its page by two types of allocation
> > 
> > 1. alloc_pages(GFP_MOVABLE)
> >  
> > It could allocate the hugetlb page from CMA area but longterm pin
> > should migrate them out of cma before the pinning so allowing
> > the pinning on the page is no problem and current code works like
> > that.
> > 
> >     check_and_migrate_movable_pages
> > 
> 
> Yes.
> 
> > 2. cma_alloc
> > 
> > The cma_alloc is used only for *gigantic page* and the hugetlbfs
> > is the very owner of the page. IOW, if the hugetlbfs was succeeded
> > to allocate the gigantic page by cma_alloc, there is no other
> > owner to be able to claim the page any longer so it's fine to
> > allow longterm pinning againt the gingantic page but current.
> > However, current code doesn't work like that due to
> > is_pinnable_page. IOW, hugetlbfs need a way to distinguish 
> > whether the page owner is hugetlbfs or not.
> > 
> > Are we on same page?
> 
> Yes, exactly. What I wanted to express is: for huge pages we have to
> make a smarter decision because there are cases where we want to
> migrate, and cases where we don't want to migrate.

Sure, maybe hugetlbfs could squeeze a bit in one of subpage of the
CMA compound page. "I am CMA allocated but allow to pinned for longterm"

Thanks.