Re: Possible regression with file madvise(MADV_COLLAPSE)

[Yang Shi <shy828301@gmail.com>]

On Sat, Oct 12, 2024 at 1:24 PM Avi Kivity <avi@scylladb.com> wrote:
>
> On Sat, 2024-10-12 at 13:05 -0700, Yang Shi wrote:
> > On Sat, Oct 12, 2024 at 8:38 AM Avi Kivity <avi@scylladb.com> wrote:
> > >
> > > On Fri, 2024-10-11 at 15:29 -0700, Yang Shi wrote:
> > > > On Wed, Oct 9, 2024 at 9:04 AM Avi Kivity <avi@scylladb.com>
> > > > wrote:
> > > > >
> > > > > On Linux 6.10.10 with CONFIG_READ_ONLY_THP_FOR_FS=y,
> > > > > madvise(MADV_COLLAPSE) on  program text fails with EINVAL.
> > > > >
> > > > > To reproduce, compile the reproducer with
> > > > >
> > > > > clang -g -o text-hugepage  text-hugepage.c \
> > > > >         -fuse-ld=lld \
> > > > >         -Wl,-zcommon-page-size=2097152 -Wl,-zmax-page-
> > > > > size=2097152
> > > > > \
> > > > >         -Wl,-z,separate-loadable-segments
> > > > >
> > > > > and run:
> > > >
> > > > Didn't clang make the page cache dirty?
> > > >
> > > > Having sync between clang and the execution made the problem go
> > > > away
> > > > for me.
> > > >
> > >
> > > I see it even with sync (and msync just before the madvise calls).
> >
> > Did you stop khugepaged? It may race with MADV_COLLAPSE. If it failed
> > due to race with khugepaged, you should see -EAGAIN instead of
> > -EINVAL.
>
>
> I did not, but I don't imagine I hit the race in all my attempts.
>
> >
> > I did the below commands in a loop for 1000 times, it never failed (I
> > modified the test program a little bit to print out failure if
> > MADV_COLLAPSE returns failure). I had khugepaged stopped and ran the
> > test on v6.12-rc1 kernel on my AmpereOne machine.
> >
> > rm text-hugepage
> > clang -g -o text-hugepage  text-hugepage.c -fuse-ld=lld
> > -Wl,-zcommon-page-size=2097152 -Wl,-zmax-page-size=2097152
> > -Wl,-z,separate-loadable-segments
> > sync
> > ./text-hugepage
> >
> > >
> > >
> > > Tracing shows this (last lines before syscall exit):
> > >
> > > >          hpage_collapse_scan_file() {
> > > >            __rcu_read_lock();
> > > >            __rcu_read_unlock();
> > > >          }
> >
> > It meant collapse_file() was not called at all.
> > hpage_collapse_scan_file() failed. A couple of reasons may fail it,
> > for example, refcount is not expected, not on lru, etc. You can trace
> > huge_memory:mm_khugepaged_scan_file to get more information about the
> > failure.
>
>
>    text-hugepage-689146 [023] 200457.073794: mm_khugepaged_scan_file:
> mm=0xffff92fc512aac00, scan_pfn=0x5a4310, filename=text-hugepage,
> present=0, swap=0, result=page_compound

Aha, it is because v6.10 doesn't support collapse non-PMD order large
folios. It has been fixed in v6.12-rc1. The patch series is:
https://lore.kernel.org/all/cover.1724140601.git.baolin.wang@linux.alibaba.com/

The subject says "shmem", but it actually works for regular files too.

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