Re: [PATCH v1 0/3] Improve hugetlbfs read on HWPOISON hugepages

[Jiaqi Yan <jiaqiyan@google.com>]

On Wed, May 17, 2023 at 4:30 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 05/17/23 16:09, Jiaqi Yan wrote:
> > Today when hardware memory is corrupted in a hugetlb hugepage,
> > kernel leaves the hugepage in pagecache [1]; otherwise future mmap or
> > read will suject to silent data corruption. This is implemented by
> > returning -EIO from hugetlb_read_iter immediately if the hugepage has
> > HWPOISON flag set.
> >
> > Since memory_failure already tracks the raw HWPOISON subpages in a
> > hugepage, a natural improvement is possible: if userspace only asks for
> > healthy subpages in the pagecache, kernel can return these data.
>
> Thanks for putting this together.
>
> I recall discussing this some time back, and deciding to wait and see
> how HGM would progress.  Since it may be some time before HGM goes
> upstream, it would be reasonable to consider this again.

This improvement actually does NOT depend on HGM at all. No page table
related stuff involved here. The other RFC [2] I sent earlier DOES
require HGM. This improvement was brought up by James when we were
working on [2]. In "Future Work" section of the cover letter, I
thought HGM was needed but soon when I code it up, I found I was
wrong.

>
> One quick question.
> Do you have an actual use case for this?  It certainly is an improvement
> over existing functionality.  However, I am not aware of too many (?any?)
> users actually doing read() calls on hugetlb files.

I don't have any use case. I did search on Github for around half a
hour and all the hugetlb usages are done via mmap.

> --
> Mike Kravetz
>
> > This patchset implements this improvement. It consist of three parts.
> > The 1st commit exports the functionality to tell if a subpage inside a
> > hugetlb hugepage is a raw HWPOISON page. The 2nd commit teaches
> > hugetlbfs_read_iter to return as many healthy bytes as possible.
> > The 3rd commit properly tests this new feature.
> >
> > [1] commit 8625147cafaa ("hugetlbfs: don't delete error page from pagecache")

[2] https://lore.kernel.org/linux-mm/20230428004139.2899856-6-jiaqiyan@google.com/T/#m97c6edef8ad0cc9b064e1fd9369b8521dcfa43de

> >
> > Jiaqi Yan (3):
> >   mm/hwpoison: find subpage in hugetlb HWPOISON list
> >   hugetlbfs: improve read HWPOISON hugepage
> >   selftests/mm: add tests for HWPOISON hugetlbfs read
> >
> >  fs/hugetlbfs/inode.c                          |  62 +++-
> >  include/linux/mm.h                            |  23 ++
> >  mm/memory-failure.c                           |  26 +-
> >  tools/testing/selftests/mm/.gitignore         |   1 +
> >  tools/testing/selftests/mm/Makefile           |   1 +
> >  .../selftests/mm/hugetlb-read-hwpoison.c      | 322 ++++++++++++++++++
> >  6 files changed, 419 insertions(+), 16 deletions(-)
> >  create mode 100644 tools/testing/selftests/mm/hugetlb-read-hwpoison.c
> >
> > --
> > 2.40.1.606.ga4b1b128d6-goog
> >

(Sorry if you received twice, was sent in a wrong way a while ago)