Re: [PATCH v1 2/3] hugetlbfs: improve read HWPOISON hugepage

[Jiaqi Yan <jiaqiyan@google.com>]

On Thu, May 18, 2023 at 3:18 PM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 05/17/23 16:09, Jiaqi Yan wrote:
> > When a hugepage contains HWPOISON pages, read() fails to read any byte
> > of the hugepage and returns -EIO, although many bytes in the HWPOISON
> > hugepage are readable.
> >
> > Improve this by allowing hugetlbfs_read_iter returns as many bytes as
> > possible. For a requested range [offset, offset + len) that contains
> > HWPOISON page, return [offset, first HWPOISON page addr); the next read
> > attempt will fail and return -EIO.
> >
> > Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
> > ---
> >  fs/hugetlbfs/inode.c | 62 +++++++++++++++++++++++++++++++++++++++-----
> >  1 file changed, 56 insertions(+), 6 deletions(-)
> >
> > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> > index ecfdfb2529a3..1baa08ec679f 100644
> > --- a/fs/hugetlbfs/inode.c
> > +++ b/fs/hugetlbfs/inode.c
> > @@ -282,6 +282,46 @@ hugetlb_get_unmapped_area(struct file *file, unsigned long addr,
> >  }
> >  #endif
> >
> > +/*
> > + * Someone wants to read @bytes from a HWPOISON hugetlb @page from @offset.
> > + * Returns the maximum number of bytes one can read without touching the 1st raw
> > + * HWPOISON subpage.
> > + *
> > + * The implementation borrows the iteration logic from copy_page_to_iter*.
> > + */
> > +static size_t adjust_range_hwpoison(struct page *page, size_t offset, size_t bytes)
> > +{
> > +     size_t n = 0;
> > +     size_t res = 0;
> > +     struct folio *folio = page_folio(page);
> > +
> > +     folio_lock(folio);
>
> What is the reason for taking folio_lock?

I intended to make this routine (mostly find_raw_hwp_page) to be
serialized with folio_clear_hugetlb_hwpoison() and
hwpoison_user_mappings() in try_memory_failure_hugetlb(). They don't
directly affect the raw_hwp_list. I can remove the lock in v2.

>
> > +
> > +     /* First subpage to start the loop. */
> > +     page += offset / PAGE_SIZE;
> > +     offset %= PAGE_SIZE;
> > +     while (1) {
> > +             if (find_raw_hwp_page(folio, page) != NULL)
> > +                     break;
> > +
> > +             /* Safe to read n bytes without touching HWPOISON subpage. */
> > +             n = min(bytes, (size_t)PAGE_SIZE - offset);
> > +             res += n;
> > +             bytes -= n;
> > +             if (!bytes || !n)
> > +                     break;
> > +             offset += n;
> > +             if (offset == PAGE_SIZE) {
> > +                     page++;
> > +                     offset = 0;
> > +             }
> > +     }
> > +
> > +     folio_unlock(folio);
> > +
> > +     return res;
> > +}
> > +
> >  /*
> >   * Support for read() - Find the page attached to f_mapping and copy out the
> >   * data. This provides functionality similar to filemap_read().
> > @@ -300,7 +340,7 @@ static ssize_t hugetlbfs_read_iter(struct kiocb *iocb, struct iov_iter *to)
> >
> >       while (iov_iter_count(to)) {
> >               struct page *page;
> > -             size_t nr, copied;
> > +             size_t nr, copied, want;
> >
> >               /* nr is the maximum number of bytes to copy from this page */
> >               nr = huge_page_size(h);
> > @@ -328,16 +368,26 @@ static ssize_t hugetlbfs_read_iter(struct kiocb *iocb, struct iov_iter *to)
> >               } else {
> >                       unlock_page(page);
> >
> > -                     if (PageHWPoison(page)) {
> > -                             put_page(page);
> > -                             retval = -EIO;
> > -                             break;
> > +                     if (!PageHWPoison(page))
> > +                             want = nr;
> > +                     else {
> > +                             /*
> > +                              * Adjust how many bytes safe to read without
> > +                              * touching the 1st raw HWPOISON subpage after
> > +                              * offset.
> > +                              */
> > +                             want = adjust_range_hwpoison(page, offset, nr);
> > +                             if (want == 0) {
> > +                                     put_page(page);
> > +                                     retval = -EIO;
> > +                                     break;
> > +                             }
> >                       }
> >
> >                       /*
> >                        * We have the page, copy it to user space buffer.
> >                        */
> > -                     copied = copy_page_to_iter(page, offset, nr, to);
> > +                     copied = copy_page_to_iter(page, offset, want, to);
> >                       put_page(page);
> >               }
> >               offset += copied;
> > --
> > 2.40.1.606.ga4b1b128d6-goog
> >
>
> Code looks fine, just wondering about that folio_lock.
> --
> Mike Kravetz