Re: [PATCH 2/2] mm: shmem: improve the tmpfs large folio read performance

[Yang Shi <shy828301@gmail.com>]

On Wed, Oct 16, 2024 at 8:25 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
>
>
> On 2024/10/17 01:33, Yang Shi wrote:
> > On Wed, Oct 16, 2024 at 8:38 AM Matthew Wilcox <willy@infradead.org> wrote:
> >>
> >> On Wed, Oct 16, 2024 at 06:09:30PM +0800, Baolin Wang wrote:
> >>> @@ -3128,8 +3127,9 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> >>>                if (folio) {
> >>>                        folio_unlock(folio);
> >>>
> >>> -                     page = folio_file_page(folio, index);
> >>> -                     if (PageHWPoison(page)) {
> >>> +                     if (folio_test_hwpoison(folio) ||
> >>> +                         (folio_test_large(folio) &&
> >>> +                          folio_test_has_hwpoisoned(folio))) {
> >>
> >> Hm, so if we have hwpoison set on one page in a folio, we now can't read
> >> bytes from any page in the folio?  That seems like we've made a bad
> >> situation worse.
> >
> > Yeah, I agree. I think we can fallback to page copy if
> > folio_test_has_hwpoisoned is true. The PG_hwpoison flag is per page.
> >
> > The folio_test_has_hwpoisoned is kept set if the folio split is failed
> > in memory failure handler.
>
> Right. I can still keep the page size copy if
> folio_test_has_hwpoisoned() is true. Some sample changes are as follow.
>
> Moreover, I noticed shmem splice_read() and write() also simply return
> an error if the folio_test_has_hwpoisoned() is true, without any
> fallback to page granularity. I wonder if it is worth adding page
> granularity support as well?

I think you should do the same.

>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 7e79b6a96da0..f30e24e529b9 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -3111,9 +3111,11 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
>
>          for (;;) {
>                  struct folio *folio = NULL;
> +               struct page *page = NULL;
>                  unsigned long nr, ret;
>                  loff_t end_offset, i_size = i_size_read(inode);
>                  size_t fsize;
> +               bool fallback_page_copy = false;
>
>                  if (unlikely(iocb->ki_pos >= i_size))
>                          break;
> @@ -3127,13 +3129,16 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
>                  if (folio) {
>                          folio_unlock(folio);
>
> -                       if (folio_test_hwpoison(folio) ||
> -                           (folio_test_large(folio) &&
> -                            folio_test_has_hwpoisoned(folio))) {
> +                       page = folio_file_page(folio, index);
> +                       if (PageHWPoison(page)) {
>                                  folio_put(folio);
>                                  error = -EIO;
>                                  break;
>                          }
> +
> +                       if (folio_test_large(folio) &&
> +                           folio_test_has_hwpoisoned(folio))
> +                               fallback_page_copy = true;
>                  }
>
>                  /*
> @@ -3147,7 +3152,7 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
>                          break;
>                  }
>                  end_offset = min_t(loff_t, i_size, iocb->ki_pos +
> to->count);
> -               if (folio)
> +               if (folio && likely(!fallback_page_copy))
>                          fsize = folio_size(folio);
>                  else
>                          fsize = PAGE_SIZE;
> @@ -3160,8 +3165,13 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
>                           * virtual addresses, take care about potential
> aliasing
>                           * before reading the page on the kernel side.
>                           */
> -                       if (mapping_writably_mapped(mapping))
> -                               flush_dcache_folio(folio);
> +                       if (mapping_writably_mapped(mapping)) {
> +                               if (unlikely(fallback_page_copy))
> +                                       flush_dcache_page(page);
> +                               else
> +                                       flush_dcache_folio(folio);
> +                       }
> +
>                          /*
>                           * Mark the page accessed if we read the beginning.
>                           */
> @@ -3171,7 +3181,10 @@ static ssize_t shmem_file_read_iter(struct kiocb
> *iocb, struct iov_iter *to)
>                           * Ok, we have the page, and it's up-to-date, so
>                           * now we can copy it to user space...
>                           */
> -                       ret = copy_folio_to_iter(folio, offset, nr, to);
> +                       if (unlikely(fallback_page_copy))
> +                               ret = copy_page_to_iter(page, offset,
> nr, to);
> +                       else
> +                               ret = copy_folio_to_iter(folio, offset,
> nr, to);
>                          folio_put(folio);
>                  } else if (user_backed_iter(to)) {
>                          /*

The change seems fine to me.