Re: [PATCH v3 RESEND] mm: shmem: implement POSIX_FADV_[WILL|DONT]NEED for shmem

[Mark Hemment <markhemm@googlemail.com>]

On Thu, 6 Jan 2022 at 17:06, Charan Teja Reddy
<quic_charante@quicinc.com> wrote:
>
> From: Charan Teja Reddy <charante@codeaurora.org>
>
> Currently fadvise(2) is supported only for the files that doesn't
> associated with noop_backing_dev_info thus for the files, like shmem,
> fadvise results into NOP. But then there is file_operations->fadvise()
> that lets the file systems to implement their own fadvise
> implementation. Use this support to implement some of the POSIX_FADV_XXX
> functionality for shmem files.
...
> +static void shmem_isolate_pages_range(struct address_space *mapping, loff_t start,
> +                               loff_t end, struct list_head *list)
> +{
> +       XA_STATE(xas, &mapping->i_pages, start);
> +       struct page *page;
> +
> +       rcu_read_lock();
> +       xas_for_each(&xas, page, end) {
> +               if (xas_retry(&xas, page))
> +                       continue;
> +               if (xa_is_value(page))
> +                       continue;
> +               if (!get_page_unless_zero(page))
> +                       continue;
> +               if (isolate_lru_page(page))
> +                       continue;

Need to unwind the get_page on failure to isolate.

Should PageUnevicitable() pages (SHM_LOCK) be skipped?
(That is, does SHM_LOCK override DONTNEED?)

...
> +static int shmem_fadvise_dontneed(struct address_space *mapping, loff_t start,
> +                               loff_t end)
> +{
> +       int ret;
> +       struct page *page;
> +       LIST_HEAD(list);
> +       struct writeback_control wbc = {
> +               .sync_mode = WB_SYNC_NONE,
> +               .nr_to_write = LONG_MAX,
> +               .range_start = 0,
> +               .range_end = LLONG_MAX,
> +               .for_reclaim = 1,
> +       };
> +
> +       if (!shmem_mapping(mapping))
> +               return -EINVAL;
> +
> +       if (!total_swap_pages)
> +               return 0;
> +
> +       lru_add_drain();
> +       shmem_isolate_pages_range(mapping, start, end, &list);
> +
> +       while (!list_empty(&list)) {
> +               page = lru_to_page(&list);
> +               list_del(&page->lru);
> +               if (page_mapped(page))
> +                       goto keep;
> +               if (!trylock_page(page))
> +                       goto keep;
> +               if (unlikely(PageTransHuge(page))) {
> +                       if (split_huge_page_to_list(page, &list))
> +                               goto keep;
> +               }

I don't know the shmem code and the lifecycle of a shm-page, so
genuine questions;
When the try-lock succeeds, should there be a test for PageWriteback()
(page skipped if true)?  Also, does page->mapping need to be tested
for NULL to prevent races with deletion from the page-cache?

...
> +
> +               clear_page_dirty_for_io(page);
> +               SetPageReclaim(page);
> +               ret = shmem_writepage(page, &wbc);
> +               if (ret || PageWriteback(page)) {
> +                       if (ret)
> +                               unlock_page(page);
> +                       goto keep;
> +               }
> +
> +               if (!PageWriteback(page))
> +                       ClearPageReclaim(page);
> +
> +               /*
> +                * shmem_writepage() place the page in the swapcache.
> +                * Delete the page from the swapcache and release the
> +                * page.
> +                */
> +               __mod_node_page_state(page_pgdat(page),
> +                               NR_ISOLATED_ANON + page_is_file_lru(page), compound_nr(page));
> +               lock_page(page);
> +               delete_from_swap_cache(page);
> +               unlock_page(page);
> +               put_page(page);
> +               continue;
> +keep:
> +               putback_lru_page(page);
> +               __mod_node_page_state(page_pgdat(page),
> +                               NR_ISOLATED_ANON + page_is_file_lru(page), compound_nr(page));
> +       }

The putback_lru_page() drops the last reference hold this code has on
'page'.  Is it safe to use 'page' after dropping this reference?

Cheers,
Mark