ceph: introduce ceph_process_folio_batch() method

ceph: introduce ceph_process_folio_batch() method

First step of ceph_writepages_start() logic is
of finding the dirty memory folios and processing it.
This patch introduces ceph_process_folio_batch()
method that moves this logic into dedicated method.

The ceph_writepages_start() has this logic:

if (ceph_wbc.locked_pages == 0)
    lock_page(page);  /* first page */
else if (!trylock_page(page))
    break;

<skipped>

if (folio_test_writeback(folio) ||
    folio_test_private_2(folio) /* [DEPRECATED] */) {
      if (wbc->sync_mode == WB_SYNC_NONE) {
          doutc(cl, "%p under writeback\n", folio);
          folio_unlock(folio);
          continue;
      }
      doutc(cl, "waiting on writeback %p\n", folio);
      folio_wait_writeback(folio);
      folio_wait_private_2(folio); /* [DEPRECATED] */
}

The problem here that folio/page is locked here at first
and it is by set_page_writeback(page) later before
submitting the write request. The folio/page is unlocked
by writepages_finish() after finishing the write
request. It means that logic of checking folio_test_writeback()
and folio_wait_writeback() never works because page is locked
and it cannot be locked again until write request completion.
However, for majority of folios/pages the trylock_page()
is used. As a result, multiple threads can try to lock the same
folios/pages multiple times even if they are under writeback
already. It makes this logic more compute intensive than
it is necessary.

This patch changes this logic:

if (folio_test_writeback(folio) ||
    folio_test_private_2(folio) /* [DEPRECATED] */) {
      if (wbc->sync_mode == WB_SYNC_NONE) {
          doutc(cl, "%p under writeback\n", folio);
          folio_unlock(folio);
          continue;
      }
      doutc(cl, "waiting on writeback %p\n", folio);
      folio_wait_writeback(folio);
      folio_wait_private_2(folio); /* [DEPRECATED] */
}

if (ceph_wbc.locked_pages == 0)
    lock_page(page);  /* first page */
else if (!trylock_page(page))
    break;

This logic should exclude the ignoring of writeback
state of folios/pages.

Signed-off-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Link: https://lore.kernel.org/r/20250205000249.123054-3-slava@dubeyko.com
Tested-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>