[patch 02/14] tmpfs: fix regressions from wider use of ZERO_PAGE

Andrew Morton
2022-04-15T02:13:31Z

From: Hugh Dickins <hughd@google.com>
Subject: tmpfs: fix regressions from wider use of ZERO_PAGE

Chuck Lever reported fsx-based xfstests generic 075 091 112 127 failing
when 5.18-rc1 NFS server exports tmpfs: bisected to recent tmpfs change.

Whilst nfsd_splice_action() does contain some questionable handling of
repeated pages, and Chuck was able to work around there, history from
Mark Hemment makes clear that there might be similar dangers elsewhere:
it was not a good idea for me to pass ZERO_PAGE down to unknown actors.

Revert shmem_file_read_iter() to using ZERO_PAGE for holes only when
iter_is_iovec(); in other cases, use the more natural iov_iter_zero()
instead of copy_page_to_iter(). We would use iov_iter_zero() throughout,
but the x86 clear_user() is not nearly so well optimized as copy to user
(dd of 1T sparse tmpfs file takes 57 seconds rather than 44 seconds).

And now pagecache_init() does not need to SetPageUptodate(ZERO_PAGE(0)):
which had caused boot failure on arm noMMU STM32F7 and STM32H7 boards

Link: https://lkml.kernel.org/r/9a978571-8648-e830-5735-1f4748ce2e30@google.com
Fixes: 56a8c8eb1eaf ("tmpfs: do not allocate pages on read")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Patrice CHOTARD <patrice.chotard@foss.st.com>
Reported-by: Chuck Lever III <chuck.lever@oracle.com>
Tested-by: Chuck Lever III <chuck.lever@oracle.com>
Cc: Mark Hemment <markhemm@googlemail.com>
Cc: Patrice CHOTARD <patrice.chotard@foss.st.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Lukas Czerner <lczerner@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

mm/filemap.c | 6 ------
mm/shmem.c | 31 ++++++++++++++++++++-----------
2 files changed, 20 insertions(+), 17 deletions(-)

--- a/mm/filemap.c~tmpfs-fix-regressions-from-wider-use-of-zero_page
+++ a/mm/filemap.c
@@ -1063,12 +1063,6 @@ void __init pagecache_init(void)
init_waitqueue_head(&folio_wait_table[i]);

page_writeback_init();
-
- /*
- * tmpfs uses the ZERO_PAGE for reading holes: it is up-to-date,
- * and splice's page_cache_pipe_buf_confirm() needs to see that.
- */
- SetPageUptodate(ZERO_PAGE(0));
}

/*
--- a/mm/shmem.c~tmpfs-fix-regressions-from-wider-use-of-zero_page
+++ a/mm/shmem.c
@@ -2513,7 +2513,6 @@ static ssize_t shmem_file_read_iter(stru
pgoff_t end_index;
unsigned long nr, ret;
loff_t i_size = i_size_read(inode);
- bool got_page;

end_index = i_size >> PAGE_SHIFT;
if (index > end_index)
@@ -2570,24 +2569,34 @@ static ssize_t shmem_file_read_iter(stru
*/
if (!offset)
mark_page_accessed(page);
- got_page = true;
+ /*
+ * Ok, we have the page, and it's up-to-date, so
+ * now we can copy it to user space...
+ */
+ ret = copy_page_to_iter(page, offset, nr, to);
+ put_page(page);
+
+ } else if (iter_is_iovec(to)) {
+ /*
+ * Copy to user tends to be so well optimized, but
+ * clear_user() not so much, that it is noticeably
+ * faster to copy the zero page instead of clearing.
+ */
+ ret = copy_page_to_iter(ZERO_PAGE(0), offset, nr, to);
} else {
- page = ZERO_PAGE(0);
- got_page = false;
+ /*
+ * But submitting the same page twice in a row to
+ * splice() - or others? - can result in confusion:
+ * so don't attempt that optimization on pipes etc.
+ */
+ ret = iov_iter_zero(nr, to);
}

- /*
- * Ok, we have the page, and it's up-to-date, so
- * now we can copy it to user space...
- */
- ret = copy_page_to_iter(page, offset, nr, to);
retval += ret;
offset += ret;
index += offset >> PAGE_SHIFT;
offset &= ~PAGE_MASK;

- if (got_page)
- put_page(page);
if (!iov_iter_count(to))
break;
if (ret < nr) {
_