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tmpfs: fix regressions from wider use of ZERO_PAGE

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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: 56a8c8eb1e ("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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2022-04-14 19:13:27 -07:00 committed by Linus Torvalds
parent 7fbd166a8f
commit 1bdec44b1e
2 changed files with 20 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
mm/filemap.c
View File

@ -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));
}
/*

27
mm/shmem.c
View File

@ -2513,7 +2513,6 @@ static ssize_t shmem_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
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(struct kiocb *iocb, struct iov_iter *to)
*/
if (!offset)
mark_page_accessed(page);
got_page = true;
} else {
page = ZERO_PAGE(0);
got_page = false;
}
/*
* 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 {
/*
* 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);
}
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) {