On 5/5/21 4:43 AM, Jan Kara wrote:

On Tue 04-05-21 16:35:53, Junxiao Bi wrote:

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On 5/4/21 2:02 AM, Jan Kara wrote:

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On Mon 03-05-21 10:25:31, Junxiao Bi wrote:

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On 5/3/21 3:29 AM, Jan Kara wrote:

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On Fri 30-04-21 14:18:15, Junxiao Bi wrote:

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On 4/30/21 5:47 AM, Jan Kara wrote:

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On Thu 29-04-21 11:07:15, Junxiao Bi wrote:

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On 4/29/21 10:14 AM, Andreas Gruenbacher wrote:

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On Tue, Apr 27, 2021 at 4:44 AM Junxiao Bi [off-list ref] wrote:

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When doing truncate/fallocate for some filesytem like ocfs2, it
will zero some pages that are out of inode size and then later
update the inode size, so it needs this api to writeback eof
pages.

is this in reaction to Jan's "[PATCH 0/12 v4] fs: Hole punch vs page
cache filling races" patch set [*]? It doesn't look like the kind of
patch Christoph would be happy with.

Thank you for pointing the patch set. I think that is fixing a different
issue.

The issue here is when extending file size with fallocate/truncate, if the
original inode size

is in the middle of the last cluster block(1M), eof part will be zeroed with
buffer write first,

and then new inode size is updated, so there is a window that dirty pages is
out of inode size,

if writeback is kicked in, block_write_full_page will drop all those eof
pages.

I agree that the buffers describing part of the cluster beyond i_size won't
be written. But page cache will remain zeroed out so that is fine. So you
only need to zero out the on disk contents. Since this is actually
physically contiguous range of blocks why don't you just use
sb_issue_zeroout() to zero out the tail of the cluster? It will be more
efficient than going through the page cache and you also won't have to
tweak block_write_full_page()...

Thanks for the review.

The physical blocks to be zeroed were continuous only when sparse mode is
enabled, if sparse mode is disabled, unwritten extent was not supported for
ocfs2, then all the blocks to the new size will be zeroed by the buffer
write, since sb_issue_zeroout() will need waiting io done, there will be a
lot of delay when extending file size. Use writeback to flush async seemed
more efficient?

It depends. Higher end storage (e.g. NVME or NAS, maybe some better SATA
flash disks as well) do support WRITE_ZERO command so you don't actually
have to write all those zeros. The storage will just internally mark all
those blocks as having zeros. This is rather fast so I'd expect the overall
result to be faster that zeroing page cache and then writing all those
pages with zeroes on transaction commit. But I agree that for lower end
storage this may be slower because of synchronous writing of zeroes. That
being said your transaction commit has to write those zeroes anyway so the
cost is only mostly shifted but it could still make a difference for some
workloads. Not sure if that matters, that is your call I'd say.

Ocfs2 is mostly used with SAN, i don't think it's common for SAN storage to
support WRITE_ZERO command.

Anything bad to add a new api to support eof writeback?

OK, now that I reread the whole series you've posted I think I somewhat
misunderstood your original problem and intention. So let's first settle
on that. As far as I understand the problem happens when extending a file
(either through truncate or through write beyond i_size). When that
happens, we need to make sure that blocks (or their parts) that used to be
above i_size and are not going to be after extension are zeroed out.
Usually, for simple filesystems such as ext2, there is only one such block
- the one straddling i_size - where we need to make sure this happens. And
we achieve that by zeroing out tail of this block on writeout (in
->writepage() handler) and also by zeroing out tail of the block when
reducing i_size (block_truncate_page() takes care of this for ext2). So the
tail of this block is zeroed out on disk at all times and thus we have no
problem when extending i_size.

Now what I described doesn't work for OCFS2. As far as I understand the
reason is that when block size is smaller than page size and OCFS2 uses
cluster size larger than block size, the page straddling i_size can have
also some buffers mapped (with underlying blocks allocated) that are fully
outside of i_size. These blocks are never written because of how
__block_write_full_page() currently behaves (never writes buffers fully
beyond i_size) so even if you zero out page cache and dirty the page,
racing writeback can clear dirty bits without writing those blocks and so
they are not zeroed out on disk although we are about to expand i_size.

Correct.

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Did I understand the problem correctly? But what confuses me is that
ocfs2_zero_extend_range() (ocfs2_write_zero_page() in fact) actually does
extend i_size to contain the range it zeroes out while still holding the
page lock so it should be protected against the race with writeback I
outlined above. What am I missing?

Thank you for pointing this. I didn't realize ocfs2_zero_extend() will
update inode size,

with it, truncate to extend file will not suffer this issue. The original
issue happened with

qemu that used the following fallocate to extend file size. The first
fallocate punched

hole beyond the inode size(2276196352) but not update isize, the second one
updated

isize, the first one will do some buffer write to zero eof blocks in
ocfs2_remove_inode_range

->ocfs2_zero_partial_clusters->ocfs2_zero_range_for_truncate.

     fallocate(11, FALLOC_FL_KEEP_SIZE|FALLOC_FL_PUNCH_HOLE, 2276196352,
65536) = 0
     fallocate(11, 0, 2276196352, 65536) = 0

OK, I see. And AFAICT it is not about writeback racing with the zeroing in
ocfs2_zero_range_for_truncate() but rather the filemap_fdatawrite_range()
there not writing out zeroed pages if they are beyond i_size. And honestly,
rather than trying to extend block_write_full_page() for this odd corner
case, I'd use sb_issue_zeroout() or code something similar to
__blkdev_issue_zero_pages() inside OCFS2. Because making pages in the page
cache beyond i_size work is always going to be fragile...

Thanks for the suggestion. I will make v2 using zeroout.

Thanks,

Junxiao.

								Honza